WSQT's new crystal controlled 40 watt rig

WSQT the Squat is guerilla radio broadcasting from the heart of Occupied Washington on 1680 AM. We just installed crystal control in our transmitter, and now have a design we feel is good enough for publication so others can use it.

Some of our shows are posted to the newswire at www.dc.indymedia.org. Until now, listener range was limited by interference from a powerful station hundreds of miles away, which whistled with the variable oscillator's signal. Thanks to a dumpstered VCR and its crystals, that whistling is now a thing of the past and our listeners can enjoy much better audio.

Anyway, on with the technical description.

Previously, we used a variable control like a receiver due to lack of available crystals for the broadcast band. This week, however, a dumpstered VCR yielded up a pair of crystals(14.318 and 16.0) that together make a beat frequency of 1682, easily pulled down to a rock-stable 1680.

If you listen to WSQT on an evening show, (where we had the worst problems with interference from a NJ station) you will no longer hear the "hetrodyne" whistle that used to plague our broadcasts in the fringe zone. On an evening show a flutter will still occur in the last four blocks of coverage, but even the big Clear Channel stations are only held to a 20HZ tolerence(to prevent whistling).

Any two commercial stations on the same channel sometimes will occasionally have this flutter where both can be heard at once, but thanks to crystal control, they will never whistle. Now we don't whistle either!

Here's how anyone can duplicate these results for the AM extended band, for channels from 1660-1700khz. Crystals for 3.579 and for 4.00 MHZ are extremely common in VCR's and other consumer electronics. Multiples of this such as 14.318 and 16.0(which we used) also are sometimes found. If you only have 3.579 and 4.0, no sweat-just feed them into the mixer stage and multiply by four. Only one additional stage is needed for that.

Next comes the real beauty of this setup(known to hams as a VXO or variable crystal oscillator). Most crystals can be tuned one part per thousand, or as many khz as their frequency in Mhz. If you multiply, that variation is also multiplied. In either case you get a range of 14.318 mhz+-15 khz or so, and 16=- 16 khz or so, for a 30 khz variation of the difference frequency in either direction.

Capacitors pull crystals up, and by pulling one or the other you can increase or lessen the difference. Our setup first came in at 1660 until a too-small capacitor on the low crystal oscillator was removed, then came in almost exactly at 1680, requiring just a final adjustment. You can also push a crystal down with a coil, but at a cost in stability.

What all this means is that you can broadcast on 1660, 1670, 1680, 1690, or 1700khz using crystals from old VCR's, and one of these will be open almost anywhere due to the limited popularity of AM. Pushing down with a coil on one you could possibly go lower, but that might require retuning for drastic temperature changes if you broadcast from an abandoned building under remote control.

Forget variable coil and capacitor transmitters-they are almost as hard to build, much harder to make work right, and these VCR crystals come from the dumpster! You will still need a trimmer or tuner capacitor to set the crystals, but you only set it once, so you don't need a string tuner or other capacitor drive.

If you are building an AM rig, the schematic published here pumps out almost 40 stable watts. Bolt the output transistors to a burned-out car stereo power amp(with its guts removed) for a heatsink.

If you were to put this on an abandoned warehouse with a 300 foot by 300 foot metal roof(which gives a MUCH better antenna ground than we have), and put up a trash bag full of helium holding 150 feet of fine wire for an antenna, it could reach for 12-15 miles. Same if you ran a 150 foot wire up someone else's tower and hooked up to their ground network. The transmitter costs about $45 plus what you get from the dumpsters.

With the right lowpass filter(like ours), a good audio limiter(like Winamp in a computer and maximum volume set not to overmodulate), and the crystals, this rig should meet the standards broadcast transmitters are supposed to meet(for spurious outputs and carrier stability), giving a signal as good as any traveller's aid station if you can get an antenna and ground(the hard part) just a quarter as good as theirs.

AM antennas are a pain in the ass with a quarter wave being 150 feet, but there is a lot more space in the AM band than in FM band and enough power can be made cheaply to match the range of a cheap 2 watt FM ring into a compromised antenna. Still, you need to use an SWR meter, get under 2-1,and make sure the best ground you can get is part of that. At least 50 feet of wire are needed unless you want either a bandwidth so narrow as to take the highs off music or so much ground loss you only get out a half mile or so.