Filters for MilAir

[G7RUX]

Morning All,

Having recently returned to the fun that is scanning in general and aircraft stuff in particular, I was wondering if anyone had any sensible recommendations regarding bandpass filters for MilAir reception. I have some bits and bobs for VHF airband filtering and they improve reception quite a bit in my area so was looking for similar approaches for MilAir. However, I haven't had much luck in finding anything sensible for 225-400 MHz...any ideas?

Regards
Piggly

 

[vagrant]

Scanner Bandpass Filters | 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

Around $80 last time I checked and handles 225-400.

 

[eorange]

I have the tiny inline mil air bandpass filter from AOR and I don't think it does squat. I get better overall results with my Mini Circuits FM band stop filter when listening to mil air.

 

[737mech]

I have the PAR 225-400 bandpass and it works GREAT!!

 

[iMONITOR]

I have the PAR 225-400 bandpass and it works GREAT!!

But that blocks the VHF air band correct?

 

[mancow]

Yup

 

[kb5udf]

I have 2 of these. Haven’t swept them, but they appear to work well. Great price On eBay.
Coaxial Dynamics Low Pass RF Filter SMA Female 2435-50Z

 

[737mech]

But that blocks the VHF air band correct?

Yes it does, solution…Use another scanner.

 

[eorange]

I have the PAR 225-400 bandpass and it works GREAT!!

Honest question... what kind of before and after differences did you notice with that bandpass filter?

 

[737mech]

Honest question... what kind of before and after differences did you notice with that bandpass filter?

Before not as much clear comms in 225-400. Las Vegas is a rf noisy place. I thought there was something wrong with my scanner. I’m 14 miles line of sight as the crow flies from Nellis AFB. After the bandpass I’m getting great results.

 

[Edelweiss]

Before prescribing a filter have you diagnosed the actual problem? DAB? TETRA? Intermod etc.?

 

[G7RUX]

Oh yes, I’m well aware of what is causing issues at my main Rx site; the receiver location is shared with a number of transmission systems and would benefit from reducing those out-of-band signals to cut down on IMD in the receiver front end. The VHF air and filter does an excellent job but I wanted to add MilAir to it too, with an arrangement of splitter, filters and combiner before a nice little LNA…I might be able to put together a good enough duplexer to replace the splitter and combiner too for a bit more stop-band goodness.

 

[Edelweiss]

Often it is only one nasty signal that causes the nuisance. You may even be able to cancel that with a coax stub. I am living next to a VHF and DAB+TETRA tower. VHF Broadcast Stop filter helped me on VHF airband. On 70 cm on one Rx is frequently hammered, still have not found the offender (which is NOT nearby in the spectrum) Digital transmissions...

 

[Bmacs]

I have the Stridsberg 225 HighPass - It kills the FM and VHF transmitters around me, but it passes both 225-400 and everything above (if you need it)

Receive Filters

Excellent performance!

Bill

 

[G7RUX]

Often it is only one nasty signal that causes the nuisance. You may even be able to cancel that with a coax stub. I am living next to a VHF and DAB+TETRA tower. VHF Broadcast Stop filter helped me on VHF airband. On 70 cm on one Rx is frequently hammered, still have not found the offender (which is NOT nearby in the spectrum) Digital transmissions...

My issue is several VHF and UHF transmitters on the same rooftop as my V/UHF air and net a so the receiver does not have a nice time. Any one of them impacts the receiver but with all of them on it’s quite unpleasant.

 

[prcguy]

I have 2 of these. Haven’t swept them, but they appear to work well. Great price On eBay.
Coaxial Dynamics Low Pass RF Filter SMA Female 2435-50Z

I have several of these filters with N connectors and the ones that say low pass cut off fairly well above 400MHz but they will pass down to the VHF air band. There is a version that says 225-400MHz band pass and that cuts of nicely just above and below that range.

 

[vagrant]

Nothing to see here. I found what I was looking for.

 

[iMONITOR]

I know the frequencies are much lower but do strong "AM" broadcast stations ever cause interference problems up in the VHF/UHF bands

 

[G7RUX]

I have several of these filters with N connectors and the ones that say low pass cut off fairly well above 400MHz but they will pass down to the VHF air band. There is a version that says 225-400MHz band pass and that cuts of nicely just above and below that range.

These look pretty good (I have one on the way) but the main issue at this location is with transmitters in 138-225 and with a system that operates just above 400. The low pass should help quite a bit but the "inbetween" part of VHF is going to be a pain so I'm still looking as well as making up some discrete component filters to see what I can do with those.

I could go down the route of making stubs to notch out the troublesome Tx frequencies but I would prefer to use a bandpass arrangement if I can so the kit can be used elsewhere too.

Here in the UK it appears that AntennaPro (related to Amphenol) can supply a pair of suitable bandpass filters so I shall see what sort of price they want for those before deciding which way to go.

 

[dlwtrunked]

I have 2 of these. Haven’t swept them, but they appear to work well. Great price On eBay.
Coaxial Dynamics Low Pass RF Filter SMA Female 2435-50Z

Note that is a low pass filter and may not help to eliminate nearby strong VHF and FM broadcast band problems.