Friday, 10 July 2009

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 to a Google Site - . 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.

Monday, 6 July 2009

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!