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WSQT Radio
WSQT Guerilla radio is now broadcasting at 87.9 MHZ from the heart of Occupied Washington. As our current location is very unfavorable from an RF standpoint, we had to up power to 10 watts to get out.
In open country the same rig and antenna went for miles and miles, with no apparent loss of signal quality over 4 miles away, but in the city local range is about a mile and max range maybe 2.5 miles. No amount of antenna fiddling could fix this, so that would leaves a choice: more height or more power.
Given that our previous location was spotted visually, power seems to be safer,given a clean signal(our tests showed the strongest spur {still in band} more than 40 db down). Believe it or not, I was able to set it up next to a TV and watch channel 7 with the antenna just a few feet away, with the same setup that went for miles in band. You had to look closely to see interference in this very demanding test.
While one watt will go three miles open country in our tests, we needed ten to get out in our current location.
Anyway, we are now getting that ten watts from a $1.99 IRF 510 MOSFET! None of the hams seem to have ever tried it that high, and turn on/off times would seem to prohibit that. Most of the rise/fall time, however, comes from the resistor in the shown test circuit-and the times are no worse than the 2n2222 and 2n3904, which work at VHF but need much drive.
Since the Q of the capacitive input(about 190pf) is low at this frequency, driving the IRF 510 takes a lot of power-about a watt to a watt and a half. The ten watts sustained, 11.5 watts cold that I've gotten is two thirds of what the AM rig can sustainably make from one IRF 520-and 7-10 db gain is no worse than expensive VHF bipolars!
Why did we return to the IRF 510 after our earlier experiments
gave poor efficienty? Because we fried our only NTE 235 in antenna tests, and the IRF 510 will take 100v peak instead of only 60. In short, its SWR-proof against open loads(shorted loads may still kill it!), a great asset in testing and tuning antennas. We got down under 1.5-1 SWR, but it su4re didn't start that way with all the reflections we were getting.
Anyway, here's how to use it: use a .01 cap to couple it to a tuned circuit with about 100pf and a four turn coil at two turns. Feed that from the top with a watt and a half or so. You will get about 7 volts at the tap and a good match to the 75 ohm out-put from the 1 to 1.5 watt output amp. On the ohter side of the cap, use a zener regulated bias circuit to give about 3.3v bias-this seems to give best output, as more delays turn-off and less delays turn-on. use a normal tuned output circuit, but couple lower than normal to allow for the losses in the device. I used a five turn coil, coupled to the IRF 510 at 1/2 turn, with 22pf at the top to couple to the load.
You will not get the full Class C efficiency, but rather in the 50-60% range at best. If this is still not enough power for a bad location, four IRF 510s with more drive (2-2.5 watts each and