Airband antenna (VHF)

[ILSAPP]

Hi all,

Last weekend I built two different airband antennas (tuned to 127MHz): a 3-element yagi and a 1/4 ground plane antennas and tested them separately.

They worked very well at low heights above roof (to a maximum of around 1.5m) but as I increased the height to 5m above the roof my scanner picked up some noise and some local FM stations (at least sounded like that, I can almost understand what was being said). The same phenomena occurred for both antennas with no noticeable difference between them in that aspect. I used (for both) the same coax cable: RG213.
I used a 6m PVC pipe (filled with wood for increased rigidity) and the antennas were attached via zip ties to that PVC mast.

Even so I still heard some distant comms but the thing is that the noise floor increased from no bar to around 3/4 of the maximum signal meter displays bars.
I also noticed that below 50MHz and above 250MHz the signal meter display indicated no bars, so no noise there.

Any idea of why this behavior and how to solve this noise/interference issue?


Thank you

 

[cmdrwill]

" I also noticed that below 50MHz and above 250MHz the signal meter display indicated no bars, so no noise there. "

This tells me that your antenna IS working in the VHF band where you wanted.

 

[iMONITOR]

Any idea of why this behavior and how to solve this noise/interference issue?

VHF-FM Broadcast 88 - 108 Filters
VHF-FM Broadcast Filters | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

 

[ka3jjz]

This has nothing to do with ham antennas...

Anyway, the reason you're getting more garbage from FM broadcast is fairly obvious. You've obviously got a number of FM stations that are slamming your front end pretty heavily with the better antenna. Mixing products can cause all sorts of strange stuff, and this is but one possibility. PAR's filters are a tad expensive but you get what you pay for. I'd say go for it. Mike

 

[ILSAPP]

Many thanks for all of you that replied.
KA3JJZ sorry, I'll close the thread to follow the rules, no bad intentions from myself.

 

[Ubbe]

It will work using lower cost filters. The difference is how much they attenuate your monitoring frequencies.
These filter are done with respect to do only air monitoring: https://www.ebay.com/itm/Airband-AI...-Excellent-rejection-SMA-M-SMA-F/282688071569
or general FM broadcast filter that do not attenuate air 120MHz as much as standard FM broadcast filters do: https://www.ebay.com/itm/FM-Notch-F...ection-75dB-Airband-applications/282829198921

/Ubbe

 

[iMONITOR]

BPF-AIR Air Band Band Pass Filter | Scanner Master

 

[ILSAPP]

Many thanks for the links Ubbe and iMonitor.
Shame that the airband BPF doesn't show any data regarding the attn/loss at various frequencies.