[Hackrf-dev] HackRF-dev Digest, Vol 21, Issue 19

[McDonald, J Douglas]

A couple of comments. I'm very new to all this stuff, HackRF and Gnuradio,
so take this with some salt.

First, I'm running HackRF with both SDR# on Windows (I got it to work OK) and Gnuradio
on the Ubuntu test disk. Both work just fine. I have not tried transmission yet.

As to frequency, feeding the HackRF from a low impedance (50 ohm) emitter
follower preamp (homemade) and 25 foot antenna or, for broadcast band a
tuned loop, it works down to broadcast band, as while sensitivity is well down,
the atmospheric and chopper power supply noise is well up there, so the external
noise is well above the HackRF noise. In some locations this may not be true.

People have comments on FM reception. As I said, I'm very very new to this,
but I worked very hard in Gnuradio Companion to make a good low noise FM stereo
receiver and finally succeeded. It was not easy, not easy at all! It took very very 
careful selection of downconversions and bandpass filters to get good reception from
the station I was interested in: 100 kW ERP 800 foot antenna 23 miles away, with
59 kHz data subcarrier, 67 kHz FM "talking newspapers for the blind" and
a full set of HDRadio subcarriers. All these generate terrible birdies on
all my FM tuners (all had been reasonably well until they added the 59 kHz). 
The key was exceedingly strong filtering of the 19 kHz pilot and its 38 kHz harmonic. 
Also it was necessary to use the narrowest bandpass (1.75 MHz) on the HackRF
and a preexisting passive preselector with a  1 MHz bandpass using helical 
resonators. I now have all of the birdies well in hand. This is a classical music station that often 
runs for minutes at -35 dB audio level, and sometimes for shorter periods
as low as -50 dB audio level. Interestingly the talking newspapers at 67 kHz
worked well with no problems.  So FM can be done well.

Doug McDonald