Dear LF Group,
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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