Does such a thing exist?

[RDowson]

Hi,

I’m looking for an attenuator (preferably a variable one) that attenuates 108 to 136 MHz but still allows the rest of the signals (mainly military UHF and 455MHz airport Ops) to pass through into the scanner unaffected.

Does such a thing exist?

 

[Hit_Factor]

Yes, they exist, not sure about variable. Band-pass filter

 

[Reconrider]

I think you're looking for a frequency filter. Attenuators help lower the strength of the signals so the scanner doesn't overload/decode better.

 

[RDowson]

I think you're looking for a frequency filter. Attenuators help lower the strength of the signals so the scanner doesn't overload/decode better.

That’s the problem I have. I’m currently using two scanners and 2 antennas. One has an attenuator to weaken the signal on VHF to stop overload. The other pulls in full strength UHF as that’s not impacted by the overloading signal.
I want to try and do everything in one scanner if I can.
the scanner can attenuate individual channels but -20db is too much so just kills the signal.

 

[RDowson]

Yes, they exist, not sure about variable. Band-pass filter

Won’t a band pass kill off the section of the signal completely? I still need it coming into the scanner, just attenuated a bit, with the rest untouched.

 

[RDowson]

Plan B option. Would this work?

The built-in attenuate function of the scanner kills the signal too much. Could I use that feature in combination with a variable amplifier to increase the gain on the entire input to the point where I get VHF with no overload?

Maybe one of these?

Variable Signal Booster - Grey

The Variable Gain Signal Booster has been designed specifically for the mobile vehicle to maximise signal strength from any digital TV or FM/DAB radio aerial.

maxview.co.uk

 

[Hit_Factor]

Won’t a band pass kill off the section of the signal completely? I still need it coming into the scanner, just attenuated a bit, with the rest untouched.

It does the opposite, only allows the band (of interest) through, attenuates the rest. You can run an attenuator inline with a band bass filter if you need to.

Are you near powerful transmitters?

 

[RDowson]

Are you near powerful transmitters?

Yes

 

[neilromesburg]

Yes they exist but very expensive.

 

[jonwienke]

You don't want an attenuator, you want a bandstop/notch reject filter tuned to the strong signal(s) causing the problem. An attenuator will reduce all signals passing through it, not just the problem signal(s).

 

[RDowson]

You don't want an attenuator, you want a bandstop/notch reject filter tuned to the strong signal(s) causing the problem. An attenuator will reduce all signals passing through it, not just the problem signal(s).

Can’t be notched for a number of reasons. The interference is across the band and also a notch filter isn’t precise enough to not kill off the channels next to the interference that I would want to listen to.
The attenuator works fine but means I need two scanners to avoid attenuating UHF which doesn’t get any interference.

 

[jonwienke]

What is the source of the interference? If it's across an entire band that's a serious issue, not just for you, but for everyone using VHF freqs in your area.

 

[RDowson]

What is the source of the interference? If it's across an entire band that's a serious issue, not just for you, but for everyone using VHF freqs in your area.

Scottish VOLMET broadcast on what appears to be multiple frequencies from the mast a few fields away. It broadcasts to all of Scotland.

 

[bob550]

Scottish VOLMET broadcast on what appears to be multiple frequencies from the mast a few fields away. It broadcasts to all of Scotland.

I see a VOLMET broadcast from Prestwick Airport on 125.720 MHz. Is this the source?

 

[RDowson]

I see a VOLMET broadcast from Prestwick Airport on 125.720 MHz. Is this the source?

Yep, its broadcast from Dundonald Hill which I’m looking at from my window.
When searching the band I can pick up the same broadcast on multiple frequencies though.

 

[bob550]

You're probably picking up multiple false images of the original frequency due to the high signal strength.

 

[N8YX]

Are you using one scanner for multiple service monitoring?

What about antennas? - single-band, multi-band, broadband (discone)?

I'd take a look at Comet or Diamond duplexers/triplexers in the frequency ranges you wish to monitor. Pick one which passes 136MHz and above, or (as I did) use a combination of various models to feed banks of VHF low-band, VHF high-band, UHF and 800MHz-up receivers. A couple of feed paths off the distribution manifold are wide-open (covering all frequencies) so I can drive my wider-coverage receivers with them.

 

[RDowson]

Are you using one scanner for multiple service monitoring?

What about antennas? - single-band, multi-band, broadband (discone)?

I'd take a look at Comet or Diamond duplexers/triplexers in the frequency ranges you wish to monitor. Pick one which passes 136MHz and above, or (as I did) use a combination of various models to feed banks of VHF low-band, VHF high-band, UHF and 800MHz-up receivers. A couple of feed paths off the distribution manifold are wide-open (covering all frequencies) so I can drive my wider-coverage receivers with them.

I’m monitoring civilian and military airband. Also, the 455MHz frequency for ground Ops at the local airport.

I have two Diamond D777 antennas and two Uniden scanners.

I’ve split the saved channels into VHF and UHF. One scanner does VHF and the other does UHF.
The one doing VHF has a veritable attenuator to kill off some of the signal to stop the overload.
Using one scanner with the attenuator results in the UHF frequencies being weakened unnecessarily and stops me picking up some of the 2/3 bar signal, such as the airport ground Ops.

The current two scanner/two antenna setup works fine, I’d just like to have it all on the one scanner if possible.

 

[N8YX]

Does the interference happen on only ONE frequency? Meaning: There's only one interfering station which operates on a fixed frequency.

 

[bob550]

I remember building a frequency-specific attenuator many years ago. As I recall, it consisted of a coax stub of a length cut to the particular frequency together with an inline potentiometer. I can't be more specific as this was over 30 years ago and the stub is long gone. I'd guess I probably saw the design in Monitoring Times in those days. Search for coax stub filter and you may find something of interest.