[Hackrf-dev] Narrowband fm - what is the secret?

[Gavin Jacobs]

I did make some progress on this, as follows:

- temporarily attached rubber-ducky antenna from handheld, onto the hackrf One; got a very faint signal

- zoomed in on frequency display and determined that frequency correction of 10 ppm centered the bandwidth on the target frequency


Once I found the signal, I tried again with & without the RF amp, I could see all the spectrum move up, but not much difference to the SNR.


Per Kevin's suggestion, I will try a different antenna and a better (higher) location in the attic. This is getting a bit off topic, but I will need an omni-directional antenna, with some gain in the 2m band - perhaps a discone?


Thanks,

Jake


________________________________
From: HackRF-dev <hackrf-dev-bounces at greatscottgadgets.com> on behalf of Kevin Reid <kpreid at switchb.org>
Sent: September 15, 2016 9:03 AM
To: hackrf-dev at greatscottgadgets.com
Subject: Re: [Hackrf-dev] Narrowband fm - what is the secret?
<snip>
* you can hear APRS packets on your handheld, and
* you can see a signal from your handheld on your HackRF One, but
* you cannot see or hear APRS packets using the HackRF One.

This suggests that the signal is sufficiently weak that it is below the HackRF One's receive noise floor, but not for your handheld. Then, some possibilities:
1. Your HackRF One does not have a good enough antenna (for the frequency) attached to it.
1a. Your handheld is more sensitive/more selective/... and so you will need a _better_ antenna to get the same performance.
2. Your gain settings are too low (signals below inherent noise) _or_ too high (overload creates more noise). Find some other signal in the band (e.g. a repeater) and adjust for maximum SNR (vertical distance between the signal peak and the noise floor).
3. Your HackRF One is damaged. Damage can be caused by overly-strong nearby signals such as transmitting with your handheld, but IIRC the most likely failure is the 14dB amplifier and that can be bypassed, so that's not the problem here given that you have the amp off. But as a general principle, don't do that unless you have an attenuator or dummy load on one or the other device.

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