I've just finished my third Ferrite Fox TRF radio.
( The original Ferrite Fox can be seen at: http://theradioboard.com/rb/viewtopic.php?t=3946
This radio was designed to explore the use of a contra-wound antenna, using a 10" Stormwise u125 ferrite core. The design was meant to create a table radio that covered the majority of the useful spectrum of a MK484 type TRF circuit, and with enough sensitivity to be used without an external antenna connection. Both of these goals were achieved.
The wiring of the tank circuit of the radio is shown below.
I wound 30 turns of 176/46 litz on the Stormwise core and measured 120uH. Then I wound a second 30 turn coil and measured the combination inductances in series and parallel configurations. I decided I needed to reduce the individual inductances, but rather than removing windings, I opted to slide the coils out toward the ends of the core. I ended up with individual coil inductances of 115uH. The series combination of the two coils came out to 250uH, and the parallel combination came out to 66uH.
I wired the entire tank circuit using the same 176/46 litz that I used for the antenna coil. I've always wanted to make a radio where all of the rf is conducted with litz ... don't know why ... it just seemed like an interesting option
The antenna coil is elevated from the bottom of the case by about 1.25" and the tall feet on the bottom of the case add another inch. So the coil is held above the table surface by a good 2". I also used nylon hardware to mount the feet and antenna, to avoid any metal being near the antenna.
Tuning is very nice, considering that a vernier is not used on the main tuning cap. The high band nicely spreads out the tuning range, with stations that you can easily tune back and forth through the peak on.
The low end of the band is also clean tuning, none of the razor thin vari-cap tweaking that I've found with my previous experiments. The advantages of a contra-wound antenna coil are now obvious to me.
Performance ... this topic is a bit tricky to explain. I don't have a way of measuring the Q the antenna, but I suspect that it is relatively low to "average". This ferrite antenna tunes differently than my previous 2 Ferrite Fox radios, both with less massive ferrites. The FF3 radio tunes with authority! By that I mean that the radio smoothly tunes into a signal, peaks up, and locks in to the station. And no amount of hand waving around the coil or cap will affect the tuning. Another puzzling characteristic is that there seems to be none of the overloading effects that I noted on the first two radios I have built ... no "clipping" or other obvious distortion. My impression is that the larger ferrite does not "saturate" as easily as smaller ferrites might when higher feedback currents are generated by the TRF circuit.
The radio also seems fairly sensitive ... the first night I fired it up, I picked up 17 stations ... 15 of them probably locals. One of the two "DX" stations was in San Diego, and the other was good ole WWV at 2500 kHz ... the top end of the tuning range. I don't have external connections for an antenna ... this radio is supposed to pull in signals with only it's internal loopstick ... which it seems to do quite well. It is definitely an excellent table radio ( it measures 12" wide ! ) for any and all locals, and has the potential to be an interesting radio for remote locations scouting out DX signals.
This will likely be my last Ferrite Fox radio ... I think I have covered the range of ferrite, and now have several radios to experiment with. This FF is by far the most interesting ... with it's large ferrite and 400-2500 kHz tuning range.
I can readily recommend trying a contra-wound ferrite on a TRF radio ... the improvement in tuning alone is worth the effort. And a larger ferrite core just might tame the overload problems commonly seen with the MK484 type TRF radios.
Postscript: The small meter on the front panel simply indicates the internal battery voltage ... so I know when to change batteries