VHF AM/Airband Transceiver Isolation

[BlueDevil]

I am looking to resolve an issue when transmitting on a VHF FM radio causing back feeding and breaking the squelch on a VHF AM radio. I would like to know if anyone else has dealt with a similar problem and what their solution(s) they found. Do I need a BandPass Filter/Isolator on the VHF AM radio antenna system? If so what is a good option that will work with a transceiver? Here is a description of what I am working with and the situation I am in.

I am working on a relatively new Airport Rescue and FireFighting (ARFF) Fire Truck. It has both VHF FM/High Band and VHF AM/Airband radios in the cab. Both radios are Icom Mobiles (F1721 and A120). They are mounted next to each other in the console of the cab. Each radio has its own antenna and they terminate on the roof and are currently running separate 1/4 wave NMO Antennas. The manufacturer, in their infinite wisdom, place the NMO mounts only a couple feet apart from each other. Any time the VHF FM transceiver transmits the squelch is broken on the VHF AM radio causing a lot of static in the cab. I am assuming the VHF FM radio is over powering the receiver of the VHF AM radio causing it to open. It doesn't matter which channel/frequencies the VHF FM radio is using, they all cause the VHF AM radio to squelch. I have even tried lowering the transmit power to the lowest setting and the VHF AM radio still squelches. The squelch setting on the Icom A120 VHF AM radio is currently maxed out.

I still need to check a couple things before I start making purchases or adjustments. I need to check the SWR of the VHF FM radio to make sure all the RF is truly make it up and out the Antenna. I also need to check the grounding of the antenna and NMO Mount. I would also like to try using a slightly higher gain VHF FM Antenna to see if I can get the RF energy up and out. I imagine that the current VHF FM 1/4 wave antenna is completely saturating the VHF AM Antenna which is the cause or at least contributing to the problem.

Any other suggestions, ideas, or recommendations?

 

[KevinC]

Is the FM radio set for full power? You probably don't need 50w, try cutting it back to 25w and see what happens.

 

[mmckenna]

I agree. Lower the power. You shouldn't need 50 watts for it to work.

More antenna separation would be my next step. The farther you can get the antennas apart, the better.

Filtering would work, but can be costly to get decent ones. And, it shouldn't be necessary if you do the above two steps.

 

[mmckenna]

I still need to check a couple things before I start making purchases or adjustments. I need to check the SWR of the VHF FM radio to make sure all the RF is truly make it up and out the Antenna. I also need to check the grounding of the antenna and NMO Mount.

Both good things to try, but likely are not causing your issue.

I would also like to try using a slightly higher gain VHF FM Antenna to see if I can get the RF energy up and out. I imagine that the current VHF FM 1/4 wave antenna is completely saturating the VHF AM Antenna which is the cause or at least contributing to the problem.

I'd be surprised if that resolves this. Issue is that too much RF from the transmitting radio is getting into the other radio. It's overloading the other receiver. Changing antenna types isn't going to make enough of a difference, if any at all.

 

[mrsvensven]

What you need is antenna separation. Vertical separation is best, if you can get one antenna at a different height it would be ideal.

Higher gain antennas concentrate the signal on the horizon, not "up and out". They will make the problem worse if anything.

 

[BlueDevil]

Thanks for all the quick replies. I have tried using all three stock Icom Power Levels (Low1, Low2, High) on the F1721 with no success. The FM radio doesn't need to operate at full power. I will probably cut the power back regardless. If I would have been involved with specs of the truck I would have made sure they put the antenna mounts much further apart. Although it seems like I never can get the manufacture to understand why antenna separation is necessary. I will even take pictures and draw diagrams for them to reference. It never fails the apparatus always shows up with minimal antenna separation.

I will look into seeing what it would take to move the antenna location. Since it is such a large truck it might be very easy or really difficult.

My thought with using a higher gain antenna was to narrow the beam width of the radiation pattern in an effort to get it up over the AM Antenna. In conjunction with low transmit power I was curious if that would take care of the problem. The VHF FM 1/4 wave antenna has such a wide beam width that it has to be completely saturating the VHF AM Antenna with RF. I will admit that swapping antennas sounded like a much easier and quicker solution which is appealing.

I will probably still check the SWR on the FM radio to make sure there isn't a ton of reflected RF coming back and into the radio which is mounted next to the AM radio in the console.

 

[mmckenna]

My thought with using a higher gain antenna was to narrow the beam width of the radiation pattern in an effort to get it up over the AM Antenna. In conjunction with low transmit power I was curious if that would take care of the problem. The VHF FM 1/4 wave antenna has such a wide beam width that it has to be completely saturating the VHF AM Antenna with RF. I will admit that swapping antennas sounded like a much easier and quicker solution which is appealing.

When the antennas are that close together, there isn't a significant difference in RF levels. You could try putting a watt meter with a dummy load at the radio end and transmit with the other, but I doubt you'd see any difference that would show up on any non-lab grade equipment.

Opposite corners of the cab might be enough, especially if you lower the transmit power on the FM radio. Also, make sure it's a decent coaxial cable. If the installer used cheap mounts with cheap coax, and bundled them together, I'd be concerned.

 

[KK6ZTE]

Dropping the FM radio power will make the biggest difference. Even if you do it on a channel-by-channel basis. The manufacturer probably expected a 7/800 MHz radio paired to the AM which wouldn't need the separation.

Antenna positioning is a lost art. Our local "upfit" car stereo shop in San Luis Obispo can't figure it out and they build cars for everyone. For instance, VHF whips at the back corners of the cabs...

EDIT: I noticed you said you tried Low1 with no difference. What's the A120 squelch setting? Have you verified with a Watt meter the output of the Icom VHF?

 

[prcguy]

Are the radios confined to one frequency or do they need to cover all the VHF air band and VHF commercial band? You can get band pass filters that will allow at least several MHz to pass and one on each radio will fix your problem. I've run high power 2m radios with antennas next to VHF marine radios and UHF amateur next to UHF commercial with these filters with no interference problems. I have one filter for the entire 2m amateur band, one that covers the entire VHF marine band, one that covers 440-450MHz amateur and another that covers 460-470Mhz.
The filters can be a bit pricey new but they work.

Here is one company I have used and they will make a filter for just about any frequency range. DCI Digital Communications

 

[cmdrwill]

And lets hope your radio does not suffer a blown receiver frontend due to the WAY TOO CLOSE antennas. Expensive repair.

DCI Digital Communications

The DCI filters are a very good band pass but expensive. And you may have to have a DCI on each radio.

 

[BlueDevil]

EDIT: I noticed you said you tried Low1 with no difference. What's the A120 squelch setting? Have you verified with a Watt meter the output of the Icom VHF?

I haven't gone as far as putting a watt meter on the Icom F1721. I believe the three transmit power levels for the F1721 are' Low1=5watts, Low2=25watts, High=50watts. I have tried all three power levels with no success. Maybe slightly less static on Low1 but not significant enough to be a solution. The squelch on the A120 is set at 24 or whatever value is max.

Here is one company I have used and they will make a filter for just about any frequency range. DCI Digital Communications

I may have to look into this as a potential solution. I believe we have two Command Vehicles that are experiencing the same problem. The F1721 needs to operate on a frequency range from about 150MHz-160MHz. The A120 really only needs to operate on two frequencies (118.500, 121.600). Thankfully the Command Vehicles only use the VHF AM radios a couple times a year. The ARFF Truck uses it all the time. We did setup a VHF FM to VHF AM Cross-Band Repeater at the Airport so we can communicate with the Tower using our standard VHF FM radios.

And lets hope your radio does not suffer a blown receiver frontend due to the WAY TOO CLOSE antennas. Expensive repair.

The feedback static is so strong that the guys operating the ARFF Truck have stopped using the F1721 Mobile and switched to using a Portable radio in the cab. However this has its own problems and issues, especially if trying to communicate during a true aircraft emergency. This has probably saved the A120 from being destroyed.

 

[prcguy]

A better solution would be to find a low pass filter around 137MHz for the aircraft radio and a 150MHz high pass for the other radio. Then they would be able to operate anywhere within their usable range and not be affected by each other. I would look for filters that would give a good 40dB of isolation at the cross over frequency around 143.5MHz and it can be a combination of the two filters, as in each gives about 20dB isolation there but both combined would achieve 40dB.

 

[cmdrwill]

low pass filter around 137MHz for the aircraft radio and a 150MHz high pass for the other radio.

I like that idea. And STILL get the two antennas away from each other.

 

[Mike_G_D]

I like the LP/HP filter combo too but here's another thought:

If the FM radio has external access to it's tx/rx status and the AM air radio has external access to audio or receiver mute then, in theory, you could insert a muting trigger circuit that will mute the AM air radio's receiver audio as soon as the FM radio PTT is depressed for transmit. Just a thought.

 

[mm]

You need what is called transmit interlock which is when one radio transmits an attenuator is switched in line with the other radio by a relay arrangement thereby lowering the tendency to overload and vice versa causing squelch breaks.

The interference is caused by phase noise from the offending transmit radios synthesizer and power amp stages.

We use this in all of my company's aircraft and I recently worked on an install in the REDMOND OREGON'S forest services FireWatch Cobra helicopters which has several VHF AM AND VHF FM/P25 TAC RADIO antennas just on the tailboom alone, some of the cobra antennas are within 19 inches of each other.

On the receiver side small 1 watt minicircuits pads are suitable as weight is an issue, but with our new Garmin AM COMMS the TX INTERLOCK attenuators are built into the receivers and they are installer selectable during aircraft bless out.

The typical attenuator value in the Cobra install is 20db for antenna spacings of 19 to 32 inches depending on the antenna use but all problematic transceivers and receivers (NAV BAND) are between 108 to 174 MHZ with up to 9 possible receivers needing protection in some aircraft.

We use a similar setup in the cessna aircraft for eliminating phase noise caused squelch breaks in the standard Cessna 206 factory installed dual top mounted ~32 inch spaced AM victor com antennas with internal radio receiver attenuation settings of 10/20 or 30 db as requiredby the installer.

 

[mmckenna]

Or, just move the antennas farther apart.

It's a fire engine, not like real estate is an issue.

 

[petnrdx]

You might try simple coax stub filters.
A low pass on the air band and an high pass on the high band.
Cheap and relatively simple.
If the problem mostly relates to the airband, then a coax stub filter that rolls off at say 140 mhz should do it.

 

[KK6ZTE]

And lets hope your radio does not suffer a blown receiver frontend due to the WAY TOO CLOSE antennas. Expensive repair.

How often does that happen? If that were an issue from a few feet apart, then every fire department command vehicle in the country would need receiver repair on a regular basis.

 

[petnrdx]

This is just a VERY quick example.
I used a type N tee (thats what happend to be on the bench), and a shorted stub totaling 13 inches from the feedline.
appx .3 dB insertion loss and 1.4 to 1 SWR at 120 mhz.
Gives 20 dB insertion loss above 150 mhz.
39 dB at 165 mhz.
So with careful assembly one should be able to make a LPF for the air band radio for the cost of a couple connectors, a tee
and a foot or so of coax.
Measured on my Anritsu S412E with phase stable cables.
Your mileage may vary.
And would be different with BNC or UHF connectors, etc.

 

[petnrdx]

You might also just want to remove the air band antenna and make sure the problem goes away.
That would tell you its coming in the coax as opposed to just coupling into the radios audio circuits or power leads.
My bet is the wide band noise from FM just opening the noise squelch on the AM radio.