KX1 internal battery using NiZn cells

Hi all,

Nickel Zinc rechargeable batteries are not common, however HobbyKing sell them. I decided to get a batch and have a look at them in the KX1.

NiZn batteries are not for everyone. Firstly, the capacity is lower at 1500mAh than the typical NiMH battery at 2400mAh. The plus side is that the NiZN carry a higher voltage. It is at 1.85V at the top of the cycle and then slowly decreases throughout the cycle, but remaining well above NiMH, and even Alkaline batteries that start at just under 1.6V and decline to 1.1V over the cycle. NiMH spend most of their cycle flat at 1.2V.

For a radio like a FT-817 that linearly regulates the voltage, there is no point using AA cells with higher voltage, as the regulator literally burns the excess voltage as heat. The 1.2V of NiMH are fine in that application. The KX1 is a different matter.

The KX1 has a capacity for 6 internal AA cells. Using NiMH cells, these would deliver only about 7.2V for most of the cycle, which is below the recommended minimum voltage required by Elecraft. The rig still works – although I have not extensively tested it. The power levels generated are very low – perhaps only good for QRPp levels of 500mW or less. I am quite happy with QRP operating, but I don’t see a pressing need to go to QRPp at this stage.

The NiZn cells represent an opportunity to mix things up with a higher voltage that the KX1 can then use to generate more RF power. It won’t give me more operating time, but it will give me more punch. Here’s a look at 6 NiZn cells installed inside the KX1 AA battery holder:

Turnigy AA NiZn cells in the KX1 internal battery holder

Turnigy AA NiZn cells in the KX1 internal battery holder

The cells come charged, and showed 1.85V per cell and a total of 11.1V installed. I used the cells for several hours in the field, and I came back with the six cells developing 10.4V across them. The KX1 would develop low 1W to 2.5W depending on the band and (reasonable) antenna configuration. This was only a little down from what power it was developing with a 12.2V external supply.

The 1500mAh capacity would mean that these NiZn cells will deliver many hours, perhaps more than 20 hours, of operating time in the field. The low RX current draw of the KX1 is certainly a great factor here, especially compared to a all mode rig like the FT-817. This kind of setup would be really useful for multiday remote area activations, where the radio part of the gear needs to be kept to an absolute minimum for weight reasons – for example when South West Tasmania or Fiordland in New Zealand are part of the Summits on the Air program.

Regards, Wayne VK3WAM

Internal FT-817 NiMH battery option

Hi all,

A while ago I wrote about options for internal batteries in the FT-817. As detailed in that post, some are trying out LiPO options in the FT-817, and HobbyKing sell a 2600mAh 3S option. Provided operators watch the voltage level on the FT-817 screen to avoid excessive discharge, that would work well.

I still prefer keeping the LiPOs out of the radio – I am more than happy to use them as an external power source and often do. However, the internal battery is not great weight bank for gram. It has a 1400mAh labelled capacity. Mine would still be delivering the vast bulk of that even though I have extensively used it. It gives about 1 hour 20 minutes to 1 hour 30 minutes action when doing a SOTA activation with TXing at 5 watts. I have a BHI noise filter which is a little expensive on the juice, the 817 is drawing about 420mA on RX.

In my previous post, I highlighted that many NiMH batteries do not make the grade in the FT-817. They might get close to their labelled capacity (2100 to 2500mAh) typical on lower currents, but they struggle when 2A is called for. I did notice that the HobbyKing 2400mAh AA cells could handle a 2A load, so I was interested in trying them. That time had now arrived.

I used the 8 AA battery holder that comes with the FT-817 and loaded it up. This holder needs to be put in the radio either empty or with 4 or less cells as otherwise it jams. Care also needs to be taken that the cable does not get jammed as well. The holder can be subject to open circuit faults and I had a little bit of trouble with this.

HobbyKing AA cells in a FT--817

HobbyKing AA cells in a FT–817

After sorting out those issues, I got on VK3REC, a local repeater, and proceeded to rag-chew at 5W FM. This would present the highest current load on the batteries. I was able to operate for nearly 3 hours before the batteries got to the end. Nice result, and the HobbyKing cells had no trouble throughout the discharge cycle.

Near the start, the 8 cells would present about 10.1V on RX and 9.1V on 5W FM TX on a multimeter. The FT-817 display would show 9.6V and 8.4V at this time. The 8.4V on TX remained at that level for about 90% of the discharge cycle. The final part of the discharge cycle was more graceful than the internal battery, with the RX voltage slowly drifting down from 9.2V FT-817 display to 8.8V. The multimeter was about 0.2V above each of these levels. The 5W TX was at 7.8V when I stopped the cycle. The FT-817 will power off if it drops below 7.5V.

On the whole, I am very happy with the AA cell’s performance and can recommend it for use with the FT-817. I would suggest charging the cells individually in a NiMH smart charger. Alternatively, two lots of 6 hour charges in the FT-817 should fully charge the 8 cells from flat. The green cable in the battery pack needs to be disconnected for the FT-817 to charge the pack.

73, Regards, Wayne VK3WAM

Matching the 2m Colinear for FM

Hi all,

I have described my efforts in building a colinear for 2m over several posts. The base unmatched colinear was sort-of ok at 146.5, but not very good at 144.1, soI set about performing a match for 144.1 using data obtained using BlueVNA and the mrs MiniVNAPRO vector network analyser. This has resulted in a fairly good match, with an SWR around 1.7 and the FT-817 showing only one SWR bar. I can live with that.

Considering a match for FM

Back at 146.5, the FT-817 shows 3 bars. This is not so good – its not a disaster match, but it is not too special either. My earlier analysis suggested that I could put a stub out a few cm further from the feedpoint, but this did not work. I think the reason was that the data I was getting from the MiniVNAPRO was not that good. Recently, I sought to rectify that, as discussed in this post, which the discussion relates not only to the extender but the base MiniVNAPRO as well.

With that in mind, I set up the 2m colinear and did some tests. Here is a screenshot of Zplots showing the data I obtained:

Zplots showing resistance and reactance of the 2m colinear at the BNC connector

Impedance at the BNC connector

Just for piece of mind, I ran the analysis with the extender, and this is what it shows:

Impedance at BNC connector obtained using extender, which shows very similar results

Impedance at BNC connector obtained using extender

The results are quite close, and any analysis done with one data set will pretty much be matched from the other.

The T piece for the 144.1 match

As well as the BNC connector for the radio (or a cable going there), I also have another convenient point to consider matching – the same place I have the stub for 144.1 I have put a BNC t piece here to connect the stub (and disconnect it when I don’t want it). I know I can’t use another stub for 146.5 right at this point on the transmission line, but I could use some surface mount components for a shunt, or add an extra piece of coax and put a shunt off that. We’ll first look at the data from this point:

Impedance at the T piece

Impedance at the T piece

The interesting thing about this is that this is an effective transmission line transformation. If I use zplots to consider the effect of 36cm of RG58 on the impedance, I get the very similar results, and there is about 34cm of RG58, plus the BNC connectors in the middle. The results are not exactly the same – the empirical results are a little flatter because of loss in the cable.

The impedance looks quite different, but if you consider the plot on the smith chart over on the right, it is simply rotated nearly halfway around the centre of the chart. This is the effect of a 50 ohm coax transmission line.

Now I can’t use a stub right here, but lets see what linsmith suggests I can do:

Match design in linsmitch showing the untransformed impedance then two steps of transformation

Match design in linsmitch

First up is to use 15cm of RG58 to get to a point where a series stub could match. The untransformed impedance is the pink dots – this is taken from the Zplots data. If you look carefully at the smith chart from “Impedance at the T piece”, you’ll notice a subset of that curve is shown here. The 15cm of coax transforms this to the green dots. Putting the series stub with a short 18cm away then transforms it to the red dots.

Series stubs are more difficult to create than parallel stubs. The inner conductor of the coax must be broken, but the outer conductor allowed to continue. The stub inner conductor must be connected (eg via solder) to one side of the line inner conductor and the shield to the other. This is logistically quite difficult.

I also provided about 5cm of extra coax with another BNC connector on the other end. This will allow me to easily put this segment into the colinear when I want to match to 146.5, or to take it out. When I am operating on 144.1, I do not want this segment with this stub.

Here is the data with the match in place:

2m Colinear matched in the FM band segment

2m Colinear matched in the FM band segment

I didn’t quite get the match quite right, the best match is around 147.3, however I can live with this. A FT-817 shows no SWR from 146.2 all through to 148.

Now why did it not quite match? I think it’s because I might have stubbed about 10mm too short. Such is life. Here’s a linsmith look at this assumption:

linsmith analysis of the match result

linsmith analysis of the match result

Note: I’ve added an extra bit of transmission line to account for the measuring point of the VNA – it’s not right at the stub point. This means the red dots are replaced with a second lot of green dots, with the new red dots after the extra coax transformation – note that these calculated red dots fairly match the empirical data shown in the zplots smitch chart in “2m Colinear matched in the FM band segment” above.

This analysis shows that I might actually be able to take off about 5mm from the stub and I’ll further improve the match. I’ll not do that straight way – I’ve got a big trip to the Australian Capital Territory coming up and the match is already fairly good – good enough that further improvement might have only marginal effect.

Anyway it’s pleasing to have a good match now at 146.5, and I’ll be using this a lot in the next week.

73, best regards, Wayne VK3WAM

Getting useful data from the MiniVNAPRO Extender

Hi all,

On a recent activation on Mt Torbreck as part of JMFD, I attempted to create a stub to tune a 70cm colinear where the antenna had been built physically too short. Tests at home showed that the feedpoint impedance was on a point on a Smith Chart where a shunt could bring it to around 50 ohms resistance.

It proved not to be. I’ve done a number of matches before, and all the theory in the world can’t be wrong. What could be wrong is the data obtained.

Overlap between the base MiniVNAPRO and Extender

The Mini radio solutions MiniVNAPRO is a vector network analyser device that operates from 100kHz up to 200MHz. It is a two port device that works in both transmission and reflection mode. Measuring antennas only makes use of the reflection mode. Transmission mode is useful for baluns and anything that does any kind of transformation.

The Extender is a device that like a transverter for the MiniVNAPRO. It effectively extends the operating frequency up to 1.5GHz. It can operate both in reflection and transmission mode, so like the base MiniVNAPRO, it is possible to both analyse antennas in reflection mode (s11 data) and transfer devices to obtain the full s11, s21, s12 and s22 data sets.

The Extender can actually report from about 50MHz up to 1.5GHz, so there is an overlap where both the base device, plus the extender can both report from 50MHz to 200MHz. This is quite useful because it can be used to check the performance of the extender. The data that is obtained should be similar to what is obtained from the base MiniVNAPRO. If the data obtained in this 50MHz to 200MHz range agrees between both devices, then data from the extender at higher ranges has at least more of a chance of being close to the mark.

Checking results on the 2m colinear

I have discussed the construction of this device before. I used the MiniVNAPRO and obtained s11 data which was then used with a smith chart tool called Linsmith to calcuate a required stub to match the antenna at 144.1 MHz. This was deployed my recent field day trip and worked well. This tells me that the data I am obtaining from the base MiniVNAPRO is good and useful for designing matching solutions.

While up at Mt Torbreck, I ran tests on the colinear which showed quite a substantial difference between the results that the base unit and the extender obtained. I did a quick test with a 50ohm standard and both the base unit plus the extender both agreed that there was a 50 ohm load. We’ll that’s fine but I need to have a good idea of what an unmatched impedance is so I can do something about it. I’m not going to need to do something that already is 50 ohms!

It would need to wait until I was back home to look at this issue further.

Environmental sensitivities in calibration

After sleeping on it for a day, I thought this problem could arise for several reasons:

1) Could be a fault with the Extender itself. Some on the net have a poor opinion about this device, but we have to rule out everything else before going here.
2) A software problem with BlueVNA. If this was true, then I could use VNA/j on the laptop and this would give substantially different results if this were true.
3) A problem with calibration in some way. It is interesting that the 50 ohm test worked ok, but non matched impedances showed substantial variances – this made me think that perhaps if something in proximity to the device when doing a calibration could impact on the open or short circuit results that could have impact practical results.

I did some tests using VNA/j and that showed some significant differences on a 2m antenna between the base MiniVNAPRO and the extender. My old calibrations were done with the unit lying on the table, but when I use it in the field, it is hanging off some antenna in free space. Perhaps I should try to do calibrations again holding the unit up in the air.

After doing this, I noticed that there was a shift in the results obtained. Calibrating the MiniVNAPRO with extender lying on the table does not seem a good idea. My table at home is wood, but has metal bracing underneath the surface. Still after holding the unit in the air did not bring agreed results, but one set was within 10%. Good, but could be better.

In the end, I grabbed on of those in car phone holders that have a suction cup to go on a car windscreen. I put one of these on my desk and used it to hold the device during calibrations. I also found that there is benefit doing an average calibation – 3 to 5 passes. Most of the benefit is obtained with 3. There is little point to a greater number of passes such as 10.

2m Jpole results using VNA/j with USB interface

Here is a smith chart plot of results obtained on a 2m jpole that is a little long – both in the results obtained here, and in real life.

Smith chart result of 2m Jpole using VNA/j with MiniVNAPRO

2m Jpole using VNA/j with MiniVNAPRO

Smith chart results of 2m Jpole using VNA/j with MiniVNAPRO Extender

2m Jpole using VNA/j with MiniVNAPRO Extender

We can see that the results are quite close. They aren’t exactly the same but they are much closer than the results I was previously obtaining. One thing I could observe is that the better dynamic range of the base unit does show through. Also, I used 20,000 calibration steps for both calibrations. On the extender, the steps are nearly 8 times further apart than for the base unit.

Still these results are close enough that these extender results are useful.

2m Jpole results using BlueVNA with Bluetooth interface

With the VNA/j results giving me greater confidence, I then moved over to look at BlueVNA. BlueVNA does not allow averaging of calibration data. I still tried doing the calibration in the phone holder. Here is what the Jpole looked like in BlueVNA.

Smith chart results of 2m Jpole using BlueVNA with MiniVNAPRO

2m Jpole using BlueVNA with MiniVNAPRO

Smith chart results of 2m Jpole using BlueVNA with MiniVNAPRO Extender

2m Jpole using BlueVNA with MiniVNAPRO Extender

Firstly, the results broadly match, both with the base unit and with the extender. The basic line on all four charts is the same. One thing that I have noticed is that the “noise” in the data is higher with these results. In terms of quality I would rank them from best to worse as follows:

1) MiniVNAPRO with USB
2) MiniVNAPRO with Bluetooth
3) MINIVNAPRO Extender with USB
4) MINIVNAPRO Extender with Bluetooth.

Having said that, all four results are quite usable, so I think I have got on top of this problem for now. Obviously I cannot compare the base unit at 70cm as only the Extender works there, but perhaps I could compare USB mode against Bluetooth. We’ll see how that goes with the 70cm colinear that I’ll still see if I can match it at 439 MHz.

73 and regards, Wayne VK3WAM

2013 John Moyle Field Day

Hi all,

The John Moyle Field Day is a Amateur Radio contest conducted once a year. It, along with the VHF/UHF field days represent the four major line of sight frequencies based contests in Australia. The John Moyle field days (JMFD) also have a substantial HF component that is not present at all on the VHF/UHF field days, and while it also allows activity right into the mm bands, typically operators are exiting stage left at 23cm or 13cm.

It’s a good contest for backpack portable because of the 6 hour section. It also rewards repeat contacts on CW and Digital modes, unlike the VHF/UHF field days. I am finding that I am able to operate chasing contacts nearly the whole 6 hours rather than lots of CQ’ing.

Mt Torbreck VK3/VN-001

Last year I participated in the JMFD from here, and I decided to do the same again this year. Mt Torbreck is the closest 10 point SOTA summit to Melbourne. It has no commercial radio equipment on the summit and is not accessible by road. There is only slight obstruction into Melbourne, so with a reasonable antenna, one can work HT stations on 2 and 70cm. The JMFD has a distance component, and Mt Torbreck is a good distance away to get many Melbourne contacts into the higher point distance bands. A number of JMFD stations also head southwest and west from Melbourne for the field day and this means that those contacts are high value. The EMDRC normally activate Mt Cowley as VK3ER for this contest and Mt Torbreck has LOS to Mt Cowley, about 222km away. Incidentially Mt Cowley is VK3/VC-022, but VK3ER drive to the top and use generators for their high powered station, so they are not a SOTA contact.

Mt Torbreck is accessible by a 2km walking track which climbs about 300m from the car park to the west of the summit. The walking track is steep and good shoes or boots are a plus. It takes about 50 minutes to climb up and 40 back + extra if carrying a lot of weight. I was carrying a lot of weight on this trip.


I normally plan to make two trips up because I have too much stuff for one trip. The stuff to be carried up includes:

  • Yeasu FT-897 all mode rig
  • Yeasu FT-817 all mode QRP rig
  • 2 squid poles for a 2m and 70cm colinear antennas
  • 6 1.5m al segments to make 3 times 3 metre poles for various antennas
  • A turnstile antenna for 6m. This uses four 72cm M10 al segments with M8 taps to screw in 4 more 75cm segments to form two dipoles that are perpendicular horizontal polorization. There is an effective ugly balun for 6m near the feedpoint with a female UHF connector
  • A quadruple quad antenna for 2m, which comprises of 10 al segments. There are two fiberglass poles about 50cm long to mount at the bottom and the top. This goes through a 2 PVC pipes that join together so that they are just over 2m long. This is mounted on the pole by a small al segment to offset it, and a bit of sticky tape to provide a shunt to ensure that the PVC remains vertical
  • A similar setup on 70cm but here I have two quadruple quads
  • 12 Turngy 3S 5000mAh LiPos. I put 3 in parallel x 2 in series when using them, so I effectively have two rounds of this
  • A 12V down DC-DC converter sitting on the output of the LiPo array
  • 3 Turngy 3S 2.2mAh LiPo packs to power the computer for a while
  • A laptop with a supply that takes a 12V input
  • 12 18650 lithum ion cells in 4 lots of 3 in series. Each one can be used to power the FT-817 for a while – my typical SOTA power setup
  • A pile of Anderson Pole terminated power cables, including four 1 to 3 Y cables
  • A collection of LMR195 and LMR400 cables. The 195 cables have either BNC or UHF connectors. The LMR400 are all N connectors. I also brought up a pile of converters. I typically use UHF on 6m and BNC/N on 2 and 70. The FT-897 has a UHF connector for HF/6 and an N connector for 2/70. The FT-817 has a UHF and a BNC connector which can be configured through the menus
  • A one man tent to keep me out of the sun and the forecast showers
  • A MiniVNAPRO and the extender to do some tests on the antennas to ensure all is ok
  • Some plyers, shifters and other misc equipment to do any small repairs to equipment if that proves necessary
  • A collection of HT radios for 2 and 70. The idea being to use these for FM on 2 and 70 for at least some of the contacts to save power
  • Some headphones
  • A CW touchkeyer
  • A Signalink USB for digital modes, plus my Phone/Digital interface for a backup if required

For those who follow my SOTA activations, you would realise that this is far more than I normally take. You may realise why two trips are needed. Practically this means walking UP then DOWN then UP the mountain before the start of the contest, then DOWN then UP then DOWN at the end.

The setup

Upon arrival after the first accent, I put up the tent and piled the stuff in. Carrying all that LMR400 cable, the FT-897 and the 12 LiPOs certainly was a lot of weight and it took 1:10 to get up. I headed back down and got the remainder of the stuff, and it took 40 min down and 50 min up. First was getting the squid poles up. My 2m colinear has seen a lot of SOTA action, and a little stub I made up for 144.1 works well. Very low SWR – nice one. I use it without the stub on 146.5, with SWR around 2. With the stub for 144.1, 146.5 has a SWR around 3.

The 70cm colinear is short, with the good oil around 455MHz. I planned a stub for this to get 439 at least in the game, but I was not successful. There will be more about this in another blog post in the future, but for now I could not effectively use this colinear – what a shame because this was going to be the main game on 70cm vertical. I put up one of my whip antennas on the squid pole about half way up and fed it LMR400 cable back to the radio.

Next up was the 6m turnstile. This is pretty quick to put up. This is not a gain antenna, but it is enough to put me in the game on 6m and it can break up quickly to go in a pack. It gives an SWR below 1.5 at both 50.15 and 52.15

Here’s a pic of the 6m antenna in the foreground with the two squidpoles in the background, plus the operating tent. You can see the trig point to the right in the trees:

Operating location at Mt Torbreck with a 6m turnstile, plus 2 & 70cm colinears on squid poles

6m turnstile, plus 2 & 70cm colinears

The 2m quadruple quad was next. Last year I had two of these in an array, but they were too phyisically heavy to put up, so I jury-rigged up one. This year I did not bother with an array, just going for one with some work to mount it more effectively. There’s a pic below. It was well below 1.5 SWR at 144.1 and around 1.7 at 146.5.

2m quadruple quad at Mt Torbreck

2m quadruple quad at Mt Torbreck

I had thought of trying a longer pole but I would need to use stronger materials. Another idea is to mount it on a squid pole. I’ll think about this for next year, but this will be asking a lot more of the squid pole than a wire inverted V, end fed or a vertical on HF. Why a quadruple quad? Because it is roughly equivalent to a 15 element yagi, especially if I can get it off the ground a bit more.

Finally it was the 70cm quad-quad array. Here’s the pic:

Array of 70cm quadruple quads on Mt Torbreck

Array of 70cm quadruple quads

One of the quads didn’t work so well, and time was running short, so I simply used the other one. Their feedpoint impedance is reported low at about 25 ohms. I’ll need to look into this some more, but I wanted to get operating at around 1:30 to 1:45pm so off I went.

The contest

It started lightly raining about 12pm, so I was a little reluctant to keep the MiniVNAPRO out in the elements. A few mad dashes and doing some analysis on the computer. My plan with the computer was to run it on the three 2.2Ah LiPOs directly until they were flat and then run it on the main supply for an hour. I could then run the computer on its own batteries for the rest of the time without them running out by the end. This worked well, but the 2.2Ah LiPOs gave me more time than expected. Nice to get more than you expect! When the LiPO monitors were reporting individual cells on the 2.2Ah batteries down to 3.55V, I pulled the computer out.

The whole 6 hours of operating was quite fast and furious. Most of the action was on phone, but VK3ER had a digital setup, at least on 6 and 2. They also had CW on 6/20/70 so there was some good triple dipping. I used Fldigi for PSK, and was more comfortable using it in the end in my one man tent lying on my side trying to type on a computer with the pouring rain outside than what I was in the middle.

I was also glad I brought the headphones, because the rain was very loud in that small tent. During the worst periods, I would mainly use the vertical antennas, which the main gun was the 2m colinear. The little whip at the end of the LMR400 cable on 70 was just no match for the 2m colinear. I need to get that 70cm colinear going for next year, these babies are just too good to ignore. The colinear being omnidirectional on the horizontal plane was good during the pouring rain because I did not need to get out of the tent to adjust anything. Same goes for the 6m turnstile (although it’s not a gain antenna). Gain on 6m might be a little hard to do given it needs to fit in a backpack along with everything else.

There was a 2 hour sunny period during the middle of the contest. This allowed me to get a bit more relaxed and I made more use of the quadruple quads. Towards the end it was raining again, but I really wanted some nice juicy contacts north into VK2, and my 2m q-quad was able to get them.

As for power, I already mentioned that the 2.2Ah LiPOs powered the computer well. I ended up not even using half of the 5Ah LiPOs, the first set of 6 were only 80% used at the end, with cell voltages around 3.75V. The “knee” on these is at 3.65V where the voltages start to fall away more quickly. I was hammering away with FM at 50 watts on 2m, but the LiPOs and the 12V regulator powering everything were stone cold. Not even lukewarm. This was a contrast to last year because my old 100 array of 18650 cells could not handle it. The LiPOs are just so much better for this usage.

Come 7:30pm it was finish time. I ended up not even turning on the HT’s. I barely used the FT-817, which is a major change from last year, where because of power constraints, I made most FM contacts on the HTs, and used the FT-817 for a fair amount of the rest. This year, the FT-897 was used for every scoring QSO. Did I mention that half of my big LiPO’s were not even touched? 🙂 I just had to make sure that at least 4 QSOs were at 5 watts so I could keep my QRP SOTA activator’s endorsement intact.

Packing up

So contest finished, and it was time to go home. Too much stuff there to just leave it – although the thought did cross my mind as to what would happen if I just walked down the mountain with all that stuff still up there! It was raining again and it took about an hour to pull down all the antennas. With all the wet conditions, I needed to be careful getting the fragile computer back down the mountain, so I thought I’ll go easy on the weight on the first trip, but still enough to hopefully not have a tonne of weight on the second. One thing I’ll need to make sure of next time is to split up some of the LMR400 cable on the trips because this stuff is heavy.

I left on the first trip down at 8:30, left the car to go back up at 9:20, packed up the tent and did my final checks to make sure nothing was being left and departed Mt Torbreck at 10:20pm for the last time. I arrived at the car at 11:15. It was slow the last time with the heavy pack and the slippy wet conditions on the way down. At least it had stopped raining. I was very tired for the drive home and needed a 15 min powernap in Healesville to keep things safe. My wife thinks arriving home at 2:30am is crazy but it was a very good day with 92 contacts and over 1440 JMFD points.

Regards, 73, Wayne VK3WAM

MiniVNAPro Extender with Bluetooth

Hi all,

After my earlier post which considered use of the MiniVNAPro Extender using a USB connection, I wanted to give the Extender a go with the BlueVNA program.

Using BlueVNA with the extender

When running BlueVNA, a picture of the device will show. This could be a a MiniVNA, a MiniVNAPro or a MiniVNAPro with the extender. I swiped left, which went through these three devices. As the pictures suggest, use of the extender requires the MiniVNAPro with extender to be shown on the screen.

As I discussed in my earlier post, I have found that I need to use a specific startup sequence to get useful data from the extender. When using USB, the USB cable needs to be connected before connecting the CAT5 cable between the MiniVNAPro and the extender. In the case of bluetooth, the CAT5 cable should not be connected when turning the MiniVNAPro on. I wait for the Bluetooth blue light to come on on the MiniVNAPro, and then connect the CAT5 cable. The extender’s sole orange light then comes on.

After that, the calibration sequence is as per usual. After calibrating with a open, short and 50ohm standard, I ran a test on a Diamond RH771 2/70 dual band whip antenna. Side note: This thing is not crash hot on 2m, it is better on 70cm.

Here’s a pic of me holding the MiniVNAPro connected to the extender with the Diamond on the RX port:

Diamond RH771 on the RX of an extender connected to a MiniVNAPro

Diamond RH771 on a MiniVNAPro with Extender

Here’s a look at a screenshot:

Diamond RH771 data at 70cm using BlueVNA

Diamond RH771 data at 70cm

With a return loss of over 16dB at 439MHz, this antenna will work well at the 70cm FM calling frequency. It is still respectable at 432MHz.

An interesting thing about this data is the presence of a few artifacts in the data. They are the “square wave” type formations that can be seen. These arise from the calibration process. BlueVNA does not allow the number of steps to be set during calibration, unlike VNA/J. With VNA/J, I set the calibration steps to the max value, which is 25000, rather than the default 2000 steps. It makes the calibration process take longer, but then actual use of the device is not any slower. What it does do is reduce the size of any “square wave” artifacts.

Suspect Bluetooth interference

If I calibrate in VNA/J using a lower, say 500, calibration steps, using USB, I also notice the presence of artefacts, but they are much less than what we see here. We are seeing the Return Loss being moved up to a full 1dB in these results shown above. This then flows through to everything else, SWR, resistance, reactance, because all of these things are calculated by the software using the return loss and phase (I have the phase switched off on the screen shot).

When using the MiniVNAPro with the extender in USB mode, things are getting a little “hairy” around 1.3GHz and above, I don’t think the dynamic range of 50dB is true at 1.5GHz. It’s still not bad, and certainly still useful in doing antenna analysis for 23cm.

I’m not sure I could say that about Bluetooth mode. I’ll do some 23cm analysis in the future, but here at 70cm, we are seeing these 1dB artefacts that are affecting the results. We can still look through them, and we can still draw conclusions about the antenna.

Perhaps there may be some value in BlueVNA allowing several calibrations and averaging the results. Same goes for the scans. Doing an average scan would slow things down, but it may help to mitigate some of these effects. Perhaps there also might be ways to better shield the Extender from Bluetooth activity.

Even with all of this, it is still better to have the flexibility of the bluetooth mode. Also it looks like the artefacts have a predictable behaviour (either adding or subtracting from the return loss an amount up to 0.5dB). It might be possible to correct for this in software.

Regards, Wayne VK3WAM

ADDENDUM: I used the extender to do some testing on a 70cm colinear I have been building. I performed a calibration outside, and the results I obtained did not show the effects that are seen above. So, there is another noise source on my desk at home that affects the extender/MiniVNAPro combination in bluetooth mode, rather than it being bluetooth itself.

SOTA One year anniversary

Hi all,

It’s been a little over one year since SOTA started in Australia, first with VK3 in February 2012. To celebrate, the Morrabin and District Radio Club agreed to host a gathering for us. We will do this close to the actual anniversary at the start of February in future, but this year it turned out to integrate with their usual Saturday morning meeting towards the end of the month.

Ron VK3AFW gave a demonstration of various antennas and activation options to a large gathering that was close to filling the club’s meeting room. It was a great turnout with most of our VK3 activators present, along with Andrew VK1NAM who was in Melbourne for family reasons. He was able to join us for most of the meeting.

After Ron’s presenation, it was out to the nearby parklands to set up a variety of SOTA stations. I brought along a MiniVNA Pro and used this, along with Blue VNA to have a look at what activators are using out there.

Endfedz EF-40/20

The Endfedz is a commercial end fed antenna for 40 and 20 rated to 100 watts. The antenna is well made and the cable is quite robust. The match box at the end is suspected to be a 9:1 balun.

The FT-817 reported no SWR at 7.09. Here’s what the VNAPro found at the feedpoint:

BlueVNA screenshot of ENDFEDZ EF-40/20


Both ends of the antenna are about 1.5m off the ground. The insulator at the far end has been moved up about 70cm. The centre of the endfed is about 6.8m off the ground at the top of a squid pole.

The interesting thing about this is that the antenna works quite well, but there is still some signal being lost. The match is only just ok right down the bottom of the band, with a SWR a little below 2. When we tested the antenna with about 10m of LMR195 between the feedpoint and the radio, we found the return loss at around 13dB. The extra loss is imposed by the cable. Not all of this loss is going to be the first 3dB, but still we are going to be burning about 30% of the TX power in the cable.

The SWR at the end of the cable was about 1.6 at 7.12MHz. The radio is at least going to want to put out most of it’s power.


Apparently the makers of this antenna stress that this setup is not a vertical with a single counterpoise. It’s supposed to be a dipole half on it’s side. Hmmmm, the conductor going up has a loading coil about a foot and half above the feedpoint, plus then further conductor above that. Seems like verticals I’ve made. The horizontal part of the dipole off the ground seems a lot like a radial to me. It’s broadly the same electrically as the vertical I’ve made, but only one radial. Verticals seem to like at least 4 radials a little in the air to work well.

Anyway, this was the initial result:

Buddistick VNAPro results before tuning


This pic shows that the antenna is resonant around 6.84MHz and has the best return loss at 6.85MHz (it is typical for best performance to be a little above resonance if the resistance is less than 50 ohms).

After moving the loading coil up a notch, we moved the resonant point to 7.05, and then played with the counterpoise to further tune the antenna.

The picture makes it look a bit worse as the range is plotted from 6 to 7.5, but the bandwidth is narrower than the end fed. Return loss tops out at about 8dB. It’s not a great match, so this thing could benefit from a tuner near the feedpoint.

This is what things looked like after a little bit of work:

VNAPro data on Buddipole after some adjustment

Adjusted Buddipole

Notice that the main reason for the SWR still remaining at about 1.8 is the feedpoint resistance at resonance of around 27 ohms.

This is after a bit more work:

VNAPro data for a Buddipole at 7.1MHz resonance

Buddipole configured at 7.1MHz

Notice that because the feedpoint resistance at resonance is well below 50 ohms, about 24 here, the best performance is frequencies a little above resonance.

This is what the above looks like at the other end of the coax, the radio end:

VNAPro data of a Buddipole plus 10m of coax

Buddipole plus 10m of coax

The 24ohms is being transformed to a little above 100ohms (but at a slightly lower frequency because the length of coax is not exactly a quarter wave length). The effects of a near 1/4 length coax can clearly be seen by comparing the two pictures.

2m SlimJim

Next up was a 2m SlimJim mounted on a 7m squid pole. As can be seen in the pic, the results of this antenna are excellent.

VNAPro results of a 2m Jimslim on a squid pole

2m Jimslim on a squid pole

It can be seen that the SWR is low across the whole band. This atnenna has a deep return loss. The low SWR means that most power is transported to the antenna. Loss there is good – it means it got radiated. At 145MHz, it is about 26dB, meaning that only a fraction of 1/100th of the signal is coming back to the radio. It’s still around 20dB at 144.1, and a still very nice 18dB at 146.5.

Homemade Endfed with a counterpoise

Now to finish off, it’s time to look at a homemade endfed. It’s interesting to compare this with the endfed that I am using:

VNAPro results of an endfed with a counterpoise

Endfed with counterpoise

While this is tuned a little high for the usual action at 7.1 and CW at 7.027, clearly the counterpoise adds value. The best return loss is around 15dB, which is a big improvement on the 9 to 10dB of the Endfed without a counterpoise. Might have to think about doing something 🙂

The wrap up

It was interesting to have a look around and see what people are up to. I missed out seeing Rik VK3KAN’s freestanding squid pole, but that’s for another day. The guys spent so much time looking at the various options out there, we even took our time getting up to the BBQ. Now: radio even over food. There is something wrong.

Regards, Wayne VK3WAM

MiniVNA Pro Extender

Hi all,

I had been waiting on an order from WiMo for the MiniVNA Pro Extender. It arrived today and I was keen to check it out.

Mixed Internet Reviews

This device has been subject to some discussion on the net. There are some reviews at eham, as of the date of writing this post, there were 2 reviews. One rated the Extender a 5, and the other a zero. It seems that some people are finding that this product works and some are finding that it doesn’t.

First go

It was with some fear and trembling that I set the device up and started to do some testing. Here is the pic of the Extender hooked up to the MiniVNA Pro:

MiniVNA Pro Extender and Pro connected together

MiniVNA Pro Extender and Pro connected together


My first attempts at making use of this device did not go well. On the calibration screen in vna/j, there was no difference between short/open/load calibrations in reflection mode. Hmmm, am I like the guy on eham who said that this product only deserves to go in the bin – a bit harsh! There has to be something more than this. I tried several combinations of connecting and using either 2000 or 25,000 calibration points in vna/j. No joy. There has to be some trick to this, because there are people out there that have made this product to work.

I used an adjustable wrench or shifter to gently ensure that the SMA connector between the Extender and the Pro was fully connected. I turned on and wella. The open and short calibrations look like each other, but there is a clear change of phase. I got real world sensible results out of this device.

Second go

I had to stop at that point and do some other things. It was quite a bit later in the day that I returned to looking at the Extender to have a look at a few antennas for 70cm. I have a 7cm Jpole at home, plus a turnstyle for 2m that has worked reasonabily well on 70cm as well. I wanted to look at these antennas on the Extender. I hooked things up, and made sure with the shifter that things were well connected – no go! Rubbish results. What is the deal with this device!

After a bit of stuffing around and making sure I remember each step in what I did, I have found this following method a reliable way of getting the Extender to work. Not following this leads me to get garbage results.

1) Connect the Extender to the Pro DET and DUT ports, but not the CAT5 cable.

2) Connect USB from the computer to the Pro. (I would also presume that in Bluetooth mode, turn the Pro on)

3) Connect the CAT5 cable from the Extender to the Pro.

This seems to work, on a repeatable basis. If the CAT5 cable is connected at Pro power on, it does not seem to properly use the Extender and garbage results seem to then occur.

This behaviour strikes me as a firmware bug, and changes to the firmware on the Pro or Extender might overcome this problem in the future. Nonetheless, it seems the three step process above is a reliable work-around.

When the device is working, it seems to get pretty steady results, and I am quite happy with it. One little thing to watch out for is on the Extender, the DUT port is the RX port in reflection mode, but is the TX port in transmission mode. This is different to the Pro itself, where DUT does not change.

Here is a look at a screenshot of the 2m turnstyle antenna at 70cm:

VNA/J Screenshot of MiniVNAPro Extender in action analysing a 2m turnstyle at 70cm

VNA/J Screenshot of MiniVNAPro Extender in action

Now I know how to use this device, I am quite happy with it, and look forward to doing some work on 23cm in the future.

73, Regards Wayne Merry VK3WAM

The Bluff/Mt McDonald trip, New Years 2012/13

Hi all,

After getting back from a camping trip, which included a SOTA activation of Rocky Peak, I was off the next day on a planned VMTC trip to a range of high mountains to the south east of Mt Buller. The plan was to take in part of the Australian Alpine Walking Track which I had not done before. I only had two takers for the trip, so there were three of us in total. The VMTC requires four for an official club trip and we were one short, so this turned into a private trip. My plan was to activate for Summits on the Air a series of mountains that were taken in by the walk.

Getting started

The day started with a 7am pickup of a walker from the train station, and then we headed up to join another walker up in the Howqua Hills at 8 Mile Gap (506792). From here, we headed down Brocks Rd to the Jameson River. This is marked as a 4wd track, but it is maintained to a high standard, so no problems with a 2wd. We then drove up Low Saddle Rd. The standard of this road is not as high, but I was able to get the Prius to point 513754, which would take a bit off the walk on the last day. We then piled into a 4wd for a car shuffle and took the Refrigerator Gap Track up to Refrigerator Gap and the carpark for the walk up to the Bluff.

The Bluff VK3/VE-013

The Bluff is a shortish, but quite steep climb up from the Refrigerator Gap car park. Over 400m is gained from the car park over about 1km horizontal distance. Towards the top it becomes a little scrambly. I had to negotiate the squid pole a few times, but it was not too bad. The plan was to have lunch here, so my fellow bushwalkers would have their break while I operated radio. One little problem was that I did not bring anything to secure the squid pole, and there was no trees at the operating location. I used the base of a rock and my backpack to secure the squid pole, as shown:

Squid pole at The Bluff secured by a rock and a backpack

Squid pole at The Bluff

I used a Wouxun KG-UVD1P handheld radio for FM and a FT-817 for other modes. Using the Wouxun allows me to save the FT-817 batteries. Because the FT-817 is an all mode radio, its amplifier is linear, and not as efficient as the Class C amplifier in the Wouxun. I use a BNC adapter on the Wouxun, so I was able to change the antenna from one to the other radio with ease.

The activation was performed with my colinear. I decided not to do the match yet (as discussed in the linked article) as I wanted to make the stub with BNC connectors which I am waiting for an order of these to arrive. The unmatched colinear has a poor, but not unusable match. The FT-817 would reduce power a little, but this level of mismatch (between 2 to 3) would not damage the finals. I estimate it would be 4dB or so down on a fully matched performance. Still worthwhile to give it a go and also to compare its performance to when I do have it fully matched.

It certainly does perform. I know some think that I would be better off with a Yagi, but for this kind of walking, this colinear does hit the spot. I tried it against a few repeaters and would give it at least 15dB ahead of a quarter wave whip antenna. One repeater was very scratchy on the quarter wave, I could barely break the squelch, but with the colinear, it was nearly (not quite) full quieting. Being omni-directional on the horizontal plane is also a bonus.

In the end, I was able to work into Melbourne, and looking back at the results, I was about 5dB down on what I would have expected from an optimised colinear. I think I’ll gain most of that with the match. I was also working a few stations well away from Melbourne.

Here are some pics from The Bluff:

Looking west from The Bluff

Looking west from The Bluff

Looking east from The Bluff

Looking east from The Bluff

Mt Lovick VK3/VE-020

After lunch, it was time to head off The Bluff and head for Mt Eadley, Bluff Hut and then Mt Lovick. There are two tracks marked passing Mt Eadley Stony, but we only found the higher track. The Bluff Hut has been rebuilt after the 2006 fires, but is not used as part of the cattle muster as cattle are now banned from the Alpine National Park. It was fairly late in the day when we made Mt Lovick and we camped almost right on the summit, far enough away for some dirt to put tent pegs in.

Camping at Mt Lovick

Camping at Mt Lovick

I tried 2m again from here. I was able to complete 2 QSOs using FM, and then 1 contact with VK3AFW using CW. I then went to 40m to get the last contact required for activation – plus a pile more.

Here are a few late evening pics from the summit:

Looking east towards the King Billies from Mt Lovick

The King Billies from Mt LOvick

Looking south west towards The Nobs and Mt McDonald from Mt Lovick

The Nobs and Mt McDonald from Mt Lovick

Mt Clear VK3/VE-018

Up the next morning, Sunday the 30th. We left about 9 and headed for the King Billies. Our first water stop was from the headwaters of the Jamison River, just underneath Picture Point at 634832. I treated the water with the SteriPen Adventurer (a UV treatment), another walker, Adrian used tablets, while the third, Clive, drank it untreated. I still reckon that the chances of getting sick are low, but because I have been sick from drinking high country water, I don’t take that chance.

We arrived at King Billy No 1 and had lunch there. I wanted to save my operating capacity (i.e. batteries) for summits that I could get SOTA points for, so this went begging. I activated this earlier in the year, late at night and in the fog. I was able to see my old operating location, just underneath the summit. Here’s a look at Mt Magdela (Marjorie VK3/VE-012) and the Cross Cut Saw to the left from King Billy No 1 VK3/VE-016.

Looking east at Mt Magdela from King Billy No 1

Mt Magdela from King Billy No 1

With lunch done, we followed the quite indistinct path to King Billy No 2, and then lost the path and made our way to the road junction at 650812. From here, there is a management vehicle track (with trees fallen over it) heading down towards Mt Clear. We drew water at 642787 and made our way to the base of Mt Clear. The management track leaves to the west, and the start of the walking only track is a little unclear. This is at 627766. From here, the track climbs quite steeply, but not quite as steep as the track up The Bluff. Still a good effort for late in the day. We arrived at the summit and camped within a few metres of the summit.

I attempted activating VK3/VE-018 on 2m, but had no success either on FM or SSB. It was then over to 40m using the end-fed, where there was no difficulty. With the end-fed, 40m is the old reliable.

Here’s a pic of Mt Buller, Mt Sterling, Mt Lovick (closer) and Mt Cobbler from Mt Clear. All of these mountains are SOTA summits.

Looking north west from Mt Clear

Looking north west from Mt Clear

The Nobs VK3/VE-040

The next day, it was up for an 8:30 walking start. First task of the day was to get a water top up. I only had a small amount left, and some of the party was dry. We obtained this at 617729, where the creek was about to descend rapidly down the side of the mountain. From there, it was following the indistinct path down from Square Top and towards High Cone. High Cone appears to have exactly 150m prominence, so it will become a SOTA summit someday, but it is not one at the moment. There is a bypass track around High Cone, so we took this. It’s more of a route as the track is quite indistinct. It follows just above a set of small cliffs. One gotcha with this mountain is where the spur comes down, the track continues to contour around. We headed down the spur at that point for about 15 minutes before we realised our mistake. We then worked our way around to the right spur, but this took more energy and time. It was a late lunch when we arrived at the Nobs.

Given the lateness, I decided that I would only do 40m on the summit. I activated the summit quickly, but disturbed a large number of insects nesting in a tree where I was setting up the squid pole.

Mt McDonald VK3/VE-026

From The Nobs, it was a quick drop down along the still at hard to follow at times track towards a 4wd track. We headed along this to where the walking track leaves it to head up Mt McDonald. We needed water for the night camp, and so we headed down the 4wd track to get water. We got it from the second creek at 543729, but we had to go in about 10m from the road and burrow into what was almost a little cave to get it. This took quite a lot of time, and meant we would get to the top of Mt McDonald very late in the day. It was a lot of effort getting up, and the effects of the long day were definitely having an impact. We lost the track at one point and tried to sidle around the side, but this was very hard work, and felt dangerous. I then climbed up a very steep section to regain the track that was on the tops. If climbing Mt McDonald from the east, try to stay on top of the ridge if you don’t know where the track is.

We arrived with about one hour of light left at the summit. I was pretty exhausted, but I still wanted the SOTA points for all this effort. I setup the tent and then the amateur radio station:

Camp site and operating location at Mt McDonald

Camp site and operating location at Mt McDonald

The views from all the summits that I had visited so far on this trip had been spectacular, but a great deal of the southern high country can be seen from Mt McDonald, including Mt Donna Buang, Mt Torbreck, Mt Baw Baw, Mt Abrupt, Mt Alexander (near Bendigo), Mt Hickey, Mt Buller, Mt Howitt, Mt Cobber, plus most stuff around Licola. Here’s a view looking back towards The Nobs and Mt Clear:

Mt Clear, Square Top, High Cone and The Nobs from Mt McDonald

Mt Clear, Square Top, High Cone and The Nobs from Mt McDonald

It was a great way to see out 2012 with the views we had from up there.

Mt Sunday VK3/VE-050

The final summit for the trip was Mt Sunday. The SOTA program works on UTC time, and a summit can be activated for points basically once a year. There was an opportunity to activate this summit twice, before UTC midnight (11am local) and then after. It was break camp at 6:50am to give myself a chance to do this. The other guys were completely disinterested in the radio part, but appreciated walking in the cooler time of the day. It was about an hour in that I realised that I might not make it to Mt Sunday in time, so I had to burn rubber. I walked very quickly down the track – which was easier to follow on this side of the mountain. I made Low Saddle at 506678 and left my pack there. The top of my pack converts into a little day pack, so there was enough room for the FT-817, the end-fed cable, a coax cable, the microphone, a notepad and a bottle of water, a LiIon battery and that’s it. It was time to head up Mt Sunday. The track is a little indistinct to get started, but then was easy to follow. It has also had the fire regrowth slashed back. This might have been quite difficult to follow two years ago, but is changed now.

I arrived, after a hard climb, at 10:45am. I operated north-east from the summit, about 15 to 20 vertical metres down. The summit has lots of trees, recovered from the fire at the very top, but burnt to a crisp elsewhere. I hung up the end-fed in the regrowth saplings and began to operate 5 minutes before UTC midnight. I was able to get 7 contacts in that 5 minutes, three of them in 1 minute. I felt like a contest station there for a bit. I continued to operate after 11am local, but it was far more relaxed. It was also nice to work VK3AFW and VK3PF who where both activating summits on both sides of UTC midnight. 4 lots of s2s! At 11:30 it was time to pack up and start the long trip to the car. I arrived back at my pack at 12:20pm which had a note from the others that they left to head for the car at 11am. I was an hour and 20 minutes behind.

It took another 4 hours and 20 minutes to walk the 11.5km to the car. I had a lunch break, but I was very low on water. I found water in several creeks. The first was a little desperate and I am very glad I could UV treat it. The others tasted better. The effects of the previous day had again caught up with me, plus the big effort to get to Mt Sunday in time. I was pretty tired when I got to the car at 4:40pm. I started 1:20 behind, but lost another hour to them. They could not keep up with me earlier, but now I could not even nearly keep pace with them!

After that, it was a shuffle back to the other car at Refrigerator Gap underneath The Bluff. I realised I had left my CW keyer at Mt Lovick, a day’s walk behind, but Adrian agreed to drive up there as it is close to the 4wd track. That track certainly is for 4wd only. He was eyeing off the place for future family camping visits, but I am thankful that I was able to retrieve the keyer.

It was time then to head for Mansfield and a much needed feed. Never has a bottle of soft-drink tasted so good.

Wayne VK3WAM

Constructing a 2m colinear Part 2

Hi all,

This is a continuation of Constructing a 2m colinear Part 1.

Repairing the Colinear

On a recent SOTA activation trip, I tested the colinear in the field. As discussed in that post, the lower SWR point seemed a little high. At the second and third activations on that trip, I was unable to use the colinear due to a persistent short. This short turned out to be in the BNC connector.

After coming back from the trip, I set about repairing the antenna and upgrading the connections to make them more robust. This was done using some spare foam dialectic from some RG6 cable I had lying around. I had previously used the braid from this for another project. This dialectic could now make itself useful as a physical shunt to give the joins of coax some structural integrity. I simply inserted the dialectic into the join area and secured it with tape. Seems to work well, and the colinear does not feel anywhere near as fragile.

Analysing the Colinear

Once getting the colinear fixed, I put it up at home in the backyard to have a look at its frequency characteristics. I used a MiniVNA Pro with the BlueVNA app running on a Samsung Galaxy S2, using bluetooth connectivity. Here is a screenshot:

Colinear screenshot from BlueVNA showing SWR, impedance, resistance and reactance

Colinear screenshot from BlueVNA

Several things to note from this picture is the SWR is unacceptably high. The FT-817 reports a lower SWR as some is being burnt up by warming the coax. Also, the resonant point is at 147MHz, when I was expecting 145. Before we get too excited, one thing to remember is this is a picture of what is at the BNC connector, not what is actually at the “feedpoint” which is through the common mode choke, up to the first coax connection, about 910mm of coax away.

Here is the same data using Zplots, which is an excel spreadsheet. The data from BlueVNA was saved to a s parameter file, which can be loaded into Zplots.

A Zplots render of the Colinear at the BNC connector, showing a graph and a smith chart.

Colinear at the BNC connector in Zplots

This zplots chart shows much the same information as the screenshot from BlueVNA, which is no surprise given it is based on the same data.

Zplots has the capability of compensating for the effects of transmission lines. I could then algorithmically have a look at what is going on at the actual feedpoint, which would be very difficult to physically access. Here is a screenshot from Zplots at the “feedpoint”.

A Zplot, consisting of a graph and Smith chart with compensation for the transmission line.

Colinear characteristics at feedpoint

This graph shows the antenna is resonant at 144.7MHz, which is not far from what the antenna was designed for: 145MHz. The gotcha is that the resistance at the point of resonance is about 14 ohms. No wonder there is bad SWR on this thing. This is a prime example of how a resonant antenna does not necessarily have low SWR.


What to do about about this antenna that is resonant where planned, but effectively gives me an SWR of 3 to 4 over the entire 2m band? I am going to have to do some impedance transformation to match to this antenna. What to do?

There is another Smith chart shown below. The purple dots represent the impedance at various points from about 144MHz up to about 146.5MHz. These points are effectively the same as the “feedpoint” data calculated by Zplots, with 910mm of 52ohm coax (RG58/U) being added to the data obtained at the BNC connector. Along the coax from the connector to the feedpoint, they essentially rotate on the Smith chart around the centre point. The centre point is, by the way, a 50ohm 1:1 SWR point. This is where we want to be, or at least within the red circle on the Smith chart.

I have more choices than right at the BNC connector or at the “feedpoint”. The first 500mm from the feepoint is a bit hard, as it is inside the aluminium sleeve of the colinear. This bit of coax is inaccessible. About a 290mm segment is also not accessible because it is in the common mode choke. There is a section about 350mm (360mm from where MiniVNA would have measured from) from the BNC connector that looks nice. This is 545mm from the “feedpoint”. This is represented by the smaller green dots. From here, I can use a parallel shunt bit of coax. If this bit of shorted shunt coax is 160mm long, then the impedance is transformed into the “matched” zone.

Impedance matching the colinear antenna using a Smith chart tool. The Smith chart shows a transmission line and then a shorted coax parallel shunt

Transformation of feedpoint impedance

One draw back is the cable will have a DC short because of the parallel shorted coax shunt, but this is a “nice” short.

73 de Wayne VK3WAM