TheRadioBoard

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

Cheers,

Tony

Trimmers with vernier are difficult to find but probably worthwhile for your set. Or maybe one of the IC variable capacitors?
There are several interesting homebrew vernier designs floating around.
Setting the variable back from the panel and adding a vernier should cure the two main negatives on this set.

Thanks for the update, Tony. At 1600 kHz your ferrite rod has a reactance of >11,000 ohms, so the circuit's squirrely behavior high in the band is no surprise. You could try switching a resistor in parallel to swamp the tuned circuit, but you'll lose a little sensitivity. I think I'd start with 47k and go lower if necessary.

That coil sounds ideal for LW reception. This late in the season you may not pull in any European BC on the 190-280 kHz band, but there are more beacons lower in the band than you can tune at present.

73,

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http://kr1s.kearman.com/
http://qrp.kearman.com/

I noticed that you don't make the Vc supply to the 7642 adjustable. When I use ZN414 or MK484, I have an adjustment pot to vary the voltage applied to the chip. Helpful on strong signals. I use an LED for the dual purpose of an indicator light and a 1.8V (more or less) regulator.

Also, the "well-behaved" MK484 circuit is a plus. It's been discussed on this forum.

Nice work on the circuit board. I make my own, but no silk screen or double-side capability.

Rich

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Homebrew Radio ex-Silicon Valley

... yes, I've read the post on the well behaved TRF. I didn't want to have to make adjustments other than tuning and volume, though ... I really thought the chip should work more stabily to be commercially viable. Of course, I realize that by fine tweaking component values, you can squeeze more performance out of the circuit ... but I was looking for a more casual operation

The circuit I used has two "selected" 1N4148 diodes to regulate the voltage to about 1.2 -1.3 volts ... I like the LED idea though - might as well make use of the "wasted" current.


Thanks for the insight ... I'd hate to think it was my ratty wiring


I may retune the radio for these even lower frequencies in the future. I was hoping to hear some signals in the 600m band ... but so far not much. I have been using my RF generator, which sits near the radio to provide a BFO function. Whichever band gives me the most interesting things to hear will be the band the radio ends up tuned to

UPDATE:
I swapped out the 80pF variable cap for a similar variable that is around 200pF ... now the radio tunes 300kHz to 550kHz ... too easy

.

I am north of San Francisco and can hear some MCW in the 300 to 500 KHz. range. Listening to 330 KHz now and can hear modulated CW but can't pick out a call sign other than "CC". Of course, my CW is rusty.

You should also hear hams running experimental rigs around 500 KHz. This will be CW, so hard to copy without a BFO.
http://www.500kc.com/
Rich

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Homebrew Radio ex-Silicon Valley

CC is actually in Concord, CA on 335 KHZ.

http://www.dxinfocentre.com/ndb.htm

Rich

PS: I can hear Fresno on 344 KHz. Not using a homebrew, however. It's an ICOM 756 with dipole.

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Homebrew Radio ex-Silicon Valley

Tony -- really nice work!

re the “well behaved” MK484 circuit I posted;

http://theradioboard.com/rb/viewtopic.p ... 91&start=0

That set-up can be a 2-knob radio suitable for casual operation.

The pot “A” adjustment is pretty much once-and-done. A trim pot works.
And pot “B” IS a volume control - adjusting for best volume & distortion for both weak and strong signals.

73, Dan

Epilog:

After several nights testing the Ferrite Fox on the low bands, I resolved that a much better ferrite antenna would be needed to get the performance I'm looking for at these frequencies ... maybe one like the 16" long C14T-260K-530K-BC from Stormwise ...



I think it would take some truly serious ferrite to get the results I am looking for. So, I will have to put it on my Christmas wish list

So, I decided to repurpose the radio back to the Broadcast Band, and I elected to do so by removing windings from the ferrite antenna to bring it into the band at a better operating point for the Ferrite Fox circuit. It is now operating cleanly, and provides background music for the bench ... and I'm happy with the result.

Cheers,

Tony