Constructing a 2m colinear Part 2

Hi all,

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

Repairing the Colinear

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

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

Analysing the Colinear

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

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

Colinear screenshot from BlueVNA

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

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

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

Colinear at the BNC connector in Zplots

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

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

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

Colinear characteristics at feedpoint

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


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

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

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

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

Transformation of feedpoint impedance

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

73 de Wayne VK3WAM

South Gippsland 6 summit activation

Hi all,

I’ve gone off again on one of these three day trips and activated 6 summits. Some say SOTA is a drug of addiction. Where do I get my next hit? Worse, am I a drug pusher because I took Glenn VK3YY and Kevin VK3KAB along, and helped them get 60 SOTA activator points each? Well, Glenn has already done quite a bit of SOTA work before, but Kevin was a first timer.

After a few hiccups, we left the south eastern suburbs of Melbourne before 6pm and headed towards the hills. After a stop, which involved a stop at a cardboard food production facility for dinner, we headed up the Licola Rd as dusk set in. Kevin drove his “Great Wall” 4WD and we made it to a campsite along the Wellington River inside the boundary of the Alpine National Park for an overnight stop. The tables were a nice touch at the campground (photo courtesy Glenn)

Kevin and Wayne at Wellington River campground

Kevin and Wayne at Wellington River campground

Mt Tamboritha VK3/VT-011

It was only a short drive from there up the hill to the start of the walk to the summit. We then headed up to the Tamboritha saddle. We were able to go along the 4WD track to the east of the summit, but parked before the rough stuff. We walked along the WX station track, but left it at the marker which is to the north of the summit. It was a steady climb from there up to the top, but Mt Tamboritha has a flat top, with the highest point at the southern end. Anywhere on the flat top would be in the activation zone.

On this trip, Glenn brought his end fed half wave for 40m with squid pole, Kevin brought an antenna made up of various Buddypole components to form essentially a Buddistick with a single counterpoise, and I brought a squid pole with two antenna combinations. One was my end fed for 40 and 20, and the other was some components from my old aluminium segment vertical with loading coil, plus a new piece of wire that could hang on the squid pole to complete the vertical. This second configuration was to be used for the first four activations on this trip. It is perhaps 30cm longer than my old vertical configuration. I used it with tuner right on the feed point, with a tapped loading coil 1.6m above the feed point. The Ugly Balun which I have described here was also in attendance. This antenna is a little behind the end fed when the other station is in the end fed lobe, but it is still a solid performer; being able to achieve 5 by 8 to the occasional 5 by 9 report. It does give a lower angle of radiation to open potential DX opportunities, however, and this is something the end-fed struggles with.

The main downside of the vertical is that it simply takes longer to put up and take down than the end fed which is 1) couple to the end of the squid pole, 2) raise squid pole, 3) tie wire ends to something, 4) connect radio. The vertical needs still to be connected together, mount the loading coil and run out the radials. I think the squid pole allows me to take off a good 10 to 15 minutes, but it still takes about 30 minutes to setup and 20 minutes to take down. Better than 45 minutes setup and 30 minutes takedown, but not as good as 15 minutes and 10.

Glenn got the good oil on contacts first. Kevin spent some time tuning his Buddi(stick) setup, but Glenns transmissions nearby upset his antenna analyser. In the end, we all got our contacts, my 4th was with Glenn after he left the summit, but then I got a 5th with a FM simplex station on 2m over 150km away setting up for a field day contest happening the following day. I left the summit with a spring in my step.

Here’s a picture of me operating from the summit (courtesy Glenn). The radials of the vertical are somewhat visible, and some of the ratchet band can be seen on the tree, with the base of the squid pole at the ground.

Operating at Mt Tamboritha

Operating at Mt Tamboritha

Here’s a look from the east of the summit.

Looking east from Mt Tamboritha

Looking east from Mt Tamboritha

We headed down much the way we came, but ended up on a track a little to the west of the marked trail, about 100m it worked out. It took us past the WX station and onto the 4WD access track, with the car parked about 700m further away.

Mt Reynard VK3/VT002

Rejoining the Tamboritha Rd, we headed up to the junction with the Mt Howitt Rd and then towards its closest point to Mt Reynard. There was a 4WD track that initially headed a little closer, and then headed south west. Still this enabled us to miss a cliff face. We parked the car at a point where the road began to move further away from the track. Based on the trip, it can be speculated that a walking track to the summit may exist from this road much further along, but I am not sure about this.

After leaving the car, we headed up and over high ground. There was a great deal of fallen timber, which slowed progress somewhat. We crossed a water course, at a point with a bearing of 100 degrees true from the summit, and then began the climb up, which was mostly a comfortable gradient – but with a few kicks. The total trip was about 3km each way, but mostly off track. We did find a track coming up to the summit from the south when we were still about 50m vertical metres down. This track runs off to the south, and it would seem (but completely unconfirmed) that this might end up on the 4WD track that we parked on, but a few km further along.

Mt Reynard has a very large flat top, more than 1.5km long, with a variance of less than 10m. We decided to activate near the eastern end of this, about 4 metres down from the top, but more than 1km horizontal distance! Again, I used a vertical configuration and Glenn his end fed. We also had a few interesting contacts on 2m FM simplex to spice things up. Something else that spiced things up was the large amount of ants at the summit. It seemed like there were nests everywhere. It does make it a bit hard to key CW when you are always trying also to brush ants off!

Here is a pic of Kevin and myself just below the activation area, with the camera looking south (courtesy Glenn).

Kevin and myself on Mt Reynard

Kevin and myself on Mt Reynard

Again, all three of us successfully activated, but this time (and for the rest of the trip), no one ran down the hill to provide a chase. We headed back down the track, but considering its southward direction, we left it and ended up using much the same access route, (bearing about 110 degrees true from the summit back to the 4WD track).

Bryces Plain VK3/VT-004

It was a little late in the day, and also Kevin was worried about the amount of fuel in the car in order to get back to Licola, but the opportunity of a third summit in the day lured us up the Howitt Rd. VK3/VT-004 is a little rise which is not far from the Howitt Rd. There is a reasonable road that actually goes all the way to the top, where there is a commercial tower with a large solar panel. Care was taken to park the car outside the activation zone, but it was only a short walk to the summit, especially when compared to all the other summits on this trip. This summit is an easy pickup, even with a 2WD. It was in the last hour before sunset when we activated. This time, there was a greater variance in our three logs, with me finally picking up some CW DX on 20 and 30 metres.

Here is a look at my vertical antenna:

Vertical antenna at Mt Reynard

Vertical antenna at Bryces Plain

And the sunset from the summit:

Sunset from Bryces Plain

Sunset from Bryces Plain

From there, we headed to McFarlane Saddle where we camped for the night. There were a few other campers here, plus a group that arrived at 9:30pm who immediately left to head down towards Lake Tali Karng. I would think that it makes for cool walking, but the views are not as good by torchlight.

Trapyard Hill VK3/VT-005

Next morning, we pushed back our planned departure time by 30 minutes, accounting for the late finish the previous evening (because we stretched for the third summit). The Forest Explorer map shows two tracks leaving McFarlane saddle, but there appears to be only one on the ground, the east one – even though there is a Parks Victoria sign on this track that shows one track – the west one (wrong one)! The track from a junction about 500m south where a track heads eastwards before Trapyard Hill is true, but the track is quite overgrown and is indistinct when heading across the saddle at point 1442, grid reference 876521 (55H 0487600E 5852100N for those who are unfamiliar with 6 digit grid references – but if you compare the two, you should be able to identify which 6 digits go into the short form). From here there is a first rise, and the track heads up towards the right of this to the flatter ground above. It climbs the summit to its left, missing the summit itself, with the track passing to the north. We left the track underneath the summit, heading up approaching from the north west. It was a steep climb, but not technically difficult. Attempting this summit is easier with a walking stick.

Once at the summit, I ended up getting all my contacts using Glenn’s radio with his end fed setup. I did get my antenna up, but my first attempt did not work with the tree that I strapped the squid pole onto. I got no joy on 20m CW, but the signal was spotted through the reverse beacon network.

Glenn got a pic of me raising my squid pole (strapped to a better tree stump):

Wayne on Trapyard Hill

Wayne on Trapyard Hill

From there, it was back to camp, but this time from the saddle at reference 876521, we headed north down to the Moroka Rd. It was only about 100m away from the saddle. We then walked along the road, with a little bit of passing car dust, back to our awaiting tents. We packed these up, with the pack carry part of the trip about to start.

Picture Point range VK3/VT-003

Now with full packs, it was time to head down the Wellington Plains towards the Picture Point range. Conditions had now become quite hot and the group started to wonder about carrying all of our gear up the mountain. Something else was playing on our minds – the VHF/UHF field day contest. This might mean that a 2/6m activation might make some sense, especially carrying up a FT-817 to give us some 144.15 SSB capability. Still, I decided to carry up my end fed wire, but without a squid pole. It is closer to the ground, and the reports from this setup are not as good, but it does get the contact. Besides, although we appreciate every contact, having a contact with the dedicated SOTAite chasers is worth more to us than some big gun field day station. Sorry, VK3ER/P. 🙂

In the end, we had a clear 2m simplex contact with VK3PF. Considering we could see the Latrobe Valley from the operating location, this made sense. Peter was going to ring around the repeaters to drum up some further action, but it was not to be. I managed a 2m FM contact with Jack VK3WWW operating VK3ER/P on 2m. Jack seemed pretty happy with the contact. HamGPS, an Android app, was used on my phone to determine the 6 digit maidenhead location of the operating QTH for contest purposes. We handed out the numbers, but we did not consider ourselves contest participants. The focus was ensuring that we all left with 10 SOTA activation points, which we did.

Here’s a pic of Kevin operating at VK3/VT-003. You may be able to make out the end fed half wave on 40m strung in the trees behind him, but you may need to click the picture for a higher res view – note you can do this on all pics on this blog:

Kevin operating on Picture Point range

Kevin operating on Picture Point range

On the way back down, we saw this view of the Wellington Plains:

Wellington Plains from Picture Point range

Wellington Plains from Picture Point range

We made our way back to the packs on the Wellington Plains track, and then headed down to our campsite, at grid reference 839462. There is a toilet and a water tank here. Unfortunately the tap on the water tank was broken, so the tank was empty. This meant we had to do a water run down to the nearby Nigothoruk Creek. There was good flowing water down here.

Using a SteriPen

I should mention that after my recent 6 summit trip around Mt Speculation, I believe that I contracted Giardisis from the water I drunk there. It took 8 days for symptoms to show. I thought first that I had Gastro, but it was much worse than that, with the acute phase lasting about 4 days. It has not been diagnosed, but apparently, it is not easy to medically diagnose as the link discusses. Whatever happened, I did not want to experience it again, so I purchased a SteriPEN adventurer Opti. These take 90 seconds to treat 1 litre of water, and I think I can live with that. The SteriPen got its first use with the water from the Nigothoruk Creek. The UV treatment does not change the taste of the water – which this water tastes fantastic.

Wellington Plateau VK3/VT-007 and the Sentinels

The next morning, we headed off early to get our last activation. The Wellington Plateau is a flat top, but there is a small rise to the highest point. We activated just to the east of this, near the 4WD track. This meant that it would be harder to get VHF/UHF field day contacts, unlike from VK3/VT-003, but Kevin did work VK3ER/p on 6m.

Speaking of the 4WD track, yes it is possible to get a car in to this point, but it would take a great deal of effort. The track is very poor, so a very high clearance vehicle would be needed. Also, the access point is through Miller Gap. It would be an adventurous drive!

After wrapping up the activation, we headed down to the nearby Sentinels, which overlook Lake Tali Karng. It’s a great view from up there, but a little hard to get in the pics:

Lake Tali Karng from the Sentinels

Lake Tali Karng from the Sentinels


We met a Victorian Mountain Tramping Club (VMTC) group that had walked up directly off-track from the lake. It took them about 3 hours to make the 600 vertical metres climb. Tough work with full packs – although these packs did not have any radio gear in them. We kept the full packs only for the trip into and out of our overnight camping spot, only carrying limited food, water, and of course radio gear for the trips to mountain tops for SOTA activating.

After the Sentinels, it was back to camp for lunch, quickly take down the tents, and a walk with the full packs for about 9km back to the car at McFarlane Saddle. It was hot work and we certainly appreciated getting back to the car, and getting to Licola for a fuel topup – that’s both petrol for the car and icecream for other refuelling purposes. It was a great trip and lots of fun for 60 SOTA points.

Wayne Merry

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:

Phone Radio circuit board image

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.
Phone to Radio interface board

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.

v1 Phone Radio interface circuit

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.
Updated interface circuit between an FT-817 and a phone device (iPhone, Android)

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

Schematic as implemented on PCB