Dear LF Group,
I am quite interested in the possibility of an NoV for 9kHz to extend the
frequency range of my transmitting antenna experiments, but am not entirely
convinced yet that it will be feasible to do field strength measurements in
the same way it is at 136k and 500k. However, as a start, to investigate
noise levels and so on, I have been trying out a portable receiver using a
breadboard loop antenna / preamp to feed the 9kHz received signal into a
laptop sound card for software-defined signal processing and display using
Spectrum Lab etc. The prototype, although thrown together, seems to work
quite well, covers about 3kHz - 22kHz and is pretty simple, so I thought
others might be interested. The circuit diagram is attached.
The antenna is a multi-turn loop about 0.8m square with the winding made as
a bundle of insulated wire. It is loaded with a shunt capacitor and the
input resistance of the preamp to produce a low-pass response with a cut-off
frequency around 22kHz. The first stage of the preamp is a bipolar
transistor with series/shunt feedback to define the gain and input
impedance, which gives an equivalent input noise of about 1.8nV/sqrt Hz. The
second stage is an "audio" op-amp, with RC feedback to give gain that rolls
off at about 20dB/decade over the frequency range of interest, to equalise
the loop output which rises with frequency, to give a roughly constant
Vout/Ein around 10dB.
In the tests I have done so far, the preamp noise level is at least several
dB below the band noise in the region of 9kHz, so the sensitivity of the
system is as good as can be used. It works successfully with DL4YHF's
Spectrum Lab in various configurations, also with I2PHD's Winrad software. I
would suggest keeping the preamp at least 1m away from the computer, and the
loop antenna several metres away - my laptop seems to be quite noisy at VLF.
The coax connecting the loop to the preamp can be any convenient length.
Running both preamp and computer from batteries is very helpful when trying
to identify sources of mains noise, which seems to make up most of the
artificial noise in this part of the spectrum.
Cheers, Jim Moritz
73 de M0BMU
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