Hello Jim,
Good work! If you want we can make a ferrite-ferrite QSO, RX wise ;-)
I also tried a high Z preamp but still didn't get good results. My BF862
has a gain of 26 dB which is to much. I have to try some source
resistance (without a parallel C) to optimise things. Maybe there were
some VHF oscillations too, not sure. Have to spend more time on this...
QRN was very high here as well. Have to wait for better conds here.
Maybe tomorrow...
73, Stefan
Am 23.08.2011 19:51, schrieb James Moritz:
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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