Notes from a Small Station
Joe Chester M1MWD wonders why there is no replacement for the single-band VHF multimodes of old.
Joe Chester M1MWD wonders why there is no replacement for the single-band VHF multimodes of old.
Would you like to have a 2m/70cm all mode transceiver? When I say all mode, what I really mean is FM and SSB – I don’t think AM is really important (tell me I’m wrong). Of course, the answer is to buy a multi-mode HF radio, one which covers 2m and maybe 70cm also. You will get a good transceiver, maybe 100W on 160-6m, and a bit less on VHF. However, it won’t quite be a mobile or portable rig. Not quite bicycle friendly. Is such a transceiver possible? Let me rewrite the specification. VHF/UHF, FM and SSB, in a lightweight portable format, maybe 5-10W max output. Think of all the uses – 2m contesting out /P, or the SSB satellites. As there will be FM on board, it could be used as a mobile FM rig. As a backpacking radio, for SOTA work maybe.
The FT-290
I once had an FT-290. It was a lovely little radio for the time. FM and SSB, a few watts output, but it was fun to operate. I put it in my car and operated mobile on my daily commute, which was a 150km round trip at the time (I butchered a coat hanger to make a quarter-wave to mount on a wing of the car!). I also used it for /P 2m SSB operations during contests, with an HB9CV gifted to me by my original amateur radio teacher. My best performance was into the south of France from a hilltop in the centre of Ireland. Today, mobile operation, unless the car is parked, has been made more difficult by the huge increase in road traffic volumes, although many operators do work mobile. And /P operation, with a big HF transceiver, is also a more difficult operation than it was back then with the FT-290, sometimes even requiring a generator. As a footnote, I found myself looking longingly at a refurbished FT-290 at a hamfest recently – not huge money, but beyond the current budget limit. Perhaps that would be the answer?
Why no Replacement?
But wait a minute. Why is it that a transceiver with this specification is not on the market today? I’ve asked around, and the universal answer is that there is no market for such a radio. Really? I can guarantee at least three customers, from a single chat about this on an 80m net recently. At hamfests over recent years I’ve spoken with suppliers and manufacturers’ representatives. They all universally sing the same tune. No one wants such a radio. The development costs are huge, and without the certainty of worldwide sales there is no business interest in such a radio.
Now I’m not a marketing expert. But I can’t see why there would be no interest in such a lightweight radio. Lots of amateur operators do portable work, and the annual 2m and 70cm contests are well supported. These are essentially SSB activities. So, my case is that there would be a demand, if such a radio was available. As it’s not, there really is only one choice. No, I’m not talking about the second-hand or refurbished market. I guess I’ll just have to design and build one.
Some Thoughts
Daft! Mad! But that’s not what happened when I told this to a few fellow operators. Rather the reverse. Suggestions poured in, and I began to lose the thread of the very technical conversation that followed.
One clever chap even suggested doing an internet campaign to raise money for the development. He was cut off sharply when we discovered his real ambition was to be away with the funds to Saint Lucia! But I digress. So back to the challenge.
This is very much what A. Einstein used to call a thought experiment. For now anyway. There are lots and lots of component suppliers, and even of subsystems suppliers out there. So, my plan is to sit down with a good book about designing radios and start from first principles. I’m also going to cheat a little, by looking at block diagrams of various radios. For, of course, the thing that makes this project viable is SDR technology, and the SDR chip sets are readily available. But I’ve leaped ahead.
Trivially, what I will need is a front end to capture VHF/UHF signals, which of course are analogue AC currents. I will then need a bit of filtering, and then convert these to digital form, and present these to a processor which will do all the demodulation. Take the output from this to
an audio processor, and finally an amplifier, and I’m done. Any questions? Stay awake at the back, as questions will be asked. Because here’s the first question. Is this a good start for my design?
Well, yes, and no. As a concept block diagram, it will probably do for now. But more research will be needed. I can see one weakness already. This concept will need a computer to run the software.
And I don’t want to lug a laptop with me when I am backpacking up Mount Snowdon. Then another surprise. Because there are very, very small computers available quite readily, which could be programmed to do what is needed. I’ve been told the CPU chips are available from you know where for £1 a dozen, or even cheaper.
But I don’t think I want to design the computer processor for this. What about a Pi, or an Arduino, or similar. Now we’re getting somewhere. Might have to learn Python, but so what.
A day later, and it’s not as mad an idea as it sounded originally. But then, surely someone somewhere must have tried this already? Well, no; all I found were HF transceivers, with 2m boards in them. Even the new rigs like the Xiegu, the Ailunce or Q900 (other possibilities exist as well). So, it’s back to the design stage.
Now the search turned to the question of the SDR chip, or system, if you prefer. “Use a Lime SDR (small) or Pluto SDR (even smaller)”, one correspondent said. Both those SDRs can receive and transmit anywhere from HF to a couple of gig, so would easily cover all the VHF and UHF bands. I spent a bit of time looking at what are in fact chipsets intended for the mobile phone industry. Many of these, like the LimeSDR and the Hermes Lite, are actually full specification transceivers, even if the output is only a few milliwatts. I settled on the LimeSDR for my design, because of a conversation with another radio amateur who had already used it for an Oscar 100 station, which seems to be a popular use for this chipset currently.
Coming Together
The supplier says that the LimeSDR is an open source SDR transceiver, covering 10MHz up to 3GHz, in full duplex. I was warned by another operator that the lower frequency limit on some of the boards may be higher than the quoted specification, but that’s for another day. Checking the Lime webpages was an interesting afternoon. It seems the Lime design philosophy is to put as many of the building blocks on the same chip as possible to make it easier and cheaper for developers. Therefore, I won’t need to assemble so many individual subsystems to get this done. This is what I now have, Fig. 1.
With this done, I really don’t know where to go next. Any help would be appreciated. OK, I know that the Lime is expensive for what I want it to do so what would other people suggest?
And I’m not a welder – last time I tried I nearly burned a hole in the kitchen table putting solder on an audio plug. But I am convinced that my ‘thought experiment’ has, with help, achieved the first stage of an achievable design.
Based on this I believe that it would be possible to design and build an SSB 2m transceiver, lightweight and very portable. Anyone like to give it a go?