John Moyle Field Day 2014 At Mt Torbreck

Hi all,

Most of my activations are lightweight, and even more so these days, using a KX1 with a random wire antenna. Once a year, I go in an entirely different direction, and that is for the John Moyle Field day contest. This is a contest with both HF and VHF and up sections. It also rewards making seperate contacts on phone, CW and digital. It has a 6 hour section that can be timed for three rounds of contacts for each station, and they can be contacted per band.

It’s the VHF and up section that interests me. Currently that interest is on three bands 6m, 2m and 70cm.

A few years ago, I identified Mt Torbreck VK3/VN-001 as a suitable site to operate for the contest, and I went there in 2012, and again in 2013. This year, I sought to improve on previous year scored. I had dispensed with the Quadruple Quads that I had used on previous expeditions and built a 6 element yagi for 2m.

Access to the summit was the usual Barnewell Plains Rd up from Eildon Jamieson Rd. It’s a little rough, but there is no real trouble getting the Prius up here. As per last year, I would need two trips up to carry the gear.

Gear included:

  • FT-897
  • FT-817
  • Laptop and power adaptor that could take 12V in
  • Signalink USB for digital modes
  • A HT for 2/70 FM
  • 8 3S 5000mAh LIPOs
  • 3 3S 2200mAh LIPOs
  • A 12 V regulator
  • Various coax pieces both LMR195 and LMR400
  • Two 7m squid poles plus the lightweight 4.7m squid pole
  • Two 1.5m al pole sections
  • Turnstyle antenna for 6m
  • Coax based colinears for 2 and 70
  • A PVC based and squid pole mounted 6 element yagi for 2m

With the removal of the Quadruple Quads, it was less gear than last year. I had also taken up less battery capacity because I used little more than 50% of it last year.

As everything was carried in by hand and was battery powered, all my contacts here were SOTA contacts.

Conditions were nice during the setup. I finished the second trip up at about 11am and proceeded to setup all the gear.

Here’s the 2m colinear:

2m colinear squid pole mounted at Mt Torbreck

2m colinear squid pole mounted at Mt Torbreck

The 70cm colinear was built the previous day before the contest, and was then tested with the MiniVNAPro and Extender. It is a little hard to get the element spacing just right – I built it a little longer than a version from last year, but it still ended up a little out of band. SWRs of mid 3s to 4 are really too high, so I’ll need to make a match for it.

Zplots view of 70cm colinear

Zplots view of 70cm colinear

The gain is still very nice, and I get away with these high SWRs because the FT-897 does not develop as much power on 70. It also cuts back the power in presence of higher SWRs. Still, I need to get the SWR below 2 to give my radio an easier time and so a match will be needed – otherwise I would have to build the thing again, even longer per segment – I’m a little over building these things now!

Physically, the lightweight 4.7m squid pole was able to take the weight of the colinear, however, given that I was also attaching some LMR400 coax, I needed to support the weight of that against a tree, otherwise the pole would start to bend so that the bottom of the colinear was not far from the ground.

Next up, the 2m yagi:

2m yagi squid pole mounted at Mt Torbreck

2m yagi squid pole mounted at Mt Torbreck

In light winds, the setup for this was fine. I am going to need to guy it for stronger winds. I can guy it just below where the yagi is mounted. The mounting is a PVC pipe t piece on the pole, with the stem of the T allowing me to put on another T piece for the beam. I am going to modify the physical construction a little more along the lines of what Peter VK3PF has done, putting the elements through the beam, rather than using a wingnut to tie them onto the top of it. This will make setup and tear down simpler.

I was pretty happy with the measured results of this antenna:

2m yagi VNAPro measurements shown in Zplots

2m yagi VNAPro measurements shown in Zplots

This gives an SWR below 1.1 at 144.18MHz. The front to back was predicted by NEC2 to be over 30dB, and it was clearly high as I moved the yagi around. Predicted free space gain was about 16dBi. I was able to use it to work VK1DA/P during the contest, although by then, winds were very high and I had trouble keeping the thing up. So improvements for next year is a guying mount and physical improvements to the element mounting. I will also put in a BNC socket just after the common mode choke. The common mode choke on this antenna is simply 6 turns of the coax around the PVC beam right next to the feed point.

Last antenna up, the 6m turnstyle from last year. It’s not the most high gain antenna out there, but it does enough to put me in the game on 6m. I’m still thinking about whether I might put in a 2 element yagi to replace it next year, however the advantage of this thing is that I don’t need to do anything to adjust it during the contest. This is handy when in the tent because it’s raining outside.

6m turnstyle at Mt Torbreck

6m turnstyle at Mt Torbreck

You may notice the dark cloud in the above photo. Unfortunately, it was a sign of things to come.

So what’s missing. I have nothing horizontal polorisation for 70cm, and my plan for that is to build a 70cm yagi for next year. I’ll take the lessons learnt from building the 2m yagi this year. I am not going to bother with any vertical polorisation antenna for 6m.

The radios were setup in the vestibule of the tent:

Radios in tent vestibule

Radios in tent vestibule

A look at the batteries – these were 3 parallel by 2 in series (effectively the cells are 6S3P) which then feeds a 12V regulator.

Mind the computer

Mind the computer

It does take a little bit of work to get the workspace clean enough in the small tent. The computer was here to provide VKCL logging during the contest and to run the PSK software (Fldigi). An alternative is to run something like DroidPSK on my phone or tablet through one of my radio interfaces.

I operated for about 4 hours in the contest. There were fewer portable stations out compared to last year, and a notable absence of picking up people participating in and around Melbourne using 2/70 FM HTs. I picked up a lot of contacts last year that way.

A number of strong showers passed through, but I heard from other contest stations further southwest that conditions were getting wild. There was sounds of distant thunder, and the radar looked bad out 100km to the west, so I pulled the plug. It felt a shame to end 2 hours early – I missed a whole available contest window. Conditions were looking not so good and there was lightning around, so I did not want to risk it.

I quickly pulled down the antennas and took the high priority gear (radios, computer, etc) down to the car first. On the way back up, a decent electrical storm came through with lightning every 3 or 4 seconds. Some strikes were within 500m. That storm passed when I got back up to the top, but now it was time to take down some of the metal gear. I actually thought about abandoning it, but banked on getting off the ridge before the next storm arrived. As it turned out, I was not quite off the ridge when the next squall came through. It was lightning every 2 or 3 seconds, the rain was driving hard and the track was a river under 30cm of flowing water. There was water absolutely everywhere. I was glad that I had taken down the electronic gear on the first trip and it was in the dry car! I still was not too happy about carrying 1.5m metal poles in the middle of an electrical storm. I felt much safer once off the ridge and about 20 minutes later the storm had passed. The rain then backed off to a more usual level. I was pretty wet for the drive home!

This was quite an experience, and hopefully the summit will be more forgiving next time and allow me to get a 6 hour activation in. I got 846 points, which I felt was not bad given the 4 hours operation, plus the lower number of portable stations. I look forward to next year with 2 yagis and a matched 70cm colinear to help extend the scores above my 2013 high.

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.

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

Portable preamps for 6, 2m and 70cm Part 4

This is a continuation of Portable preamps for 6m 2m and 70cm – part 3.

After some comments from Bob, KV3VO, I decided to make these preamps narrow band by using a shunt LC tuned filter. It increases the component count by two, but there was room on the board for it. The reason for this change is by having the preamp broadband, it effectively reduces the radio’s ability to reject signals by the same amount of the gain of the preamp – because these strong signals are going to be amplified by the same amount as the desired signal. By using a LC filter, the signals in the desired amateur band will still be amplified, but other signals will be subject to attenuation. I have experienced this problem, not with the FT-817 or FT-897, but with a Wouxun KG-UVD1P. When doing SOTA activations from a summit with commercial RF towers nearby, the Wouxun can be overwealmed. The FT-817 and FT-897 have been used in the same environment and their ability to reject signals is much better, but again, it is better if this preamp is not weakening that capability.

So, here is the insertion loss at 6m with a 39nH inductor and 240pF cap, both 0805 size:

LC shunt filter insertion loss at 6m

Here is the insertion loss at 2m with a 6.4nH inductor and 180pF cap, both 0805 size:

LC shunt filter insertion loss at 2m

And again for 70cm with a 2.7nH inductor and 51pF cap, both 0805 size:

LC shunt filter insertion loss at 70cm

The PCB is little changed. I have added the filter on the front end of the amplifier.

Preamp PCB

Wayne VK3WAM

EDIT: It does not take long for bugs to be found! I have to move the shunt filter C6 and L4 to the other side of the coupling capacitor C5, otherwise I create a DC short across L4. Simple change to the PCB, will be able to get it in before it is loaded for fabrication on the first batch.

Portable preamps for 6m 2m and 70cm – part 3

Hi all,

This is continuing on from Portable preamps for 6m, 2m and 70cm – Part 2.

I have updated some board designs for both the preamp and the sequencer.

Here’s a look at the preamp board:

Preamplifier board

And here is the sequencer/dual bias tee:

Preamplifier sequencer and dual bias tee

As stated in the previous post, this bias tee is designed to operate both of the FT-817 and FT-897 antenna ports. The caps exposed to full brunt RF are much more expensive than the X7R caps that can be found on ebay, but this is part of the cost and convenience of having something that will bias the coax and allow 100 watts SSB to pass. The caps are rated to 5 or so amps, so this imposes a SWR limit. It is on the low impedance side, but of course the actual current varies along the cable in the presence of SWR due to the standing waves. The FT-897 will reduce power in the presence of high SWR, but this board could have its ratings exceeded if driven with 3 or higher SWR at full power. Of course, I have also considered some margin for the inductors that will face high RF power as well.

Away from the high stakes, I can use cheaper (much cheaper) caps. The relay is also a little expensive, but if you wanted to order 1000 boards plus components, I’m sure that just about all the components will get much cheaper!

There is no need to actually use this board for two bias tees, by not installing two inductors one capacitor and the two BNC jacks associated with one channel. All the other components remain required.

The 4401s are a few cents, along with the resistors. Both boards are designed for a BNC through hole jack. It is still possible to run alternative connectors, but these would need to be case mounted – eg a UHF or N connector, with a short run to the holes on the board for where the BNC jack would go. The planned BNC jacks also provide for case mounting, so the board will be held to the case through the jacks.

The preamp board has less scope to use cheap components. The relays selected there must be rated for the RF current going through – the AXICOM HF3’s are rated to 60watts continuous RF power and 2 amps. Most of the caps and inductors again must face high power RF.

I’m in the process of ordering parts and getting some boards printed to build this thing and give it a spin. I also look forward to increasing my contacts next John Moyle field day.

73 de Wayne VK3WAM

Portable preamps for 6m, 2m and 70cm – Part 2

Hi all,

This is a continuation of a post at: Portable preamps for 6/2/70 part 1.

Part 1 showed a schematic for the preamp component. This would live on the antenna, near the feedpoint. Given the bias tee, it will only need to be fed by coax. It does not need any control lines or separate DC cables going up there.

Here is a picture of the PCB design:

Preamplifier board

I’ve got this under 2 inches by 1.5 inches, so I am happy with the size. I have two BNC connectors on board, these can be obtained from rfsupplier. I used microwave design techniques on this board with as much as possible of the RF being microstrip with 50 ohms characteristic impedance. Of course, with this approach, there is no using vias for anything apart from ground connections. In line with Minicircuits recommendations, the PGA-103 is surrounded with grounding vias. I also ensured that there were grounding vias near each of the relay RF inputs and outputs.

Sequencer design

The sequencer needs to do several things:

  • Provide the Vdd supply for the preamp and put this on the coax
  • Only put Vdd on the coax when the radio is Rxing – otherwise the preamp will be exposed to RF far beyond spec, and magic smoke will not be far behind. As the preamp uses non latching relays, any removal of DC from the coax will cause the relays of the preamp to switch to bypass, allowing TX RF to safely pass.
  • Allow for a delay between the radio commencing TX and high power RF going out the cable. Modern full mode rigs have TX inhibiter inputs, and the FT-817, 57 and 97 are no exceptions. The microprocessor can respond in a few microseconds, but relays need a few milliseconds to switch. The plan is to allow 25 milliseconds to be safe.
  • Also provide for a short delay when TX stops before switching back to RX. This would be about 10ms.
  • The local bias tee to put the DC on the cable needs to withstand 100 watts ssb. This will mean this bias tee will not need any relays to switch the RF. Of course there are relays in the preamp at the antenna end of the coax, where they are needed.

Microchip manufacture the PIC range of controllers. The task needed here is basically a timer and a state machine to deal with the various scenarios. Others who have built sequencers have used PIC controllers. In a previous life, I wrote real time control systems on PCs. The requirements here are far more simple! Looking at the range, the entry level PIC10 series will do the job. While not in the data sheet for the device, the Microchip web site also details that this device has current source/sink capability of 25mA. This is quite nice, and with a 5V supply, should give lots of drive and sink options.

On the FT-817 and 97 there is an ACC port. This has:

  • TX GND (a sink when the radio wants to TX, otherwise floating – i.e. open collector),
  • TX INH signal. This input feeds the base of a BJT through a 47k resistor in the radio. The transistor needs to sink 8V through a 3.3k resistor to prevent TX. This is 2.4mA. We need to provide 1/Beta of this current, and Beta should be 40+, so only 53uA or so is needed.
  • A 13.8V (or actually whatever the voltage that is feeding the radio is – so on a FT-817 this could be 9V for instance. There is a 10ohm 1/10th watt resistor on the radio in series with this source, so this source is practically limited to about 80mA. We will not want to power things from this, but we can present high impedance to it and use it as a signal to indicate the presence of the radio.
  • A GND

Here’s a look at the circuit:

Preamp sequencer for 6/2/70

EDIT: This diagram is updated to correct a small drawing bug.

I thought about whether I would use a transistor to switch the 5V supply to the bias tee and onto the coax, but there could be 300 to 400mA being pulled when on, and this is a bit to sink when it needs to be off. Instead, I introduced a relay to switch the supply voltage to the bias tee.

There are actually two bias tees. One for 6m and the other a compromise between 2 and 70. Both the FT-897 and the FT-817 have two antenna connections. The 897 is not selectable, one is HF/6m and the other is 2/70. I have largely gone with this configuration, however the 6m optimised tee could drive 2m. The FT-817 is fully switch-able, so either of its antenna connections can be used for any band. I have selected bias tee components that can be switched, so one of these circuits could have the left optimised for 2, the other for 70, or have both bias tees the same configuration. Again, I selected bias tee components to sink the fraction of power that will go through the tee from TX power at 100 w ssb, CW or 50w PSK. This is actually one of the key reasons for two inductors on each tee. One is a ferrite coil, the other is a chip inductor. Both are from Coilcraft and the part numbers are going to be quite specific. It will not be possible to use any inductor of a similar inductance value, because the actual inductance at the target frequency can be quite different to expectations. There is also the need to actually sink some of the power that comes through here as inductors are not known for having high Q – Q effectively being a ratio between inductor resistance and reactance.

The relay needs around 12mA to drive the coil. I am using two 4401’s with the coil on the collector of the first. Q1 has a PIC output on the base. If this is taken high, then Q1 will conduct. 2mA will be drawn from GP0 on the PIC, well within the 25mA limit. Q2 has the radio 13.8V output on its base. The resistor is sized again to provide for a 2mA draw. If the radio is connected, then Q2 will conduct, otherwise not. This setup is effectively and AND operation. Both the PIC output and the radio 13.8V signal (which just needs to be any meaningful non-zero voltage) are required for the coil to switch from NC to NO and provide the bias voltage onto the bias tees. No radio connected to this sequencer, then no Q2 conducting, and therefore no DC bias on the coax. This results in the preamps at the other end being safely deactivated and bypassed.

GP3 is an input only pin and it is connected to TX GND. This is an open collector pin, acting as a current sink when activated, with very low resistance. About 1mA will be pulled out of the GP3 chip when low. The PIC has TTL inputs, and also provides weak pull-ups, so no external pull-up resistor between TX GND and Vdd is required.

GP1 has a 1k resistor, which is effectively in series with the 47k resister in the radio. This should provide about 110uA of drive, again a tiny fraction of the 25mA PIC capability, and about double what is needed by the radio for TX INH. The 1k resistor is there mainly to protect the PIC in-case of an interface cable short – only a little less than 5mA would flow in case of a short.

The circuit has a number of 1nF capacitors. For the ones not associated with the bias tee, I can use some cheap X7R dialectic caps, but little compromise can be made with the 1nF cap in the bias tee. ATCeramics make some good high RF power rated caps for that job, I can use cheaper Ebay caps for the rest.

Finally, the code in the pic, which I will need to write, basically needs to implement the state machine. This code should be pretty straight-forward.

Next step is to design the board, and then it will be time to get some boards made. I have also designed a FT-817 to phone interface for PSK – see post here – and so I will get a batch job done with a few of each of these boards.

73 de Wayne VK3WAM

Part 3 can be found here.

Portable preamps for 6m, 2m and 70cm – Part 1

Hi all,

One project that I have been working for a little while is the design and eventual build of preamps for the 6m / 2m / 70cm bands.

My requirements are:

  • Relay switching for TX, by the presence of DC supply
  • Preamp activated by DC power, otherwise off and bypassed
  • 100 watt TX capability, but only at 50% duty cycle
  • DC supply for the preamps to be on the coax, so bias tees will be needed
  • Said bias tees must also work in the presence of 100w TX power

Why preamps?

The question needs to be asked. These bands are often quite low noise, certainly on many SOTA (Summits on the Air) mountain tops. Most of my SOTA activations have been HF, however there have been a few that have been on 2m. During the John Moyle Field Day contest this year, I operated from Mt Torbreck, VK3/VN-001. While I got a good score, there were a few QSO’s going missing who could hear me, but I could not hear them. Some of them were using more power than me. Some have reported that they hear much better with preamps. So, there seems to be enough of a reason to do it.

DC power supply from the coax

It takes some design work, but clearly it is going to be convenient to have DC power come down the same cable as the coax from the radio. Near the radio, I will need a bias tee to put the DC on the cable, and it will only do this when not TXing. When it gets to the preamp, there are three possible states:

  1. The radio is TXing and there could be lots of RF energy on the coax, no DC
  2. The radio is RXing and there is DC on the coax
  3. The radio is RXing and there is no DC on the coax

State 1 needs to ensure that there is not large amounts of energy that gets on the Vcc rail in the preamp. An inductor blocks (in part) AC, including RF. It does it through reactance, and because its Q is not infinite, some resistance as well. Unless the Q is very low, it is by far through reactance.

State 2 needs to get the DC to the Vcc rail of the preamp. This will flow happily through the inductor, but the inductor does have some DC resistance – so there is going to be a voltage drop. The DC should not continue further down the coax towards the antenna, so a DC blocking cap is needed – this needs to have very low effective series resistance and very low capacitive reactance at the RF frequency. It also needs to be able to handle the RF current during State 1.

State 3 has no special circuit requirements for state 3 as no DC power is on the coax, and the inductor is going to block the RX RF energy from going up into the preamp, instead it needs to go to the radio where hopefully it results in received intelligence 🙂

The impedance of the inductor needs to be high, much higher than 50 ohms at the RF frequency. It needs to have low DC resistance. It also needs to take into account the fact that inductors are imperfect beasts at RF frequencies – they have resonances and act like capacitors above the resonant frequency. I was able to use some design formulae from RF Circuit Design by Chris Bowick. These give a whole range of impedance matching values derived from S parameters. It can also be used to find actual inductance and capacitance values for a given component at a given frequency. The nominal inductance of an inductor is not a fixed value, but changes with frequency. Different inductors change in different ways, based on their construction.

Getting the right inductors has been a major challenge. Even if I have an insertion loss of only 0.2db, there is still about 3 watts getting into the preamp circuit if there is 100 watts of RF outside, and some of this will have to be dissipated by the inductor’s resistance. Also, some of these inductors are quite large, but if they are too big, their resonances will be at far too low a frequency to be useful for the targeted 6m, 2m and 70cm bands.

In the end, to deal with power dissipation requirements, I used a specific bias tee inductor and some chip inductors. If I used just one inductor, I would lose a little too much DC across the tee (x2 of course for the fact that I need to put this DC on the coax back near the radio!)

Ripple on the Vcc rail

So now, I have got my DC when I want it, and are not likely to release magic smoke when at 100 watts 50% duty cycle modes. I still have some ripple on the supply, and I need a bypass cap to sink it to earth. The inductors suck up most of the heat dissipation. If I use a single cap, this leaves a lower frequency pattern, so I have found that having two caps, one around 1nF to 5nF based on the band (higher band, lower cap), and a second 10uF cap seems to get the ripple down to less than a millivolt. In (a different) RF Circuit Design by Richard Chi Hsi Li, the point is made than rather than having a capacitor forest of low to high values, a single cap should be selected around self resonant frequency (SRF). This exact frequency is going to vary, and the cap cannot be ordered with some precise, eg 2.2235nF value, so I target a little below SRF for the small cap. Some simulation tests show that these caps are only dissipating a few milliwatts, so it is looking good.


The next thing to discuss is getting the amplifier that I am going to use out of the way during TX, or I will release the magic smoke. I have found a suitable RF surface mount relay that looks suitable for the power rating required from Axicom. It has a continuous 60 watt rating, so 100 watts SSB or even 100 watts CW should be a piece of cake. Lets not try 100 watts PSK though! I will need two of these relays, one each side of the amplifier. Normally closed is no DC voltage on the coax, state 2 and 3. This would bypass the amplifier with the signal going directly from one relay to the next. Normally open will connect the amplifier.

One issue is that using the same relay, the RF paths have to cross. If I use a two sided board with a ground plane with the RF signal going on microstrip, I cannot cross the signals on the board. I am going to have to have a short jumper on the board for one of the signals – I have chosen the normally open (preamped signal).

The amp

From the Antenna side relay normally open the RF comes into the amplifier. Now, I could just use a JFET or MOSFET, or even a MMIC, but Minicircuits have a device called PGA-103+ which should make this part real easy. It has three (well four, but ground shares two) pads. One for RF in, one (two) for ground, and one for RF out and DC bias. This device works on either 3V or 5V. The device needs a DC blocker cap on the input, and one on the output. The bias voltage needs to get in there somehow, and so this calls for another bias tee on the output pin, before the output DC blocker cap. Another bias tee (that’s three now), but at least this one does not need to dissipate large amounts of current – it’s I^2R that causes the heat – and current is squared – damn!!!

Having three tees with the DC voltage drop causes quite a lot of design issues, and it took some time to identify the right components to do the job. There are chip inductors, air coil surface mount inductors. Coilcraft even make some core based smt inductors. The challenge is to get something with a high enough inductive reactance that does not take up too much space, and has not effectively become a capacitor because it is above SRF. Some devices are designed to be used above SRF and that’s ok, but most inductors are not, so this does provide a limit. The air coil ones just don’t have the reactance, even though they have low DC resistance.

The amp provides 26dB gain on 50m and 25dB on 2 and 70. It has a noise figure of 0.5dB. That is really nice. This should be a good quality preamp.

The 5V supply requirement of the amp meant that I selected 5V relays. I did not want to add some BJT device with a zener regulator to bring down 12V (or 13.8V or so) to 5V. Extra complexity for the preamp, duplicated on each one. It will be far easier to get a 5V DC-DC step down converter off ebay, plug some Anderson Powerpoles on it and run that from a 12V-15V or even wider supply. So this is the plan.

The DC current requirements of the amp and the two non-latching relays are 160mA. I needed to ensure that across all the bias tees to the amp (thats three remember!) that I was getting at least 4.75V there. I did contemplate using the amp in 3V mode, but that needs a zener plus a BJT, so I tried to avoid that. In the end, I can get 4.87V to the amp, and that is well within spec.

Here’s a look at a Schematic for the preamp.

Schematic for preamplifier for 6, 2 and 70

It should be noted that component values will change for each band. It does not seem possible to retain simplicity and have a preamp for all three bands, but at least I should have a common PCB board design.

I will need a separate schematic for the radio side bias tee. I will talk about this more on a future post, but it is going to need to take the TX indication out of the radio, use a PIC chip to TX inhibit for 30ms or so, and switch the DC – through a BJT and then onto the bias tee onto the coax.through two)

Part 2 can be found here.