It's wonderful to watch your ideas take shape, and weird to watch the shape change as the ideas mature into workable products. The latest radio is no exception. I went through a panic stage four days ago, and the direct-conversion receiver / cw rig turned into a Pixie II, and back again. Do a search for the Pixie II, it's a fascinating exercise in minimalism. The PA transistor doubles as the receive mixer; keying the PA emitter shorts the receive path and puts the transistor into full drive. On key-up, recovered audio is passed to an LM386, and into a pair of 32-ohm headphones. Usual power source is a 9V PP3, and the housing can be anything from an Altoids tin to al fresco.

I did say it turned back again, and it remains a variant of the Rev. George Dobbs G3RJV's famous 'Sudden' receiver. I'm using Micrometals iron toroids in the receive preselector, and a 3.58MHz ceramic resonator as the frequency controlling element. I need tighter control than George's VFO original, because it's also transmitting. I've settled (for the time being) on a 30kHz band, from 3.55 to 3.58. This includes the QRS 'sandpit' at 3.555 and the QRP centre of activity at 3.560. The radio has a 700Hz transmit offset, given by arranging for a pair of resistors to be switched in and out of the frequency control voltage 'totem-pole', which feeds the LED varactor. Not only am I using a LED as a varactor, but I find that forward bias gives more linearity. This method is not original, but has been used with success in the radio home-brewing world for some years. The other feature of the forward bias is that the LED can be brought out to the front panel, adding interest as the radio is tuned. The LED dims and brightens, glowing well at the lowest frequency.

I'm currently waiting for parts, and I have two evenings ahead of me where I'm committed elsewhere, so it'll be just ideas until the latter part of the next weekend. With a following wind, the prototype may be running inside a week, and taking reports from Southern England. We'll see.

Morse Code Advocacy

This Saturday gone was hard work, but great fun. I delivered a talk and teach-in to forty-eight 10 - 12 year-old Junior Sea Cadets, along with their twelve adult and senior Cadet team leaders at an Adventure Camp. I presented a potted history, a practical demonstration of current amateur radio usage and then let them loose with keys and oscillators. The kids loved it. They were given a copy of the code, on an A5 sheet, and they sent me their names. I divided them into three-member sub-teams and gave one group the task of composing a distress message, while the other three were shown more about amateur radio. When ready, the distressed group sent their message, and the 'helpers' decoded it and responded...

There was a wide spectrum of interest; some Cadets wanted to know if they could talk to aliens, others wanted to know where they could get keys and beepers. I think a couple of them may be hooked, and we might hear them in future when they have their Foundation Licences.

I had to return home late Saturday evening for domestic reasons, but I've had reports that there were flashing-light Morse signals passed from tent-to-tent after lights-out. Seeing how Morse can capture the imagination of these youngsters, I'm wondering if more could be done to raise the profile of what remains one of the most efficient ways of moving information around our planet. I wrote a short pamphlet about it, and I'm considering writing an extended monograph for just this purpose.

As I often say in this blog, more follows...

A New Station - M6BMO

My youngest son, Billy, recently qualified as a Foundation licensee. He studied with the Worthing and District Amateur Radio Club, to which we are both very grateful for their help and support. Now Bill is an M6, he needs a radio, and there lies a problem. I can't afford a 'black box' rig for him, and as a Foundation ham he can't use my homebrews. He can, however, use a radio made from a commercially available kit. Some of Tim Walford G3PCJ's kits ( www.users.globalnet.co.uk/~walfor ) are suitable, and maybe I'll buy Bill one for Christmas. Until then, I'll simply have to become a vendor myself!

What I have in mind is a simple kit based on Billy's immediate desires, and which may prove useful to others in the same boat (impoverished, Foundation licence). He wants to use 20m, and cw. This fills my heart with gladness; 20m is my favourite band, the rig is technically unchallenging and can be made in a variety of ways. First thoughts are with either a Pixie II clone or a Sudden / VXO / Pebblecrusher mix. What ever happens, I hope to have a prototype going quickly, and a kit advertised soon after. Then, Bill can build a kit bought with his pocket-money (I'm hoping it'll be that cheap), and get going.

Somewhere new to rest my case

New website!
Yahoo! Geocities is due to close its ports later this year, so I'm migrating the best bits of http://uk.geocities.com/egnaro937@btinternet.com/index.html to a Google Site - http://sites.google.com/site/g1inf4u/home . I've already added a page about the Poundshop DC receiver, along with basic info on the 24MHz theremin and the closed-circuit RF trainer projects.

The new site will take some little time to build, but I thought I'd give a heads-up on the move.

Closed-Circuit Radio

I've wanted to do this for some years, and I've finally taken the plunge. All the non-radiating radio network trainers I've come across have used some form of multi-port telephone system, where the students and tutor are connected together and may talk amongst themselves. They have PTT switches, headsets and boxes full of electronics, but they lack the one thing real radio networks have - a radio spectrum to explore.

I've prototyped a closed-circuit radio network. It's a very basic DSB (dual-sideband) system, using only three active devices in each 'station'. Little effort has been put into emissions control, because there are no emissions. The network is contained in a loose web of 75R TV coax, and the signal levels are tiny.

I've used my favourite integrated circuit, the Philips SA602 mixer-oscillator. This eight-pin device contains all you need to make a basic DSB 'modem', able to modulate and demodulate voice onto a RF carrier which is produced by its internal oscillator. Frequency control is by a ceramic resonator, whose resonant frequency is changeable by adding capacitance between it and ground. This 'pulling' of the resonator's frequency is analogous to a blues harp player lowering the note of their harmonica by increasing the volume of the mouth on some of the 'draw' notes.

Controls are simple; PTT, tuning and AF gain. Power for the student stations comes along the coax, and is segregated from the signals by a simple inductor / capacitor diplexer (much like a HiFi speaker crossover filter). I plan to add a couple of feature to the tutor station, namely a variable noise level for the system, and an interference simulator (piped in from an MP3 player?) to add realism.

The really big thing here is the necessity for each student to learn how to tune their radio, netting-in on the signals they hear, and maintain the setting. It is quite possible (and may even be useful) for several nets to exist simultaneously, given that several tens of kilohertz of bandwidth are available. Multiple tutors could train the students; interfering, breaking-in and calling.

So why bother? The system outlined here would cost little more (if any more) than an AF system, has much more realism, and would be FUN to use. People who require controlled experience of HF communications without actually using our precious ionosphere before they are qualified would benefit greatly.

Once I've hardened the design, I intend to publish it, and perhaps provide kits for interested parties. The photo shows a matrix-board prototype which offers some idea of how a PCB layout would look. It measures 37 x 70mm, and it should be noted that the microphone socket is missing from the bottom left. I have successful prototypes constructed on stripboard and ugly-style over a copper-clad groundplane. I plan to publish layouts for stripboard, and downloadable Gerber files for those who wish them.

No promises on time-frame here. That's one of the pleasures of doing electronics as a hobby, and doing other stuff for a living!

Forty-Metre SSB Superhet Transceiver

Yet another HF superhet rig

I like doing unusual radios, so when I decided to produce yet another HF superhet, I felt compelled to give it a twist.

The current project uses two SA602 mixer/oscillators, and a DPDT relay to swap the signal path around to take the rig from receive to transmit. I used the fortuitous layout of the SA602's inputs and outputs, along with the equally serendipitous pinout of the miniature relay to form a very compact and efficient SSB 'modem'.

So far, I've managed to get it running as a receiver, and added an AGC circuit to soften the blow when a strong signal is encountered. The AGC is pretty standard; just a charge pump and capacitor driving an NPN transistor, but the control devices are unusual. I've put a pair of red LEDs in a stack from the +12V line down to the AGC transistor, and coupled them into the RF input to the first mixer. As well as smoothly attenuating the signal, I get a pair of signal-strength LEDs as a bonus. Two birds, one stone (well, two bits of silicon).

I've housed it in a pair of 2oz tobacco tins, mounting the circuitry in the lids, which I've soldered together. This makes it compact and lightweight; ideal for backpacking. The photo shows an early incarnation, before I boxed it. The two mixers, the relay and the three-crystal filter are at the top. The fourth crystal is for the BFO / carrier, and the 12MHz ceramic resonator at left is for the 4.915MHz / 7.1MHz conversion. This choice of IF meant I could terminate the filter directly to the 1500R ports of the SA602s, and still maintain great passband shape even with just three crystals.

A simple direct conversion receiver

Back to my roots - simplicity
I tried a little experiment. I hooked-up my Poundshop receiver (Google it...) to my netbook, and fired-up the wonderful SAQrx Panoramic Receiver software. I found I could use SAQrx as a digital signal processing AF subsystem, and tune-around inside the Poundshop's gigantic audio bandwidth. I was satisfied with the results, but the instability of the Poundshops's 7088 chip meant it drifted too quickly for comfort when the passband was narrowed. I needed crystal control, and a fixed reference.

Why use batteries?
Most of my projects run from 12V lead-acid gel-cells, or 9V alkalines if low-powered. The current project has two features; a single active device with a 4.5V - 8V supply range, and a close association with a small computer. Why not use the power from a USB port? It's 5V, and capable of supplying 100mA (more if care is taken). So, I found a redundant USB cable, an old earbud cable (the kids tread on the earpieces!) and scribbled out a plan on the Schematic Editor.

My favourite chip
It had to be a SA602 mixer-oscillator. These have poor big-signal handling, but are very easy to use. I attached a 7030kHz crystal, a smattering of capacitors and a loop antenna (one metre length of four-core cable), and hooked-up the computer power and signal cables. It worked as soon as I'd turned the SA602 over; a blunder which deserves it's own paragraph...

A creature of habit
Nearly all my projects are produced in 'ugly' style, where chips are turned-over, and used legs-up. So, when I came to use a 'familiar' chip in a piece of stripboard, I reversed all the connections. Happily, the SA602 survived this abuse; I simply desoldered it and mounted it in the solder-side of the board. Success!

A little deaf, but it works
It's a starting point. I may try to improve the antenna, and / or add another SA602 as a RF amp stage. Because the antenna and mixer inputs are balanced, this is an ideal choice. I get an extra 15 - 18dB of gain, controllable by applying a bias voltage to the oscillator emitter pin (6). This works well. I have a home-brew guitar amplifier which uses this scheme as a tremolo effect, with pin 6 fed from a sine-wave generator via a 'level' pot.

I'm getting restless.
The station is essentially dormant; stored in the garage in boxes. However, I'm missing it already...

Today, I hooked-up my Poundshop radio (a 40m DC receiver made from a £1 scanning fm toy radio) to a simple and magical piece of software; SAQrx. This tiny Windows application (it runs under Wine on my Acer Aspire One) turns the Poundshop into a very useful CW receiver. I set the radio up on 7MHz (the band edge), and use the DSP filter in SAQrx to probe around the band. I can watch the whole 22KHz going into the PC on a real-time FFT display, and slide the filter onto any juicy-looking carrier. I would now like to make a dedicated receiver, using a 7MHz crystal for stability and USB power for convenience.

Downsizing and other nonsense

Beyond amateur radio, I have other things to do. Oh, well...

Since the last post, I've discarded a lot of radio stuff. Old, unfinished and abandoned projects, items which 'might be useful one day', most of my surface-mount stock, and a pile of magazine cuttings. It had got thoroughly out-of-hand, and the carnage may yet continue. I have several A4 ring binders full of datasheets, all of which exist as PDFs or other electronic documents. Why keep all that paper?

An adventure at a local carnival convinced me that a lean and portable radio station was all I require, and anything I couldn't carry around in a shoulder-bag was a liability.

I'm a Petty Officer in the Sea Cadet Corps, and I instruct in communications at our local Unit (Worthing). As part of a recruitment exercise, I took an FT101-B, a field antenna, a tuner and a large (24Ah) gel-cell battery to Broadwater Carnival. I cannot use our Sea Cadet frequencies away from the Unit, as the transmitter permit won't allow that, so I operated as G1INF/P. All this should have been easy, and provide an interesting display.

The antenna, a 33-foot copper screw-together pole, was a success (it supported our Ensign). The tuner wouldn't load the thing on 20m, and there was no activity on 40m. The Yaesu's tuning knob seized (thirty years of zero-maintenance finally too its toll), so the Cadets were reduced to running 'errands' with PMR446 hand-helds, and helping to locate lost children in the crowds.

The whole affair was grossly overweight, and unmaintainable. The tuning gear on the radio needed a strip-down, clean and lube (gun-oil is excellent), something I couldn't hope to do with my 'field tool-kit'.

Regen receivers, again - I find the idea of regenerative so seductive, I had to try again. The thing works, but I have reservations. It's unstable below 8MHz, squarking loudly if oscillation is approached. It needs more gain, and a lower-Z output, as it currently uses a crystal earpiece. But it works; I've recovered SSB and cw from both 20m and 40m, and a host of commercial stations in between.

I'm going to try another, based closely on G0KJK's HF9 design, as published in Sprat 82. I'll take my time over it, and follow the design closely, using lessons learned in previous regens. Microphony, feedback and hand-capacitance all need careful attention. Stiffness, layout, and screening!

The 'simple' regen is pictured here; it's a variant on the G4KJJ 'Smid-Gen', itself a subset of the HF9. The top-left is the main tuning, under this is the bandspread. Top right is AF gain, with reaction below. Left switch adds 100pF to the tuning, an the right switch adds 9V to the radio. A random wire is seen attached as an antenna.

Onward, I hope to make a better regen receiver. I also want to make the add-on box for my Racal Minical (see previous posts), and that radio was designed for a shoulder-bag!