Bleedover Issue

[RDowson]

Hi,

I have just mounted my Diamond D-777 on the roof and I’m getting excellent civ and mil airband reception.
I live very close to a mast which broadcasts Scottish Volmet on 125.725mhz and this bleeding over into the other VHF frequencies.
What are my options to fix this?
Thanks!

 

[WA0CBW]

Well... Sounds like strong signal overload. So reduce the interfering signal; filters, attenuator, better receiver.

 

[RDowson]

Well... Sounds like strong signal overload. So reduce the interfering signal; filters, attenuator, better receiver.

Cheapest option for me is to reduce the attenuation but that would be across the board. Will this result in some loss of range?

 

[Whiskey3JMC]

Or simply don't use the rooftop antenna when monitoring close to the mentioned frequency range

 

[Ubbe]

What are my options to fix this?

I would try and mechanicly shield off the angle to that volmet transmitter. Or use a better receiver. Any other solution would make you loose to much signal from the stuff you want to receive.

/Ubbe

 

[iMONITOR]

Contact Bob and have it make you a custom filter.

PAR Electronics
Bob Par

Scanner Filter FAQ | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

PAR Electronics Inc. Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave

www.parelectronics.com

 

[KevinC]

Contact Bob and have it make you a custom filter.

PAR Electronics
Bob Par

Scanner Filter FAQ | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

PAR Electronics Inc. Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave

www.parelectronics.com

I'd contact Dale Parfitt. 😁

 

[WB9YBM]

Well... Sounds like strong signal overload. So reduce the interfering signal; filters, attenuator, better receiver.

That'd work assuming there are no other signals in that particular band the op wants to listen to (at least as far as filter & attenuators go because of their bandwidths). If the op wants to still hear other signals in that band, maybe relocating his antenna to the other side of the property (get some distance separation--not sure if that'll be enough, though).

 

[iMONITOR]

I'd contact Dale Parfitt. 😁

Brain fart! I got Bob from ProScan confused with Dale from PAR.

 

[RDowson]

So if I was to upgrade my scanner is there one that would be best? Does it need to have a notch filter too?
Am I cheaper looking as SDR where I can apply a software notch filter?

 

[majoco]

If you want to get a custom filter to reject or reduce the offending signal, you may want to list the frequencies that you do listen to. That way, Dale can optimise both functions, "accept" and "reject". It's not going to be cheap, a standard three-pole filter "ain't gonna cut the mustard", two in series might just do it. Do you know someone with a proper receiver or a spectrum analyser to get some actual signal level figures. You don't necessarily want to get rid of the signal, just reduce it to a level that your receiver can handle, but what you spend on filters may be a good part of buying a better receiver, although 'in band' signals are hard to remove.
As Ubbe says, you may want to try moving the antenna, to the other side of your roof for example, so that it doesn't see the transmitter.

Looking on the bright side, you will always know the temperature and atmospheric pressure!

I'd lend you my field intensity meter but it's a bit heavy to put in the post........

 

[RDowson]

PAR are’t able to make make a notch filter without it interfering with the other nearby channels that I listen to.
where do I go from here? I’m willing to upgrade my handheld Uniden to something better suited for home use but don’t want to spend lots of money for it not to work.
I assume the Icom 7300 would work as it has a notch function? Is there anything cheaper that would also work?

 

[Ubbe]

I assume the Icom 7300 would work as it has a notch function?

All notch filters are probably too wide. What Uniden model do you have? A SDS scanner will be easily overloaded but you probably need it to monitor simulcast systems. You could get a second $100 scanner just for airband monitoring.

Also consider that it is the total amount of RF signals that causes overload. If you haven't tested using a FM trap filter then it might be a FM broadcast transmitters that are the last drop that overfills the cup and that would be the first thing to try and it doesn't cost much either.

When I mentioned using a mechanical shield I where thinking of using 1ft standoffs at the bottom and top of your antenna and then one inch wide metal or perhaps electrical wire going between the top and bottom maybe 5-6 turns at the end of the standoffs to produce a protection from RF at a very narrow angle. That angle needs to be set precisly at that volmet transmitter. Probably needs you to listen to one of the bleedover frequencies while adjusting the angle.

/Ubbe

 

[RDowson]

Also consider that it is the total amount of RF signals that causes overload. If you haven't tested using a FM trap filter then it might be a FM broadcast transmitters that are the last drop that overfills the cup and that would be the first thing to try and it doesn't cost much either.

I didn’t realise this. I’ve ordered an FM filter to try and I’ve also ordered an SDR dongle to see if I can get any better luck with SDRuno and it’s filtering features.

 

[chief21]

I assume the Icom 7300 would work as it has a notch function? Is there anything cheaper that would also work?

The Icom 7300 is an HF/6 meter radio. It doesn't do VHF or UHF. You need a scanner or a communications receiver to monitor the VHF and UHF aviation bands.

 

[Ubbe]

...I’ve also ordered an SDR dongle to see if I can get any better luck with SDRuno and it’s filtering features.

Filters in software are only for the demodulator and audio and not for the RF overload that happens in the front end of a receiver. The SDRPLay series of dongles has hardware filters in the front end and are also less prone to overload than RTL-SDR but are also more expensive. I have a msi.sdr that are a clone of SDRPlay but have some issues with it's different filters that reduce sensitivity considerable in some bands.

SDR dongles are always (as far as I know) worse than any scanners receiver in performance of sensitivity and overload handling.
But they are also a good tool to see where the strong signals are and to quickly see if any transmission exists that you might not be aware of. But see to that the gain are set low enough so that it doesn't overload and show signals at frequencies where there are not any.

/Ubbe

 

[RDowson]

I have fitted an adjustable attenuator (link) and dropping the signal by about 10dB has eliminated the interface on the other channels that I have tested so far. It’s quiet at the airport tonight so tomorrow will be a better test.
Downside is I’ve obviously lost some gain so I don’t seem to pick up the ground vehicles/ops/fire crew on the 455mhz frequencies but it’s not the end of the world.
As long as I can hear the airport tower and approach frequencies along with the military UHF frequencies I’ll be happy.
Got my FM filter coming tomorrow to see if that makes any difference too. Also got an SDR dongle so I can have a play with that so I can see visually what’s going on.

 

[RDowson]

That'd work assuming there are no other signals in that particular band the op wants to listen to (at least as far as filter & attenuators go because of their bandwidths). If the op wants to still hear other signals in that band, maybe relocating his antenna to the other side of the property (get some distance separation--not sure if that'll be enough, though).

The antenna is already on the side of the property that is furthest away from the tower but as its high up it has direct line of sight.
Would lowering the antenna down below the roofline, therefore blocking line of sight, be a possible solution?

 

[WB9YBM]

Would lowering the antenna down below the roofline, therefore blocking line of sight, be a possible solution?

That would attenuate all signals (including the weak ones). Might be worth an experiment, though; maybe there's a certain "sweet spot" somewhere in the middle...

 

[Ubbe]

You could try a 3.3pF or 4.7pF capacitor in series with the coax. You might be able to open up the attenuator and solder the capacitor between the input and output connectors. It would reduce the attenuation to only 2-3dB at 450MHz while still being 10dB at 125MHz. If you have a spare CATV $5 splitter you could modify the inside of that to make use of connectors and the shielded box and only have that capacitor in it and use that in series with the signal without the attenuator.

/Ubbe