In the early 1960s some aircraft in the RAF were fitted with Marconi ADF
sets. The loop antenna was a slab of ferrite, which if my memory serves
me right, was some 200mm x 120mm and about 30 or 40mm thick. The
windings were just a few turns of litz wire.
As the equipment had to cover the whole of the NDB beacon band I guess
it was a broad band technology.
In the early days of 73kHz I followed the example of Mike, G3XDV, in
constructing a loop antenna and FET preamp from a defunct transistor set
to make up a mobile receiving station. These were used to check out the
relative field strengths of our primitive early transmitters. My
transmitter was a audio signal generator, a 10w solid state vehicle
sound system amplifier and a big coil
Peter, G3LDO.
Having said quite a lot about ferrite rods on the reflector recently,
I thought I better try one as a quick "feasibility study"!
I have a large bar of ferrite I had assembled from surplus "U" cores
some years ago as part of a portable SAQ receiving system (it's a long
story...). The"rod" is about 470mm long, with rectangular cross
section about 30mm x 35mm. This is rather larger than the rods used by
DK7FC and DF6NM, but the ferrite is more lossy - probably 3C8 or some
similar "power" grade. 30 turns of litz wire gave L about 360uH and Q
of 150 (so using litz probably wasn't justified, but I had that length
to hand). This resonates with about 3700pF at 137kHz, giving an
equivalent parallel resistance of about 45kohm and a bandwidth a bit
under 1kHz.
The preamp is a compound JFET/bipolar follower using a J310 biased to
6mA and a 2N3053 biased to 50mA (probably similar performance to a
"mini-whip" preamp at 137k). The high Z input is connected directly
across the tuned antenna winding. With the high QRN on the band this
evening, it is hard to get a good view of the noise floor, so I used a
"dummy antenna" consisting of a pot-core inductor wound with the same
inductance and Q, substituted for the real rod antenna to simulate a
zero signal and external noise condition. The noise output from the
resonated dummy antenna was 10dB or more above the RX noise floor
(SDR-IQ). The preamp with input shorted gave a noise level below the
RX noise floor. So this preamp arrangement gives sensitivity limited
by the thermal noise of the antenna, and adequate gain for the
"reasonably good sensitivity" SDR-IQ (also for my RA1792).
I made a rough estimate of the overall sensitivity of the complete
system by measuring the level of DCF39, which is a fairly stable
800uV/m here during daylight. On an arbitrary scale on the SDR-IQ
spectrogram, DCF39 was -19dB, while the noise floor at around 137.7kHz
with dummy antenna was -109dB with 0.75Hz FFT noise bandwidth. This
works out to an antenna noise floor of about 0.03uV/m per sqrt(Hz),
several dB below quiet 136kHz band conditions. So sensitivity of a
receive system with this antenna should be limited only by the band
noise; it should also be possible to reduce the amount of ferrite
somewhat, especially if lower loss ferrite antenna rods are used. I
await lower QRN levels for a "live" test !
Cheers, Jim Moritz
73 de M0BMU
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