Com antenna on bottom?

[ilovecornfields]

I’m having an issue when I transmit on COM 2 that tower will tell me that I’m “bleeding over” from ground to tower frequency. This has happened at a couple of places. I spoke to an avionics shop and he thinks it’s “re-radiation” from the two com and ELT antenna being so close together and says that moving a com antenna to the bottom of the plane should fix this problem. Has anyone tried this? Where did you put the bottom antenna?

[larryb]

The shop theory does not sound correct to me. What kind of radio are we talking about? What are the frequencies involved, tower and ground? Before going to the expense of changing the antenna I would want to know that the radio transmitter is “clean.” That involves taking it out and checking on the bench with a service monitor.

[MIm20c]

We have ours on the bottom because of the skywatch antenna.  It’s mounted just behind the wings just slightly to the left of center. I wouldn’t change yours unless there was a really good reason. 

[ilovecornfields]

Radios are GTN 750/650. All nav/com/ELT antennas are on top. This only happens on the ground, usually when I’m talking to ground they tell me it “bleeds over” onto tower. Had this happen at 3 different places. Doesn’t seem to matter what frequency I have tuned into COM 1. Issue is only with COM 2. Shop said he’s seen this multiple times before, recommended moving the com 2 antenna to the bottom. Said this usually improves reception and fixes the problem. Claims he’s seen this on an Ovation before.

Also suggested unplugging the ELT antenna and seeing if that fixes the problem. I’m assuming he meant on the ground.

Seemed plausible to me. 

[carusoam]

Disconnecting one antenna at a time seems like an easy test... for antenna being too close...

Of course, broadcasting on one frequency and having it go out on another sounds a bit more funky than that...

how close, what frequencies, are your ground and tower?

We have an antenna engineer, but I don’t know if he is listening... @Piloto.... antenna question...

Best regards,

-a-

[Ned Gravel]

Hmm.  Less likely caused by antenna proximity and more likely caused by RF emissions closer to the backplane of the radio. 

I have an "eye poker" (belly com antenna that causes difficulty to maintainers during inspections) on my Mooney too, but it is for the handheld, when all else fails.  Com1, Com2, Nav1 and Nav2 are all up on the top - both navs to the whiskers on the vertical stabilizer.  The com antennae are separated by about four feet.  Running receivers off the same antenna is not a real issue - no power being generated.  Transmitters off the same antenna can cause a bit more difficulty. 

However, RF pushing out of the transmitter (especially near the source of radiated power) can affect unshielded conductors for other systems that may be very close by - and that is what this sounds like to me.  

The sneaky one for me was very dirty NARCO transmitters keying both sets of radios because my Garmin audio panel had next to zero shielding and one transmitter was causing wash to the other through the audio panel.  Fixed that with a PS Engineering audio panel - much better built.

Just an amateur Signals Officer.  No MUFs, or LUFs were harmed in the making of this amateur observation.

[EricJ]

7 hours ago, ilovecornfields said:

I’m having an issue when I transmit on COM 2 that tower will tell me that I’m “bleeding over” from ground to tower frequency. This has happened at a couple of places. I spoke to an avionics shop and he thinks it’s “re-radiation” from the two com and ELT antenna being so close together and says that moving a com antenna to the bottom of the plane should fix this problem. Has anyone tried this? Where did you put the bottom antenna?

How close are your antennas?   For VHF comm they should be at least 3ft and preferrably farther apart.    The farther the better, but no need to go crazy once you're past about 4-5 ft.

The re-radiation theory is practical for closely spaced antennas, as what happens with a typical radio architecture is that the oscillator from the non-transmitting antenna that gets radiated out a little bit mixes with the transmitted signal.   What happens is that there is bleed and interference between the two channels.    This is one of the several reasons why comm antennas should not be closely spaced.   Spacing further makes the amplitude of the radiated oscillator much smaller and eliminates the effect pretty quickly as the distance is increased.
 

4 hours ago, carusoam said:

Disconnecting one antenna at a time seems like an easy test... for antenna being too close...

That will help determine whether it is an LO (local oscillator) bleed issue or not, which might also be tested just by tuning the second radio to a different frequency and pulling its breaker before contacting one of the offended stations.

Another issue with antennas spaced too closely together has to do with near-field effects which are often non-linear and difficult to predict.   This is why keeping the near field (within more or less 1/4 to 1/8 wavelength for these antennas) around an antenna free from interfering objects, like other antennas with similar resonant frequencies (i.e., the same band) can be important.   This can significantly affect the radiation pattern of both antennas.

A friend was just going through some similar stuff on his Cherokee after one of his comm antennas was vibrating a lot and causing a lot of worry about generating a fatigue crack.   I warned him about selecting a new location not too close to his other antenna (which has evidence of a previous mounting within a few inches of it).   He subsequently sent me pics of a lot of airplanes on his field with VHF comm antennas spaced pretty closely, in one case probably about a foot or so.   Don't do that. 

All that said, there could be other things that could cause the problem, but I wouldn't start moving antennas until the culprit is located.   Does this happen regardless of which radio is used to talk to the offended station?   

 

[ilovecornfields]

3 hours ago, EricJ said:

Does this happen regardless of which radio is used to talk to the offended station?

No. Just on COM 2.

[MB65E]

Check the settings on the 650/750. They will act really strange if they are not set up correctly. ELT antenna shouldn’t matter-it’s not transmitting. I think the avionics shop is flawed. 

I’ve never had so much trouble than with a 650. In a wood aircraft. Ground plane was a challenge, it’s still not 100%. 

-Matt

[ilovecornfields]

Any settings in particular? There are a lot...

[MB65E]

That’s the issue! Start with stock settings and work from there. Some one may have already been in there and playing with settings. 

-Matt

[201Mooniac]

I have two GTN's installed for years without issue and I've just recently had the same comment a couple of times that when I transmit with COM2 on ground that it is bleeding over to tower.

[OR75]

What kind of Audio panel do you have ? I am betting it is a PS engineering 

[Jerry 5TJ]

18 minutes ago, OR75 said:

What kind of Audio panel do you have ? I am betting it is a PS engineering 

I happen to know he has a GMA35 remote audio panel.  

[201Mooniac]

2 hours ago, OR75 said:

What kind of Audio panel do you have ? I am betting it is a PS engineering 

In my case, it is a PSE 8000BTi, why, do you know some correlation with the audio panel?

[OR75]

I believe it has to do with the Transmit Interlock

Typically, what the transmit interlock does is lower the sensitivity of the transmitter that is not transmitting (so no bleeding)

The interlock functions works via the audio panel so you connect all the COMs via the audio panel (obviously)

The TX interlock function still exist in the GNS series but probably disappeared in the GTNs 

I speculated wrongly that it had to do with audio panel but it actually may have disappeared in the Audio and/or at the COM     

[201Mooniac]

Interesting but I've had these GTN's and the audio panel for about 6 or 7 years (since the GTNs were first introduced) and this problem never occured until the last 2 months or so.

[OR75]

Those RF waves patterns are a big mystery.  Since it is easy to do, you can try to swap the COM antenna coax cables behind the GTNs and see if it makes a difference.  

[larryb]

Below is the text from the GTN install manual on transmit interlock. This affects the airplane receivers, not the transmitter. I cannot see how this would affect the bleed over problem we are discussing.

I do have this set in my GMA 345 audio panel, the switch called "MUTE ON COM TX." Before this switch was set when I would transmit on one radio I would hear a bleed over signal on the other radio, if the frequencies were very close. This would happen with departure on 121.3 when monitoring guard on 121.5 for example. 

GTN install manual excerpt:

2.4.13 Transmit interlock and Split COM Operation
In small aircraft, COM and NAV receiver interference is affected by both the distance between antennas and the tuned frequency separation. With transmit interlock activated in the COM transceivers, split COM operation between a flight crew of more than one pilot is affected.
In aircraft that have a transmit interlock feature, when either transmitter is keyed, all other receivers are muted so that they won’t pick up interference from the active COM transmitter. This is the preferred option for single pilot operation.
For aircraft with two flight crew members, transmit-interlock would likely interfere with communications. When the pilot or the copilot transmit, no audio is heard on any other receiver. This means that if the pilot is communicating with ATC while the copilot transmits on another radio, all pilot reception is cut off during the time of copilot transmission.
If the installation does not have transmit-interlock activated, all the receivers are listening all the time whether any radio is transmitting or not. Split COM performance varies significantly across installations. If the transceivers interfere with each other, transmission

[ilovecornfields]

2 hours ago, 201Mooniac said:

Interesting but I've had these GTN's and the audio panel for about 6 or 7 years (since the GTNs were first introduced) and this problem never occured until the last 2 months or so.

Same issue here. Weird. I actually wouldn’t mind having an antenna on the bottom. Seems like I would get better reception in the air that way.

[201Mooniac]

16 hours ago, OR75 said:

Those RF waves patterns are a big mystery.  Since it is easy to do, you can try to swap the COM antenna coax cables behind the GTNs and see if it makes a difference.  

Thanks, I'll try that

[MIm20c]

14 hours ago, ilovecornfields said:

Same issue here. Weird. I actually wouldn’t mind having an antenna on the bottom. Seems like I would get better reception in the air that way.

You will for long transmissions. Com 2 on the bottom (650) has always had more reach and sensitivity than the 750 transmitting on top. 

In rural areas (Alaska et al) I have the 750/650 both monitoring the same frequency and transmit on the 650. 

[bradp]

I connected my 430 to a bottom com antenna.  I get much better reception in the air (at some altitudes, I used to swear the wing was shielding  line of sight to the ground station).  Have only had one issue is my reception on the ground stunk when I visited ATL recently.  Their tower was so tall that there was no line of sight not blocked by the fuselage

[EricJ]

2 hours ago, bradp said:

I connected my 430 to a bottom com antenna.  I get much better reception in the air (at some altitudes, I used to swear the wing was shielding  line of sight to the ground station).  Have only had one issue is my reception on the ground stunk when I visited ATL recently.  Their tower was so tall that there was no line of sight not blocked by the fuselage

Should get decent reflection off the ground.   Line of sight isn't required, just adequate signal strength at the antenna.   If a strong enough reflection off of something is received, that works, too.   That said, since the Mooney sits so low, it could still make a pretty shadowed area for a bottom-mounted antenna.

[OR75]

So you would be using a CI-122 com antenna ?