VHF antenna noise

N

nhraracer90

Filing Flight Plan
Joined
Apr 30, 2020
Messages
26
Display Name
Display name:
nhraracer90
Good evening! I recently had my new radio and ADSB installed (wired), I did the hardware mounting and etc. everything works great with the intercom and such (Becker AR6201). I had to buy a new VHF antenna (Comat CI-122). I mounted that antenna myself and made my coax lines (RG400). When I hook up my coax to the antenna , all I hear is static noise. I can’t even hear the airport WX ASOS over the static. I’m only a few miles from the airport. Am I missing something obvious? The aircraft is a helicopter. The ship is currently going through a restoration so I can’t fly it to the avionics shop (we trailered) it to and from. I attached a photo of how I mounted the antenna. Any input would be great. Thank you!

cb1e9ca96fa971a3dc2a088495acebb9.jpg



Sent from my iPhone using Tapatalk
 
Two things that would stand out to me, the antenna is vertically polarized, but mounted horizontally. And lack of ground plane for the antenna. Was there not an antenna before?
 
I hope the cable hanging down in the picture was connected when you had the static. If it was hanging, all you should get is static.

Does the antenna have a solid ground to its mounting plate? Overall, the antenna looks good.

I agree on the polarization being wrong, but if there is a strong signal, that is not a problem. The range will suffer, though.
 
Last edited:
Ryan Klems said:
Two things that would stand out to me, the antenna is vertically polarized, but mounted horizontally. And lack of ground plane for the antenna. Was there not an antenna before?
Click to expand...

The antenna before was mounted back on the tail and was poorly done with a cheap antenna. How much would that affect mounting is horizontally based vertically? I did that because it sits low to the ground vertically. Is the piece of metal I attached it to not big enough? The metal that the antenna is attached to clear from paint in the mating areas.


Sent from my iPhone using Tapatalk
 
geezer said:
I hope the cable hanging down in the picture was connected when you had the static. If it was hanging, all you should get is static.

Does the antenna have a solid ground to its mounting plate? Overall, the antenna looks good.
Click to expand...

Yes the cable was connected when I have the static lol. Yes the mating surface on the antenna to that little mounting plate is clean.


Sent from my iPhone using Tapatalk
 
nhraracer90 said:
The antenna before was mounted back on the tail and was poorly done with a cheap antenna. How much would that affect mounting is horizontally based vertically? I did that because it sits low to the ground vertically. Is the piece of metal I attached it to not big enough? The metal that the antenna is attached to clear from paint in the mating areas.


Sent from my iPhone using Tapatalk
Click to expand...

I am not an antenna guru, I'm sure there are plenty of HAM and radio experts on here that can answer that better, but I generally understand them to want a decent amount of ground plane. AC43.13-2B says minimum of 1/4 wavelength of the frequency which for 125MHz (~the middle of 118-133) would be 24"x24". And you will still get reception with improper polarization, it's just not as effective and as noted above you'll lose range (unless it's a fixed transmitter and fixed receiver, you'll never be in position for always receiving in the ideal manner anyway).
 
Ryan Klems said:
I am not an antenna guru, I'm sure there are plenty of HAM and radio experts on here that can answer that better, but I generally understand them to want a decent amount of ground plane. AC43.13-2B says minimum of 1/4 wavelength of the frequency which for 125MHz (~the middle of 118-133) would be 24"x24". And you will still get reception with improper polarization, it's just not as effective and as noted above you'll lose range (unless it's a fixed transmitter and fixed receiver, you'll never be in position for always receiving in the ideal manner anyway).
Click to expand...

Okay. So if I mounted it directly to the aircraft skin, that would give a proper size ground plane?


Sent from my iPhone using Tapatalk
 
nhraracer90 said:
Okay. So if I mounted it directly to the aircraft skin, that would give a proper size ground plane?


Sent from my iPhone using Tapatalk
Click to expand...
The antenna should be mounted so the element is vertical.

Static may be coming from something in your shop, LED lights, switching type power supplies, etc. Turn off the shop breakers one by one to identify the source.
 
The ground plane issue is not your main problem. Even though your antenna is not mounted on the fuselage shell, you should be able to pick up your airport ASOS instead of static, even though your antenna mount is horizontally polarized. Check for opens and shorts in your coax and check your antenna as well. I cannot tell from the photo, but after you find your problem, make sure your antenna base is grounded to the fuselage or frame as close to the antenna base as possible. A long ground wire connection will in effect detune your antenna. Lots of newer com antennas are bent these days...sort of half vertical and half horizontal. Vertical polarization is preferred. Good luck.
 
Doug Reid said:
The ground plane issue is not your main problem. Even though your antenna is not mounted on the fuselage shell, you should be able to pick up your airport ASOS instead of static, even though your antenna mount is horizontally polarized. Check for opens and shorts in your coax and check your antenna as well. I cannot tell from the photo, but after you find your problem, make sure your antenna base is grounded to the fuselage or frame as close to the antenna base as possible. A long ground wire connection will in effect detune your antenna. Lots of newer com antennas are bent these days...sort of half vertical and half horizontal. Vertical polarization is preferred. Good luck.
Click to expand...

^^^ that. A strong signal will sound fine without the ground plane and won’t be affected by polarization.

Something is physically wrong with the RF path. Don’t transmit before finding it, either. At best hard on the transmitter, at worst you’ll toast the RF amp transistor(s).
 
Check your coax. Center pin to center pin should be shorted, bayonet to bayonet should be shorted, and center pin to bayonet should be open. There are faults this won’t find, but this will find most of them. If you happen to know an amateur radio op, they often own gear that can find the more rare cable faults if better mounting and this basic cable test doesn’t find your fault.
I also looked up the antenna spec, and while a lot of data isn’t there it does claim vertical polarization when belly mounted. It also states its height as 9 inches, and a clearly bent element which leads me to believe that the total element length is around 20 inches, and pretty much 1/4 wave. So it is expecting/needing a ground plane. Since most of the current is in the first third of the antenna, which is sort of close to vertical when belly mounted, it will be predominantly sort of vertically polarized. It looks to me like as it is installed, the horizontal section of the element is closer to the belly than the 9 inches it is designed to be, so that likely affects efficiency and SWR. But all that said, as wrong as the antenna is installed, it should still receive, so I’m expecting a cable fault as well.
 
denverpilot said:
^^^ that. A strong signal will sound fine without the ground plane and won’t be affected by polarization.

Something is physically wrong with the RF path. Don’t transmit before finding it, either. At best hard on the transmitter, at worst you’ll toast the RF amp transistor(s).
Click to expand...

I tried two different coax cables from the radio to the antenna , same thing happens. I only have a 3 ft run from the radio down to the antenna.


Sent from my iPhone using Tapatalk
 
-KLB- said:
Check your coax. Center pin to center pin should be shorted, bayonet to bayonet should be shorted, and center pin to bayonet should be open. There are faults this won’t find, but this will find most of them. If you happen to know an amateur radio op, they often own gear that can find the more rare cable faults if better mounting and this basic cable test doesn’t find your fault.
I also looked up the antenna spec, and while a lot of data isn’t there it does claim vertical polarization when belly mounted. It also states its height as 9 inches, and a clearly bent element which leads me to believe that the total element length is around 20 inches, and pretty much 1/4 wave. So it is expecting/needing a ground plane. Since most of the current is in the first third of the antenna, which is sort of close to vertical when belly mounted, it will be predominantly sort of vertically polarized. It looks to me like as it is installed, the horizontal section of the element is closer to the belly than the 9 inches it is designed to be, so that likely affects efficiency and SWR. But all that said, as wrong as the antenna is installed, it should still receive, so I’m expecting a cable fault as well.
Click to expand...

I checked the coax when I assembled it too and all seems to be good. I tried two different coax cables and they both have the same effect. Static. Now when I have the coax plugged into the back of the radio the multimeter will “ring” when I touch the center pin to the outside casing. Is that normal?


Sent from my iPhone using Tapatalk
 
Does the level of the static change considerably when you connect/disconnect the antenna from the coax?
 
nhraracer90 said:
avionics shop (we trailered) it to and from
Click to expand...
nhraracer90 said:
I recently had my new radio and ADSB installed (wired)
Click to expand...
Not enough info. Did the shop function check their work prior to releasing the aircraft to you? Did it work for them? Is this an aviation "avionics" shop? What specifically did they "wire?" What did you specifically install after receiving the aircraft back?
nhraracer90 said:
When I hook up my coax to the antenna , all I hear is static noise.
Click to expand...
Are you using a headset to listen? With antenna connected, main power ON and the radio ON, do you hear static? With the radio OFF and main power ON, do you hear static? Describe the static: is it steady or oscillating? How are you powering up the aircraft and radio?
nhraracer90 said:
Any input would be great.
Click to expand...
With radio on, antenna connected, get an ELT, tune radio to 121.5, place ELT near antenna and momentarily activate ELT. Do you hear the pulse in your headset? For the purists here, only activate the ELT 5 minutes before/after the hour. Since you're in the middle of maintenance, check all the grounds from the main power source to the radio and antenna. Once all the grounds are verified, specifically for the radio per the install schematic, with power ON, key the mic. What happens to the static?
 
Last edited:
-KLB- said:
Does the level of the static change considerably when you connect/disconnect the antenna from the coax?
Click to expand...

Yes. When I disconnect the coax from the antenna, the static goes away. Naturally don’t hear any radio communications


Sent from my iPhone using Tapatalk
 
Bell206 said:
Not enough info. Did the shop function check their work prior to releasing the aircraft to you? Did it work for them? Is this an aviation "avionics" shop? What specifically did they "wire?" What did you specifically install after receiving the aircraft back?

Are you using a headset to listen? With antenna connected, main power ON and the radio ON, do you hear static? With the radio OFF and main power ON, do you hear static? Describe the static: is it steady or oscillating? How are you powering up the aircraft and radio?

With radio on, antenna connected, get an ELT, tune radio to 121.5, place ELT near antenna and momentarily activate ELT. Do you hear the pulse in your headset? For the purists here, only activate the ELT 5 minutes before/after the hour. Since you're in the middle of maintenance, check all the grounds from the main power source to the radio and antenna. Once all the grounds are verified, specifically for the radio per the install schematic, with power ON, key the mic. What happens to the static?
Click to expand...

Yes the shop checked everything. They did the wiring for the radio and transponder (built the harnesses) and put the jacks on for the headsets. Yes it was an “aircraft” avionics shop recommended by our A&P. Only thing I installed so far has been the VHF antenna as shown in the original photo. The static is steady and doesn’t change. Radio on and main power on static yes. Radio off and main power on NO static. I don’t have any reason to doubt the work done by the shop, I do believe it may be a simple issue.


Sent from my iPhone using Tapatalk
 
nhraracer90 said:
Radio on and main power on static yes.
Click to expand...
Is the radio squelch adjusted properly? Is this a new radio? Can you do the ELT check? Short of a SWAG it sounds like the radio is picking up its ground from the antenna. Best to call the shop before you let out any unnecessary smoke.
 
Bell206 said:
Is the radio squelch adjusted properly? Is this a new radio? Can you do the ELT check? Short of a SWAG it sounds like the radio is picking up its ground from the antenna. Best to call the shop before you let out any unnecessary smoke.
Click to expand...

It’s a new radio. Becker AR6201. I can’t do the ELT check. What do you mean SWAG? When you press the Squelch button the static goes away (according to the manual, it suppresses weak signals) when active.


Sent from my iPhone using Tapatalk
 
nhraracer90 said:
Good evening! I recently had my new radio and ADSB installed (wired), I did the hardware mounting and etc. everything works great with the intercom and such (Becker AR6201). I had to buy a new VHF antenna (Comat CI-122). I mounted that antenna myself and made my coax lines (RG400). When I hook up my coax to the antenna , all I hear is static noise. I can’t even hear the airport WX ASOS over the static. I’m only a few miles from the airport. Am I missing something obvious? The aircraft is a helicopter. The ship is currently going through a restoration so I can’t fly it to the avionics shop (we trailered) it to and from. I attached a photo of how I mounted the antenna. Any input would be great. Thank you!


Sent from my iPhone using Tapatalk
Click to expand...


Where is the ship when you were doing this? Inside a shop/hangar with stupid noisy florescent lighting turned on?
 
bnt83 said:
Where is the ship when you were doing this? Inside a shop/hangar with stupid noisy florescent lighting turned on?
Click to expand...

Yes the ship is in my building and the lights were on.


Sent from my iPhone using Tapatalk
 
nhraracer90 said:
When you press the Squelch button the static goes away (according to the manual, it suppresses weak signals) when active
Click to expand...
Exactly that is the purpose of the squelch. Without the squelch "activated" you will get constant noise, i.e., static. It is also adjustable toward sensitivity. Leave the squelch ON and push aircraft outside with a clear view toward your ASOS signal. While I doubt you'll hear it with a bottom mounted antenna, see what happens.
nhraracer90 said:
What do you mean SWAG?
Click to expand...
Scientific Wild Azz Guess.
 
Bell206 said:
Exactly that is the purpose of the squelch. Without the squelch "activated" you will get constant noise, i.e., static. It is also adjustable toward sensitivity. Leave the squelch ON and push aircraft outside with a clear view toward your ASOS signal. While I doubt you'll hear it with a bottom mounted antenna, see what happens.

Scientific Wild Azz Guess.
Click to expand...

SWAG-haha good one. Okay that makes sense. I had the squelch on at first and couldn’t hear anything at all.


Sent from my iPhone using Tapatalk
 
Bell206 said:
Scientific Wild Azz Guess.
Click to expand...

The order of guesses, from best to worst are ESWAG, SWAG, WAG, and Rectorandom Estimate. Educated Scientific Wild @$$ Guess, Scientific Wild @$$ Guess, Wild @$$ Guess, and just pulling a number right out of your @$$.

I'll address the antenna question later when I'm done with the annual I'm in the middle of.

Jim
 
What we know:

1. New radio install with new antenna and new coax in a new position on the airplane.
2. No noise (static is a relatively precise term) with the antenna disconnected and noise with it connected.
3. No reception from a station on the field a couple of miles away.

So let's proceed ...

With a handheld radio and rubber duckie antenna in a similar position near the new antenna (and the shop lights on) do you hear the AWOS clearly with no noise?

Answer: ?????
 
By mounting the antenna spaced away from the groundplane (aircraft skin) and running the element parallel to the skin is creating some capacitive coupling. The angle of the element will change the SWR, among other things...

All these things affect efficiency and allow for noise.
 
FYI: while we haven't heard back from the OP as of yet, he may have had his squelch deactivated which may have caused the continuous static he heard. Last report was no static with squelch activated. Film at 11.
 
Bell206 said:
FYI: while we haven't heard back from the OP as of yet, he may have had his squelch deactivated which may have caused the continuous static he heard. Last report was no static with squelch activated. Film at 11.
Click to expand...

So turning the lights off in my shop helped out a lot and moving the antenna from how it was mounted. I mounted it temporarily somewhere else and was able to hear ASOS and talk to the tower. Once I get it mounted to the aircraft skin somewhere and get it out of my shop, I think I’ll be good.


Sent from my iPhone using Tapatalk
 
nhraracer90 said:
So turning the lights off in my shop helped out a lot
Click to expand...
FWIW: Was that with the squelch activated or deactivated? You may want to see if the squelch settings are too high which may be blocking the signals you wish to receive while at the same time suppress your lights or other RFI.
 
I have one fluorescent light fixture I cannot hear anything on the radios when it’s turned on despite being on the airport I’m trying to hear ATIS, tower, ground, departure etc
 
Bell206 said:
FWIW: Was that with the squelch activated or deactivated? You may want to see if the squelch settings are too high which may be blocking the signals you wish to receive while at the same time suppress your lights or other RFI.
Click to expand...

Both. I did have to lower the “minimum” settings in order for it to pick up. I do have it closest to the lowest setting. Radio still shows a weak signal but I can at least talk to the tower now.


Sent from my iPhone using Tapatalk
 
weirdjim said:
Let me guess. You have a noisy fluorescent fixture inside of a metal hangar/shop, which is where the aircraft is.

Jim
Click to expand...

Yes that would be correct


Sent from my iPhone using Tapatalk
 
nhraracer90 said:
Yes that would be correct Sent from my iPhone using Tapatalk
Click to expand...

OK, let me see if I can explain this. A fluorescent light works by striking an arc inside of a tube filled with mercury vapor about 120 times a second. That's little lightning bolts being turned on and off 120 times a second. You would EXPECT to hear "static" (noise) from a lightning bolt that hits your hangar hundreds of times a minute, so you would therefore expect to hear noise in a radio next to those lightning bolts, no? Now we TRY and filter out some of that fluorescent noise using transformers inside the fluorescent fixture with filtering inside of that, but you can only do so much. And, as the tubes and filters age, that noise just keeps getting louder.

Now, we filter out other "lightning bolt" noise called spark plug noise with "shielded cable", which is a braided metal jacket around the spark plug wire. That keeps the noise INSIDE the shield but the reverse is also true. It keeps any RF energy from OUTside the cable from getting into the central wire. Inside of a metal hangar it is the same effect. It keeps any transmitted signals from your radio from getting outside of the hangar, but it keeps any signals from OUTSIDE the hangar (like, say, tower or AWOS) from getting into the hangar and therefore into the antenna.

By the way, "Polarization", "Capacitive Coupling", "Groundplane" and "VSWR" have really little to do with this simple question. Performance when you get the sucker up and running? You bet. But right now in your location, a piece of limp spaghetti in a copper septic tank would work at the range(s) we are talking about.

Did that work?

Jim
 
Last edited:
weirdjim said:
OK, let me see if I can explain this. A fluorescent light works by striking an arc inside of a tube filled with mercury vapor about 120 times a second. That's little lightning bolts being turned on and off 120 times a second. You would EXPECT to hear "static" (noise) from a lightning bolt that hits your hangar hundreds of times a minute, so you would therefore expect to hear noise in a radio next to those lightning bolts, no? Now we TRY and filter out some of that fluorescent noise using transformers inside the fluorescent fixture with filtering inside of that, but you can only do so much. And, as the tubes and filters age, that noise just keeps getting louder.

Now, we filter out other "lightning bolt" noise called spark plug noise with "shielded cable", which is a braided metal jacket around the spark plug wire. That keeps the noise INSIDE the shield but the reverse is also true. It keeps any RF energy from OUTside the cable from getting into the central wire. Inside of a metal hangar it is the same effect. It keeps any transmitted signals from your radio from getting outside of the hangar, but it keeps any signals from OUTSIDE the hangar (like, say, tower or AWOS) from getting into the hangar and therefore into the antenna.

By the way, "Polarization", "Capacitive Coupling", "Groundplane" and "VSWR" have really little to do with this simple question. Performance when you get the sucker up and running? You bet. But right now in your location, a piece of limp spaghetti in a copper septic tank would work at the range(s) we are talking about.

Did that work?

Jim
Click to expand...

Thanks for the explanation


Sent from my iPhone using Tapatalk
 
You must log in or register to reply here.
Back
Top