Issue with hardware frequency

[tre...@silverfieldstech.com]

I'm playing with my HackRF and am having trouble understanding something.  One of the posts here https://groups.google.com/g/gqrx/c/bVGQ6wDVghg helped.  But now I'm seeing something I don't understand.

If I have the hardware frequency set to an FM radio band, with the offset set to 0, then I can not hear that band.  I can hear other bands by setting the offset to that band.  But when I change the hardware frequency off of the FM band, and correct with the offset, then I can hear it.

For example, I set the hardware to 89.7 MHz, a local NPR station. Offset is 0.  I hear nothing.  If I set the offset to -400 kHz, then I hear the 89.3 station, as I'd expect.  If I set the hardware to 89.3 MHz, offset 0, then I can no longer hear 89.3 MHz.  But if I set the offset to +400 kHz, I can then her 89.7 MHz.

What I do not understand is why can I hear the band the hardware is set to with no offset?  I'm sure I'm just not understanding something.

Any help appreciated.

[Jeff Long]

This could be due to a spike at the center (tuned) frequency, caused by DC offset in the receiver. Offset tuning is commonly used to avoid this spike. Turning on DC Remove may help. Also, hit the Squelch "R" button to make sure it's not just squelching.

--
You received this message because you are subscribed to the Google Groups "Gqrx SDR" group.
To unsubscribe from this group and stop receiving emails from it, send an email to gqrx+uns...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/gqrx/89181be2-915b-4ddf-a4ce-83ec725faa08n%40googlegroups.com.

[Jeff Long]

Verified with a HackRF that there is a large DC spike. With the RF gain off, a local station could not be received with 0 offset unless DC Remove is enabled.

[tre...@silverfieldstech.com]

Thanks; when I set the DC off It now does what I'd expect.  Thank you!

Now I'll research what the DC bias on the antenna is for.

[Michael Edwards]

I think the hardware frequency is just the offset from the center of the bandwidth that you are set on. I just ignore it and click the frequency I want to listen too, and it works fine. I think the hardware frequency could be hidden and it would make life easier. I attached my bookmarks and default config. I am listening to many repeaters in the VHF/UHF band.

[Roger David Powers]

>  Now I'll research what the DC bias on the antenna is for.

Hmm, not sure what this is unless it is a way for the radio to apply voltage to the antenna connection so it can be used by an amplifier (LNA).   See description of "Software Selectable Bias Tee" in https://www.rtl-sdr.com/rtl-sdr-blog-v-3-dongles-user-guide/ for one example.

The caution is that one needs to make sure the antenna does not have a "DC short circuit" i.e. an inductor/coil across the terminals if one is using the bias tee with nothing to protect it i.e you disconnect the LNA and directly attach such an antenna.  Such coils are found in various antennas with transformers such as end fed half wave.  

Bottom line is you should only turn it on if you have an external amplifier that expects power from the radio. 

Regards,

RDP