Re: Low loss receive filter for 23cm

John Quarmby

Hi Neil

One of my locals, John G4ZTR has had the same problem. You can find a good account of what he did to fix it here:

I use a preamp design by YU1AW which includes a tuned stripline front end which doesn't suffer from DTV overload at this location.

Sam's VLNA's are very good but you really do need a filter in front for terrestrial use if you are anywhere near a DTV transmitter or a mobile phone base station.


John G3XDY

On 25/09/2016 22:25, Neil [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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