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1986 M20J. I keep losing radio contact on trips. It seems to be getting worse. Garmin 650xi. Installed about a year ago. When I am close by a center or tower, I hear fine. Sometimes a little scratchy. My comm 2, old king works great. Have had the plane in the shop. They checked everything and it all checked out. They did lower the squelch break % and it helped, but it is not fixed. Long cross countries with imc, it sure is nice to be able to talk to someone. Any ideas? Shop wants to put new antenna on and re bond. Thank you. 

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Shops that don't know how to troubleshoot like to replace stuff. Sooner or later they will replace enough stuff to solve the problem and the risk is all on you since you are paying for their mindless approach. I once bought a new Acura TL. The AM radio would stop working after about ten minutes. Took it back on warranty. Predictably, they replaced the radio. Ten minutes after driving off the lot the radio failed. Then they said it must be the antenna. That's embedded in the rear window. So they replaced the rear window. Ten minutes later -- no radio. Then they said it must be the cable between the antenna and the radio (Duh, what else is left?). Bingo! Three visits (how much would this have cost me if not covered under warranty?) to fix one simple problem.

If the shop that wants to replace the antenna is the shop that installed the radio, it should be on them to make it right. Antenna's are simple passive devices and don't generally fail. The coax should meet current Garmin specs. Perhaps the radio is defective. If they insist on replacing the antenna, I'd make them agree that if this doesn't fix it, you don't have to pay for it. Bet they become a lot less interested in the antenna.

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Thank you for the reply. The shop has mentioned replacing the coax. Unfortunately it is not the shop the did the install. I bought the plane 10 months ago north of Detroit and had them do the install. I am in Mississippi. love the plane. 10 months in and have 120+hours in it. Trying to work out the bugs. 

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This may be overly simple, but have they checked that the unit is tight and secure in the housing?  My 430W was acting similar about 6 months into having it installed and a quarter turn with an allen wrench eliminated the issue. 

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Welcome aboard 86,

Sounds like you need a good radio shop to run some basic trouble shooting...

An ordinary part of getting to know what you have...

Could be a simple low cost issue...

Check your logs to see what was installed... radio, wires, antenna, extra things...?

Try to determine if the radio was installed following the proper guidance, or if it was a quick swap-in...

PP thoughts only...

Best regards,

-a-

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It started of course after the test flight to pick up the plane. It began on the way home from picking up the plane. We stopped at an airport halfway home and had an avionics shop take a look. He adjusted a setting and it worked great. Said the setting was set like you would set the older 650. I don’t know what he changed. I was brand new to ownership at the time. We have lowered the squelch break % and it has helped, but not cured it

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I had the same problem. There was nothing wrong with the antenna, but the steel tubes it is mounted to were all rusty and nasty. Some genius mechanic in the past had insulated the antenna from the steel tubes by putting tape on the standoffs. Anyway, I cleaned it all up repainted the tubes, put star washers under the standoffs so they would bite into the metal, verified I had continuity on all four mounting screw to the skin and the steel tubes. This cured all my problems.

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  • 3 weeks later...

Rich is most likely on to something.  I’m a HAM radio operator and former COMM guy at CAP, and aircraft frequencies are very close to HAM VHF 2 meter frequencies (140 MHz or so) and are similar to issues encountered in mounting a VHF antenna on a vehicle.  I suspect you may be having a ground issue at the antenna mount, which would hugely lower your available signal by raising your SWR (standing wave ratio), which is a six-dollar way of saying that too much of your signal is staying in the radio, coax and antenna and isn’t radiating the way it needs to.  You may also have a poor antenna or radio coax connector- those can be tricky to soldier correctly, and could also cause the symptoms you describe.

It should at least be checked, because most Issues like yours that I’ve had with weak output strength have been caused by a grounding issue or an issue with the coax, as was described above.  It’s not rocket science.  Hope this helps.

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I think Rich and Ross are correct.  The scenario you describe sounds like a degrading ground plain.  Has the aircraft been painted in the past three years?  Often the stripper takes that long to migrate under the antenna and isolate the ground.  I used to find this routinely and is one of the leading causes of poor range.  A simple ring out will not reveal this.  The best bet, and often least expensive is to remove the antenna and clean things up.  Let us know what you find.

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On 1/6/2021 at 8:26 PM, 86M20J said:

1986 M20J. I keep losing radio contact on trips. It seems to be getting worse. Garmin 650xi. Installed about a year ago. When I am close by a center or tower, I hear fine. Sometimes a little scratchy. My comm 2, old king works great. Have had the plane in the shop. They checked everything and it all checked out. They did lower the squelch break % and it helped, but it is not fixed. Long cross countries with imc, it sure is nice to be able to talk to someone. Any ideas? Shop wants to put new antenna on and re bond. Thank you. 

I had the identical problem when I installed at 430AW years ago. Constant lost comm at longer ranges or with weather around, but the old King continued to work fine. I ran into another Mooneyspacer when there was a PPP in Olathe and we got a tour of the Garmin facility. He asked about the problem with his 430AW, they said to go talk to his installer. So I realized I was not the only one. Overtime, (1) switched the antennas between the King and the Garmin and got the same result (old King better than new Garmin), (2) temporarily switched units with another 430AW and got a little better reception, (3) sent the old unit in to Garmin, it was benched and the report was it was out of spec, they repaired it, then it quickly reverted to form, (4) swapped it with another unit from Garmin permanently and got the same result, old King better than new Garmin and loss of comm at range and with weather. If you followed all that, the old King nav continued to outperform the 430AW no matter what we did. I recently installed a GTN 750, kept the 430, and that resulted in removal of the old King. I had the installer run new low resistance coax throughout the plane. I have not flown it much, it is currently in the shop for the annual, but it will get a work out this summer. Reception seems to be more sensitive, but whether the range problem persists or not remains to be seen. What I repeatedly heard from mechanics and avionics shops was that the newer Garmin digital radios just are not as good. They have heard the complaints before. I read through the posts, above, and I agree that there are some things worth checking out, but what stuck with me is that your old King works fine no matter what testing is done, the new Garmin that does not no matter what testing is done. The King was a 12 watt, the 430AW is a 16 W. 

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  • 2 months later...
On 1/23/2021 at 6:43 PM, Ross Statham said:

 I’m a HAM radio operator and former COMM guy at CAP,

Ross, what your call sign? I'm KM0ONY.

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1 hour ago, 86M20J said:

Turns out I need a new antenna. 28 days is the fastest I can get one. Anyone have one lying around they want to sell??

doubtful.  What was the SWR?   Rerack the radio several times.   Pull in and push out.

SWR is Standing Wave Ratio.   It is measured with a meter that is put between the radio and the antenna.   More than likely the antenna just need to be taken off and cleaned and put back on.  You need a ground plane ie skin of the airplane for the antenna to work well.   So the Antenna needs to have good contact with the skin of the airplane.

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SWR should be a lot lower than that, approaching 1to1? I believe from other radios 1.5 to 1 is good, above 2 not so good?

I’ve never seen it change from one end of the Co-ax to the other, but then I haven’t measured both ends, but if it is, wouldn’t that point to Co-Ax?

As I usually work on older aircraft. if I change an antenna the Co-Ax is replaced too as a matter of course.

Probably some knowledgeable person could make a case that Co-Ax has a finite life. I’ve not had many antennas go bad, I’ve replaced them because the base was corroded and looked bad, but the antenna worked fine,but those were “stick” antenna’s not blades like on my J.

I assume our antennas require a ground plane, and if so then the aircraft supplies that ground plane, then the electrical bonding between the antenna and fuselage may should be checked?

I have question marks as I’m not stating facts, but things I believe to be true

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Comm antennas are monopoles which means that the ground plane (aircraft skin in metal aircraft) effectively forms half of the antenna. Good electrical bonding between the antenna base and the skin is crucial. If the base is not well sealed, moisture can cause corrosion increasing the impedance between the base of the antenna and the aircraft skin degrading the efficiency of the antenna.

Maximum power transfer to the antenna requires that the antenna and coax characteristic impedances match. SWR is (usually VSWR - Voltage Standing Wave Ratio) is a measurement of the impedance mismatch.  Mismatch will cause some of the signal energy to be reflected back toward the transmitter instead of being radiated by the antenna. The reflected signal will combine with the primary signal creating standing waves on the transmission line and the ratio of maximum and minimum voltage of these standing waves can be measured as an indirect indication of the mismatch. 

Comant specifies a maximum VSWR of 2.5:1 for its VHF Comm antennas.

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