Some time ago I posted some info on a really interesting Ferrite Loopstick I had found. It is a 1125uH, high Q wound coil, with a 4 turn auxilliary winding. At that time I thought I would make a crystal set with it, but after some thought, another idea emerged.
The ferrite coil looks like this:
The idea that I formulated was to use this ferrite to experiment with the TRF IC's that others had been experimenting with. To that end, I researched the various TA7642 circuits that were around. I came up with a circuit that used the TRF chip with a 386 amplifier ( adapted from a circuit used in the Elenco AM780K ). I designed a circuit board to simplify my experiments...
I ended up with 9 boards in the run, which should cover my experiments, as there are several flavors of this radio I would like to try. Since the circuit is basically designed to work with ferrite antennas, I dubbed it the Ferrite Fox ( I seem to have to come up with a catchy name for all my projects
) The completed radio looks like this:
This radio was constructed with a 80pF variable cap to allow the ferrite coil to be tuned across the broadcast band. I mounted the ferrite high and away from everything, and have a reasonably good tuning cap. The RF wires are on one side of the radio, and the audio circuits are on the other. The wires from the PCB to the tank were longer than I might have liked, but were the best I could come up with. I used the aux. winding on the loop to provide an antenna and ground connection ... but found that they aren't needed ... the ferrite really does all the work of picking up stations. I also provided the radio with a recorder output jack ...
This is my first radio with this chip, and I have read that the circuit can be squirrely ... and is prone to overloading with strong signals. My radio proved no different. All of the local stations seemed to overload during the daytime ... but at night it was a different story. From my kitchen in Las Vegas, with no external ground or antenna connections, I was able to easily, and clearly, pick up three 50kW clear channel stations: KFI ( 640kHz ) in Los Angeles, KTNN ( 660kHz )in Window Rock, AZ, and KNBR ( 680kHz ) in San Francisco. So, I was impressed that this radio circuit was sensitive, and had good potential.
The only instability that I had was when I was tuning at the high end of the band. Once the cap was down to 10-20pF, the stations were harder to tune exactly right, and hand capacitance proved to be a significant factor affecting the radio. There was also a tendency to oscillate when tuned near a station. I realized that the circuit would work better, if there was at least 20-30pF of capacitance in the tank at all times. Of course, this means that the 1125uH coil could not be able to tune the high end of the BC band...
So I decided to make the radio a 600m radio, and added 50pF across the tank circuit. The next night I experimented with the radio again. I was able to pick up 2 beacons ( unknown to me - no ID ). The modified radio covers 450kHz to 550kHz. I don't know if I'll find anything "interesting" in that bit of spectrum, but I have a radio to scan it with.
The next radio I plan on building with the Ferrite Fox circuit will be another broadcast band radio. But this one will use a tank comprised of a Miller 6300 permeability tuned loopstick, a 50pf cap, and a 10pF trimmer. The radio will be configured as a retuneable fixed station monitor ... and this time I should be able to easily tune anywhere in the BC band - and be stable