Does such a thing exist?

[jonwienke]

I use a stub filter tuned to block cellular in the 600/700MHz range. I used a nano VNA to tune it to minimize attenuation of UHF T-band and the upper 700/800MHz band. @majoco is correct that a stub filter will also block odd multiples of the base frequency, so blocking 125MHz will also block 375MHz and so on. It may be desirable to use a diplexer to separate VHF from UHF, and put the filter on the VHF side of the output to keep it from notching UHF airband as well.

 

[RDowson]

I have this coming tomorrow:

Do I just cut 50cm of coax and stick an F connector on one end?
I plan to connect to an SDR dongle and see if I can see the notch. Then just cut small amounts off the end of the coax until the notch is on the desired frequency.

Does this plan seem ok?

 

[RDowson]

I use a stub filter tuned to block cellular in the 600/700MHz range. I used a nano VNA to tune it to minimize attenuation of UHF T-band and the upper 700/800MHz band. @majoco is correct that a stub filter will also block odd multiples of the base frequency, so blocking 125MHz will also block 375MHz and so on. It may be desirable to use a diplexer to separate VHF from UHF, and put the filter on the VHF side of the output to keep it from notching UHF airband as well.

I don’t think UK military frequencies go beyond 372MHz so I assume it’ll be ok without a diplexer?

 

[jonwienke]

Just be aware the stub filter will block odd multiples of the base frequency. If none of the multiples are of any interest to you, no worries. But if any of them are of interest, you'll need to get creative.

 

[majoco]

I plan to connect to an SDR dongle and see if I can see the notch. Then just cut small amounts off the end of the coax until the notch is on the desired frequency.

Exactly - and if your offensive transmitter is on all the time, this should be a doddle as long as your cabling is all RG6. By 'small amounts' we're talking about 5mm at a time.Your SDR software should have a meter of the signal strength if you tune to the station and you can watch it reduce as you get nearer to the sweet spot. You may not get rid of all of it, but as long as it stops cross-modulating your receiver then it's job done. This may not sound very ethical, but I have found that a good way to get a nice clean cut through coax is to use an old wood chisel and cut through it, smacking it with a reasonably heavy hammer onto a block of wood. Sounds brutal - but it works. I've not tried it on RG6 which has a copper-plated steel inner conductor.

 

[Ubbe]

There shouldn't be any problem using any ordinary cutter. Check the signal while cutting and it should jump in signal level when you shortcircuit with the cutter and then go back to normal after the cut. Or the cutter isn't sharp enough and some part of the braid are just tugged and not cleanly cut and make contact with the inner lead.

/Ubbe

 

[RDowson]

So how exactly does the diplexer work? Do I need one to split the bands and then another to join them back to the one cable for connection to the scanner? Or is it something different I need on the scanner side?

 

[majoco]

I wouldn't worry about a diplexer at the moment. let's get rid of one problem without creating another.

 

[RDowson]

This morning I moved my antenna down into my bedroom. Doing this has almost completely removed the intermod. I can only assume this is because there are more internal/external walls that are stopping the strong signal from the nearby mast getting to the antenna. When I tune to the local ATIS, I now get full 5 bars of strength. When I had the antenna higher up and had to attenuate the signal to remove the intermod, the strength would drop to 2 or 3 bars. Why would there be a difference?

So now I have something working well. I’m not bothered about losing range as I’m mostly interested in traffic for the local airport and the surrounding area anyway.
Only downside is that the radials on the Diamond D777 kinda get in the way when I’m a bedroom!

Can anyone recommend an antenna that’s as good but doesn’t have radials so that it can live in the bedroom without me tripping over it?

 

[Ubbe]

So now I have something working well. I’m not bothered about losing range as I’m mostly interested in traffic for the local airport and the surrounding area anyway.

Then you can unscrew the radials. If you don't like having the antenna in the room, then move it back to its original place, without radials, and wound some aluminium foil around the antenna to reduce the signal level from that VOR. Take your scanner with you and connect to the antenna while testing how much alu foil are needed.

/Ubbe

 

[RDowson]

Then you can unscrew the radials. If you don't like having the antenna in the room, then move it back to its original place, without radials, and wound some aluminium foil around the antenna to reduce the signal level from that VOR. Take your scanner with you and connect to the antenna while testing how much alu foil are needed.

/Ubbe

Thanks. The tin foil along with pointing it at a funny angle seems to have done the trick. Just need to do some testing now to make sure it’s still picking up everything else ok.

 

[Ubbe]

Just need to do some testing now to make sure it’s still picking up everything else ok.

Without the radials the receive elevation angle will raise, hopefully above the VOR antenna, and still pick up aircrafts. The angle for VHF will probably raise higher than on the higher frequency mil-air band. Begin covering the top of the antenna with tin foil and work downwards to keep the bottom part intact.

/Ubbe