Re: Comment on my BFM spectrum please... #ettusb210 #uhd #sdrconsolev3

Jim Smith G0OFE

I'll have a look at this when I get time, but first the OP needs to have a listen on a frequency within a block, but no other signal, and in BFM mode, and let us know if he hears anything (a horrid-sounding hum) to (probably) rule out an emc issue or otherwise.

Jim, Bournemouth, Dorset. IO90BR.
Elad FDM-S2, Airspy HF+, SDRPlay and Funcube Dongle SDR Recievers
8-element OP-DES for Band 2, 8-element LFA for 2m, 3-element LFA for 6m, 20m Windom for HF,. Wellbrook 1530 Loop for LF

On 21/05/2019 08:32, Allan Isaacs wrote:

Most SDRs are wide open to signals across the whole radio spectrum.

If you have an SDR Play you can see an effect of this here in the UK by looking at the broadcast FM band.

The SDR Play allows adjustment of the Local Oscillator and with this set to 144MHz I can see very strong signals from 88.5 to 108 MHz perfectly duplicated in the range 376.5 to 396MHz. These are images and are much the same strength as the proper signals.

Jim in Bournemouth might like to check this and we can compare notes?


Your interference appears to have blocks around 4MHz wide so tune your receiver until you see these on their correct frequency.

For example the digital radio band here uses well defined blocks of about 1.5MHz in the 220MHz band.

By tuning around you’ll see several instances of these (digital) blocks and no doubt there’s a mast close by that’s transmitting your interference, maybe a mobile phone mast, TV or whatever. Sometimes a smaller mast is designed to look like a tree so may not be too obvious.

Your best option is to use a filter tuned to attenuate the interfering signals or, if you wish to mainly listen to the FM band, use a bandpass filter for that band or both filters.

Unless you attenuate your interfering signal it will mess up most of your reception, not just BFM, perhaps by raising the noise floor even if those blocks are not obvious.

Allan G3PIY


From: [] On Behalf Of erik@...
Sent: 21 May 2019 06:40
Subject: [SDR-Radio] Comment on my BFM spectrum please... #sdrconsolev3 #uhd #ettusb210



As you can see from the attached picture, I have 3 large 'humps' in the BFM range, with stations appearing out of the top of each (for the most part). You do not see any noise floor inbetween these three groups, like you do at the top end of the spectrum.

Why is this?

I can only guess; are these groups of stations each coming from a common wide-band amplifier?

Thanks in advance for your thoughts.


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