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
Jim in Bournemouth
might like to check this and we can compare notes?
interference appears to have blocks
around 4MHz wide so tune your receiver until you see
these on their correct
For example the
digital radio band here
uses well defined blocks of about 1.5MHz in the
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
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
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
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
Thanks in advance for your thoughts.