Re: Low loss receive filter for 23cm

Richard GD8EXI

   Your rounded noise peaks 15-20MHz wide in the 1.15 to 1.35 range wide are probably mixing products from a local DTV transmitter.

 I use a notch filter tuned to 650MHz between my antenna and VLNA to keep out local DTV signals which mix in the VLNA to give what sounds like white noise right across the band but looks like equally spaced humps about 15MHz wide on a spectrum analyser. The filter has a loss of ~0.1dB at 1296MHz and 40dB of rejection at 650MHz the centre of the three DTV multiplexes and ~20dB on the flanking multiplexes. It’s simply an open coaxial line on a N type T adaptor set up using a tracking generator and spectrum analyser.

The transients may be 4G mobile phone signals.

I tried making a similar notch filter for the local 4G phone signals tuned to around 850MHz from memory however it had too much loss at 1296MHz about 1dB. So I use a 5 pole interdigital filter after the VLNA to stop the amplified mobile phone signals overloading my TS790. The VLNA can cope with the mobile phone signals but the TS790 was overloaded after they were amplified by over 30dB in the VLNA.

Another source of big transient signals near 23cms is the air traffic control radar beacon system on 1030MHz from the ground and 1060MHz from the aircraft. I also have one of these 5 miles away but fortunately behind a hill so not an issue.


On 25/09/2016, 23:25, "Neil neil@... [ukmicrowaves]" <ukmicrowaves@...> wrote:


I have major problems with overload of my 23cm VLNA when my mast is
fully extended.  Not sure if it is digi TV or something else, but I get
around a dozen sharp peaks of noise from many directions, some more than
30dB over band noise.  I'm using a 44-ele Wimo. If I insert 3/6/10dB
attenuators before the VLNA, there is very little improvement. With 20dB
attenuation, the problem goes away almost completely, but so do most DX

Looking at the output of the VLNA on a spectrum analyser, I see 15-20MHz
wide rounded noise peaks between 1.15 and 1.35 GHz in some directions
with the mast up high.  I can also see some *very* big signals (-20dBm
according to the analyser) in the 1.2 GHz range which shift about a lot,
too fast to see on the analyser other than occasional spikes.

With the mast lower, the problem is much reduced.  I built a W6PQL-style
two-post bandpass filter in a diecast ally box, using 10mm copper tube
posts and brass tuning screws, with coupling loops on SMAs, but I am
unable to get the insertion loss less than 0.65dB.  Also the loss is
very sensitive to the contact between the case and lid.  Using a couple
of G-cramps to jam the lid shut helps a bit.  I suppose machining an
enclosure from aluminium bar and using a lot of screws on the lid would
help, but it all seems a bit sensitive and delicate and lots of screw
thread to metal connections to go off-tune. Would a PCB stripline filter
or some sort of machined coaxial filter do a similar job?  I can
probably manage with less stop-band rejection so long as the insertion
loss is tiny.

With the filter in circuit before the VLNA, the QRM problem is cured
*completely*, so the problem is definitely out of band overload.

Question: What is the lowest loss filter I can make/buy which will drop
600-800MHz TV signals and cellphone stuff by at least 20dB, but have
minimal effect on 23cm signals and stay nice and stable?  No point in
having a super VLNA if I'm losing 0.6dB or more in a filter.  Happy to
use HPF, band-reject, bandpass, whatever will give me enough rejection
but lowest possible insertion loss.  As the problem only happens for
terrestrial use, is there any point in trying to achieve better than say
1.2dB system NF at 23cm?  (VLNA 0.45dB-ish, 1.5m total coax, big Radiall
coax relay, 3 x N plug/sockets, 3 x SMA plug/sockets and filter ahead of
the VLNA).

I've read the splendid ITU noise doc several times...!!PDF-E.pdf

Neil G4DBN

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