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.

Equipment

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:

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

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

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

FT-817 (& FT-897) Phone Audio interface part 3

Hi all,

This is a continuation of FT-817 Phone Audio interface part 2.

It has been a while since I have posted on this topic, and the principle reason for that is that I was patiently, and then not so patiently waiting for boards that I had ordered. They arrived while I was away on a series of SOTA activations, but I have populated one of these boards and tested. Apart from getting one of the cables wrong – which I needed to fix, everything works quite well. The markings of the rectifier diodes are wrong, but I already knew that upon ordering as that is a bug in the device files in my design software.

There is quite a lot of latitude with the potentiometer settings, both audio in to the phone and audio out. I ended up using a volume level about 2/3rds on the phone. There is going to need to be individual adjustment of these, as it is a function of the phone used, the digital mic setting on the radio and also the need to ensure that the signal is not being splattered. If you do not have access to two radios with one having an existing audio interface – eg a Signalink for testing, then you will require some on air reports to ensure that your audio is at a good level without causing splatter.

Here is a pic of both an unpopulated board and a populated board. For an Samsung Galaxy S2, I used a 12K resistor for R10. Apple devices might need a lower value, around 4.7K to raise the current that the interface draws. It seems like around 600uA is the threshold for Apple devices, but the Galaxy S2 is happy around 300uA.

Populating the board is quite straightforward. Although most components are surface mount, they are either 1206 or 0805 size, and I did not even need to use a magnifying glass. I recommend using a temperature controlled iron with a fine tip.

I have a few spare boards, and can make these available for $12, or $25 with the parts, either $A or $US plus postage. Send me an email at vk3wam@gmail.com if you are interested.

It should be possible to adapt these boards for use with other radios as the connectors at each end are straight inline. On the phone side, most modern devices have ground on pin 3 and mic on pin 4, but many older non-Apple devices are the other way around, ground on pin 4, mic on pin 3. If you have such a device, then you’ll need to wire it that way. On the radio side, RX audio (phone in) TX audio, GND and a PTT grounded to TX signals are required and most radios should support these. The Yaesu radios provide a constant audio input on their data port, but some radios (I believe the Elecraft KX3 falls into this category) will change the output audio based on the AF gain (volume) setting.

73 de Wayne VK3WAM

Designing a 20/40 band CW rig – Part 2

Hi all,

This is a continuation of Designing a 20/40 band CW rig – Part 1.

Shown below is a schematic of the TX and initial RX parts of the rig. I have been successful in simulating all of these parts, and have a reasonable amount of flexibility if the real world performance of the components does not match the simulated performance.

TX Driving

This being a CW orientated rig, all that is needed is a oscillation on the desired TX frequency. CW merely turns it on and off. I did mention that I wanted to retain cabability of PSK and FSK modes. PSK needs the phase of the oscillation to be changeable, while FSK modes need the frequency to be changeable. An Analog Device AD9834 fits the bill. Steve KD1JV is using this in his Mountain Topper. This device is not a PLL and VCO combination, rather it generates the output digitally, and then feeds a DA convertor to generate the waveform. The output is needed for two things – it is the Local Oscillator for a mixer, and can be used directly for a TX frequency. The AD9834 has a number of capabilities that I plan to use, but this will be for another post.

Unlike Steve, I have decided to take the analog output of the AD9834. This output will drive the initial mixer, and the device selected has a 50ohm load. I’ll talk about this device on another post, but I will be using a different approach to the other designs I have seen because I want more dynamic range and better inter-modulation distortion performance.

The input to my driver is going to be taken off this output. The AD9834 can provide a balanced or unbalanced output. I need unbalanced. This signal is shown as OSC. It then feeds the base of a MMBR941, Q1. This is a RF BJT device in common emittor configuration. One of the nice aspects of BJT devices in common emitter is there is a fair degree of flexibility in setting the input impedance and the output impedance of the device. I decided on a 150ohm input impedance. I am also driving the device fairly hard, but not too close to device limits. The gain on the device is 26dB.

To save power when not TXing, I use Q2 as a switch. This 4401 device is being used as a current sink, switched on during TX, and off when not. It deprives Q1 of its DC ground, so no DC current will flow through the device. The bias network is shut down, but is still kept alive on AC so that the load presented to the AD9834 is not significantly changed. The effect of Q2 kills nearly all of the output power from the collector of Q1.

TX Power

My objective is to deliver 5W into a 50ohm load (antenna). There are quite a lot of choices to go about this, but I want to keep things as simple and as cheap as possible. I considered initially using another BJT in emitter follower mode, by using Q1 to deliver the desired peak to peak voltage AC signal and then the emittor follower would supply the current. I used an inductor on the emitter to improve the efficiency, but at the end of the day, it is still being used in Class A mode. This is a CW rig, so Class C amplifiers await!

The schematic shows three BS170s that are driven by the output of Q1 on their gates. These are N-channel MOSFETs that can dissipate about 800mW of power. If they are run 66% efficient, then that means that each one can deliver 1.6 watts. The efficiency is a function of the peak to peak voltage supplied by the MMR941, and what level the bias is set at. The bias mid point needs to be below the pinch off voltage on the BS170’s for Class C operation. The further away, the better the efficiency, but it cannot be set too far away, as there are limits to the peak to peak output I can get from the driving device. After a fair amount of experimentation, I set the bias level using a voltage divider resistor network. R9 and R10 (the R10 going to ground) form this network. (The other R10 nearby on the base of Q3 has a new identity as R18). This level ends up being around 1.2V, around a volt below the BS170 cutoff to ensure Class C operation.

The job of Q3 is to act as a switch, on during TX, so that R9 and R10 do their job as described. Off during RX, so R9 is taken out of the circuit, and R10 pulls the base of the BS170’s to ground so that they are cut off from doing anything. The BS170s then act as open circuits.

Presenting the output to the load

Class C waveforms have less than half of the initial sine wave present. The BS170s also present a changing load to Q1, so what it produces is not a great looking sine wave either. We actually have quite an ugly looking waveform on the output of the BS170s. This needs to be cleaned up. Also, the load is not 50 ohms. It can’t be if this circuit is going to operate off 12V. The best that can be hoped for, in terms of an output wave form is 24V peak to peak, due to the action of the inductor L2. I used the complex model of L2 in my simulators, rather than using a perfect inductor. The complex model output was practically indistinguishable from the ideal model. Coilcraft make some good inductors, and I plan to save builders of this rig from having to wind their own inductors, by using these Coilcraft chip inductors.

24V peak to peak really only allows for 12 ohms load impedance directly on the finals. This needs to be transformed to 50 ohms at the antenna connection. All rigs need to do this transformation. Steve’s Mountain Topper Radio provides a network for each band. The FT-817 provides one for each of its bands that it can TX on, and they are switched by relays. The FT-817 set of finals are operated in a push-pull configuration, but off a 8V rail. This means that the load impedance that these are driving will be lower than 12 ohms, perhaps about 9. I should analyse the inductor/capacitor networks for a given band that Yaesu have put in there. If you were to look at the circuit diagram, these matching networks take up most of the power board module schematic.

I reckon a bit of convenience is a good thing, so I am going to use a relay to switch between the two bands. This relay is K1. It is DPDT, so I can use the one relay for both ends of the matching network.

The match network has a LC tank which does most of the job of restoring a clean sine wave, and then a LC series, presenting a high impedance to remaining harmonics. This second LC series begins the process of impedance matching, where I only need two more capacitors to complete the job. This is per band of course.

Now reality bites, and there is the need to use real world components. Also, for these networks, X7R dialectic is not acceptable, but for cheap capacitors, 1nF or above tends to be X7R. So to get around this, I have paralleled up some caps to get the values I want, and to continue to use NP0 dialectic caps in 0805 SMT size. There was one cap where this approach was too long in the tooth, so I use a 3.3nF ATC cap that is 1111 size. It’s a much more expensive cap, but I’m only using one!

After these matching components, I have a sine wave output at 50 ohms supplied to the antenna. It is the job of the operator to take it from there.

Receiving

I have to receive as well, this is a transceiver after all! Now, approach 1 could take the signal straight off the antenna jack, but there is the matter of the TX output to deal with. At 50 ohms, this is not going to be a 20V peak to peak output any more. It is closer to 83V peak to peak. Of course, all of the caps need to have dialectics rated to 50V, as 83V gets 41.5V from ground, each side.

Now, I could use a relay to switch between RX and TX, to shut out the TX signal from going anywhere but out the antenna jack to the antenna. The FT-817 does. One problem – full break in on CW. It is not too good to have a relay clatter (twice) every time you send a dit or a dah. I want full break in capability on this rig, so this means using a transistor approach. A transistor is going to have to hold back this 83V, but that is too much.

Instead, I have taken a similar approach to Steve KD1JV, by putting a blocking circuit at the 12 ohm area. This cuts down the voltage that the blocking network needs to resist on TX. It also has the benefit of providing a filter through the same LC networks that dress the TX output for the antenna.

The main transistor to block is Q9, a N-channel MOSFET 2N7002. I have used a biasing network, that is assisted by BSS84 Q10 and a 4401 Q11.

Q11 is a current sink. When switched on, it takes the gates of both Q9 and Q10 to near ground. Q9 is a NMOSFET, so a ground on its gate will block any signal that is about 0V or higher. Near ground on Q10 is a PMOSFET, so it is switched on, there being nearly -12V between its gate and source. Q10 switched on takes R15 out of the circuit, leaving a voltage divider of R13 and R14, meaning Q9 is biased over 10V. The input signal is going to swing between 0V or so and 20V on the drain of Q9, with 0v at its gate. If something makes it to the source of Q9, it won’t be much below the gate, meaning Q9 should stay switched off, switching off TX signal from sensitive RX circuitry.

When wanting RX, Q11 is switched off. This then means the gates of Q9 and Q10 are pulled to 12V by R17. 12V on Q9 gate will tend to switch it on. Q10 will be switched off, meaning that the bias network is now R15+R13 against R14. The drain of Q9 will be biased at around 1V. There is 12V on the gate now, and around ground on the source. Q9 will be switched on as hard as I can make it, so get as much of the RX signal through.

Unfortunately, when TXing, quite a bit of the signal still makes it through Q9. Too much by itself to be safe. The transistor itself is ok, it is not being used anywhere near maximum limits, but the mixers awaiting later will not like what gets through. I want to block this signal, so I use another LC series resonant device to block the harmonics. This is because what does get through Q9 is horribly mangled with most of the power in the harmonics, not the fundamental. This cuts things down nicely. Also this network starts the process of impedance matching from 12 ohms back up to 50 as the mixer I want to use wants 50 ohms as a source impedance. Now this matching network is band specific, so this means a second relay. This relay, like the TX relay is only switched during band changes, so there will not be any relay clatter here. Q12 and Q13 are little helpers. They are switched on during TX to shunt away any TX signal that still makes it through this far, but because of the LC network, it ain’t much. The job of Q9 is also easier to block the TX signal when it is mostly high impedance sitting behind it.

Wrapping up

This about wraps up this post. Next up I will look at the audio processing part of the rig. The other major parts are the RX mixing and filtering and the microprocessor. I’ll look at these with later posts. My plan is to get one of these rigs built in about a month to two months as a prototype, and we’ll see how it goes from there.

Regards, 73, Wayne VK3WAM

This topic is continued at Designing a 20/40 band CW rig – Part 2.

FT-817 Phone Audio interface part 2

Hi all,

I have continued to work on the interface, described in Designing a Phone/Radio interface, and I think things are looking pretty good. I have changed some component values slightly to fit in with easily obtainable parts. One approach is to use thru-hole parts for everything, but this results in quite a big board. Now SMT does scare people. but if we stick to the larger part sizes, it should be easy enough. This does however means steering clear of 0402 and 0201 tiny sized components.

My first try at drawing a PCB used SMD capcitors while keeping everything else thru-hole. Using 1/4 watt thru-hole resistors ends up taking up a lot of space. I decided to change over to SMD resistors as well, but leaving the diodes, transistors and the trimmer potentiometers as thru-hole. This allowed me to get the board size at 2 inches by 1 inch. My resistors are generally 1206 size (this is .12 inches by 0.06 inches, or about 3 by 1.5mm. Most of the caps are 0805 (about 2mm by 1.2mm) size. These are reasonably easy to manage with tweezers.

People have asked what components are used for the transistors and diodes. I am using 2N4401’s for the BJTs and 1N5819 for the schottkies. Both these components should work very well.

In some of my simulations, I have tried quite “out there” scenarios. The circuit actually still works with a 1V power supply, especially with higher audio frequencies on the digital mode being used. It works with 8V, actually very well. I have not bothered higher voltages, perhaps I should give 12V a go, however no phone is going to be supplying that kind of voltage level. I suspect that they (i.e. the various phone manufacturers) are either stepping up the cell voltage output – typically a LiIon cell that will range from 4.2V down to 3.5V or so, and stepping it up to 5V, or just feeding it in unregulated, with a 5 to 10K resistor in series to current limit the supply on the microphone pin. Any of these scenarios will work with this circuit. If anything like a 3V or higher supply is involved, there will be over 50uA to drive the second transistor to sink 450uA of current on the PTT pin. Even if other radios to the FT-817 have different loads on their PTT pins, I can’t imagine it’s orders of magnitude!

On the radio side, there are clear variations. The FT-897 has a lower impedance output on the fixed audio, and a lower Vpp level. I still expect the circuit will work fine. I also had a look at the Elecraft KX3. This does not have a data port, but rather relies on the speaker output and microphone input. The speaker output will be at a higher level than what either the FT-817 or the FT-897 will drive this circuit. The trimmer potentiometer can be turned down to help. Also the radio audio control will change the voltages seen on the output. As for the input, again the potentiometer will have to be turned down, because the circuit is feeding something approaching a line level. Microphone is a good 15dB to 20dB down on that. I would presume that the KX3 would have some forgiveness about the input impedance, as cheap mics are high impedance (10K plus), while high quality mics can be as low as 100 ohs. I’ll need someone to investigate if there is any DC on the microphone input as well, as this could be there for the same reason that there is DC on the phone microphone input. If someone has a Oscilloscope and a KX3, it would be interesting to see the audio output levels, but you would also need to know what load resistor you used.

I have a picture of the circuit for you to enjoy below:

Btw, the diodes are back to front, and this is a consequence of whatever bug is in the schottky files used by GEDA. It is easier to just highlight it as an errata. The circuit will not work if the diodes are not correctly put in place.

I used the geda suite to design the circuit, using gschem to draw schematics, ngspice to do simulations, and PCB to draw the circuit art. These programs can be a little hacky and the help files are not for the uninitiated, but they certainly get the job done. I feel pretty comfortable with these, and I am now also looking to do a design for a bias tee and a preamp for 6m/2m/70cm. More about that later.

Send me an email at vk3wam (at) gmail (dot) com if you are interested in getting one of these interfaces. If people are interested, I could sell the boards at $10 US. I’ll need to look through the cost of the materials if people are interested in kits.

73 de Wayne VK3WAM

EDIT: Here is a slightly updated board design, optimised to remove some of the via holes, improve some spacing and comply with various production house design rules.

Designing a Phone/Radio interface

Hi all,

I have been interested in digital modes for some time. I purchased a SignaLink USB in the early days, even when I was an F-call, anticipating my full call would be issued before it arrived. This turned out to be the case. It would be a bummer if I could not use it! Note: in VK, F-calls may not transmit anything aside from SSB, AM, FM, PM. and hand generated (inc using an electronic keyer) CW.

My early days were focused on Easypal, a DRM based SSTV mode. I also did a little bit of playing around with RTTY, PSK and other narrow band based modes. Eventually SOTA started in VK, and I began to think about the possibility of doing digital modes on summit, but not taking a computer and the Signalink.

I use a Samsung Galaxy S2, and I began to think about what would be needed to use this for PSK and RTTY. Wolphi has some andriod apps on Google play for these modes, and also of interest, sells an interface. He also has put some designs up on the web here and here.

Version 1 of these interfaces uses a transformer to boost up tx audio from the phone device, so a transistor can be switched to pull down the PTT on the radio. Version 2 removes the transformer and instead relies on a voltage that is present on the microphone pin. There is also a bug in the schematic of v2, as the rx audio does not go to pin 1, it needs to go to pin 4, plus add a capacitor.

Separate to this, VK3XJM developed an interface between his FT-817 and his ipad. His initial try was broadly similar to the v1 Wolphi interface, and I then made him aware of v2. His next revision is similar to the Wolphi v2.

My design effort

Well, homebrewer extrodinare, I wondered if I would buy one of these interfaces, or build my own. Answer: design and build my own!

The first issue to consider is that line devices are supposed to present high impedance to the audio out device. While the mobile(cell) phones will think that they are driving headphones, these still will typically be 200ohms plus. The idea to use a transformer to boost up voltage is going against the principle of trying to minimise the load. So, I will not use the transformer approach. Both VK3XJM and Wolphi have found that this approach is barely viable anyway.

So the next step: time to find out more about this microphone voltage. It has been highlighted that this varies quite a bit between devices, and this can cause some of the above interfaces to not work so well. What is this voltage? It turns out that most devices will put this voltage on as part of the “Plug-in Power Supply” system. This is to provide power for Electret Condenser microphones. These devices have a permanently statically charged film over a metal plate. This forms a variable capacitor, which changes based on captured sound. The tiny signal is placed on the gate of a common source JFET, with the signal taken from the drain. “Plug-in Power Supply” provides the voltage on the drain to drive the JFET. Typically the voltage is 3V with a 6.8Kohm resister in series in the supplying device. This is not supposed to be a high current power source!

Unfortunately, there is no one standard for the voltage, plus the series resistance of this source. On the net, there are stories of voltages anywhere from 1.5V to 3V. Hmmmmmm, this variability could be a problem.

I took a hybrid circuit based on a combination of the VK3XJM and the Wolphi v2 design and put it together in gschem. After a bit of stuffing around to find libraries for various devices, I used gnetlist to use the gschem saved file to generate a spice netlist for spice simulation.

The first thing I found is that I was sucking just way too much current from the voltage on the pin! If the supply voltage was high, the circuit still worked, but it was marginal. Things needed to be improved. I also found on birdwatching web sites (these guys are using these electric condenser microphones) that many devices will not supply much more than 300uA, and my circuit was pulling far more than that. Time to put this circuit on a diet!

The biggest pull of current was the first transistor Q1. It was time to get the Ic down, to 200uA tops! Using some design principles in Ludeman Introduction to Electronic Devices and Circuits, I ended up selecting a 20k collector resistor. This then flowed through to a 1.1k emitter resister, although my choice here was a little arbitrary. I was still trying to keep Ve low, when Ludeman recommends Re to be 1/3rd of Rc. From there, the Ib can be determined, and Vbe found using a formula. This then sets what the two resistors used as a voltage divider should be. My resisters used were far higher than the initial circuit. Ludeman also provides formulas for the input, output and emitter bypass capacitors, based on what low pass is required. I chose 100Hz. Its quite low, but it means that there should be no problems doing PSK at 500hz AF. I was prepared to fall back to 200Hz if required, but the really high impedances made capacitor choices easier. Note that Ludeman points out that the emitter bypass capacitor is not just set by Re, but there is also a AC path out of the base of the transistor. This path can be quite significant in the capacitor sizing.

The ngspice simulators showed the Q1 switching nicely on TX input, while presenting very low load to the signal. Next up was dealing with the PTT switching transistor Q2. The FT-817ND has a 11k load from a 5V source on the PTT pin based on the circuit diagram. Both the initial circuits basically rectify the AC output of Q1, filter it with a cap and feed this to the base of Q2. I first used Schottkies like VK3XJM. Hey the lower drop across the diode should help? I could not get the ngspice model working properly, so I went to ordinary 1N3891 diodes. I got these working nicely. I also bumped up a resister on the base to 20k, but also Q2 effectively provides a high resistance to the tiny weany current that is going through the base. This also allowed me to slice the filter cap right down to 100nF. This is the only cap that actually has to be charged in this circuit, and the early high current versions of this circuit required much higher values of this cap to work. It also meant that this cap was presenting too much load to the “Plug-in Power Supply” system.

So what I have now will work quite happily on anything from 1.2V to 3.5V, and I have not bothered testing outside these ranges ’cause it should cover just about everything. Once the voltage is below about 1.5V, it takes around 80ms to pull down the PTT on a 500Hz AF source, but it will pull it down. I think it would even work with a 1V supply! At 3V, it basically pulls it down in 2ms, which is on the first signal waveform! I am happy with these results, because this circuit should be very robust.

Current requirements: At 3.5V, it pulls around 80uA average, 140uA peak from the supply. Well within the 300uA for a mono electric! When the supply is 1.2V, it pulls 70uA peak and 20uA average. This circuit has certainly come out trim after its diet!

Where there any consequences in using such a tiny filter cap at 100nF. Well, yes, but they don’t matter. At 3.5V supply, the ripple is 200mV, but the signal at the base is plenty high enough, and the thing is so current limited that it does not matter than Vbe is 0.9V to 1.1V!

When the voltage is 1.2V supply, the ripple is hardly anything. So 100nF is perfectly fine for the filter cap on the base of Q2.

I have a screen shot of the circuit below. Click for full size.

I plan to build one of these on variboard. If people were interested, I could design either a through hole or a surface mount circuit PCB for it and make a kit available.

Regards, Wayne VK3WAM

EDIT: I replaced the ordinary diodes with Schottkies as their lower voltage drop across the diode helps the robustness of the circuit. The updated circuit diagram, with light background, shown below, can be clicked on.

EDIT2: Updating of the schematic as microphone audio in (to the phone) is taken from pin 4 as discussed in the text. I had kept the old one up, but Gerald DL3KGS noticed the difference.

Schematic as implemented on PCB

Updating VKCL to v3.5

Last night, my little project was to get VKCL working for the upcoming Trans Tasman contest on 160m phone and 160/80m CW and digital. The last time I used VKCL was during the John Moyle Field Day contest. On that day, I started using a then current v3, but this crashed upon entering my first QSO. Upon a bit of frantic testing, I was able to use a late v2 of the program to complete the logging of the contest. I had to manually adjust the scores later in accordance with the 2012 rules.

The Trans Tasman has changed this year, with 4 previous contest days being merged into 2. The 80m phone contest remained separate, but all the other 3 were combined into one contest, which is taking place tomorrow night (Saturday 21st of July). I would need a new version of VKCL for this.

Upon installing the latest version, I noticed that it immediately wanted to pick up my John Moyle Field day log and it was not happy. If I pressed the config button, the configuration screen would come up, but completely empty. Pressing the “X” close button would close this, but shortly afterwards, the program would crash through an unhandled exception. What to do? Firstly, I was running VKCL under Wine 1.2 on Ubuntu, so I upgraded Wine to 1.4 to see if that would make any difference. No it didn’t. Next, I got a Windows XP laptop and tried it there. No difference. So it is not a Linux or Wine thing, it doesn’t work under Windows XP either.

I thought that this was getting a little ridiculous, surely the software must be more robust than this. I thought that perhaps the author, Mike VK3AVV had not thought to test this program installed over the top of a v2 VKCL, so I needed a way for the program not to see the John Moyle log.

Under the C:\VKCL3 file (/home/<username>/.wine/drive_c/VKCL3 when using Wine) there is a file VKCL3.ini. In this text file, there are two dbPath entries, under [Select] and [Config] respectively. If these entries are edited to point to a directory that has no pre-existing log file, then everything seems to work fine. Care needs to be taken to maintain the double backslash “\\” as the directory names. Changing this to a single backslash will not work. This is because the text is being fed into a computer language that uses backslash as an escape character (The C language does this). Once these two dbPath are changed, then VKCL3 can be run and a new log created wherever you desire by using the configuration screens in the program.

Also, the QSO crash bug seems to have been fixed. Hopefully no surprises tomorrow night.

Regards 73, Wayne