Mystery comm failure after 30 ,minutes

[motoadve]

Radio number 2 in my 182 its a Narco MK12D, recently during flight the first 30 minutes tower, control, approach, flight following, everyone can hear my radio calls, after 30 minutes in flight no one can hear me, only planes near by or the tower on final approach, so we thought it was the antenna, installed a new one, same problem, maybe the radio, so I borrowed another radio, same issue.

Any ideas?

 

[Jim Carpenter]

Audio panel?

 

[AV8R_87]

after 30 minutes in flight no one can hear me, only planes near by or the tower on final approach

Can you hear any sidetone when you transmit? That's generated by the receiver listening to the transmitted signal. Compare the quality of the initial sidetone with what you get after 30 minutes.

Anything else looking odd with the radio after 30 minutes? Does it behave like it has low operating voltage?
I'd check the voltage at the circuit breaker, and maybe at the radio connector as well, under load. Circuit breakers do go bad and develop high resistance. That could cause excessive heating and even higher resistance, causing significant voltage drops when attempting to transmit.

 

[WDD]

When you installed the new antennae, did you install new connectors as well as a new cable? I had a problem with the antennae cable connector on my radio. It was simply old, wasn't holding the parts/wires together as it should. Gave intermittent transmission and receiving problems. Easy to fix.

 

[dmspilot]

Sounds like a voltage issue, either with the radio itself or the aircraft. Would be interested to know how well the radio works if you take off and fly around for >30 min. with it turned off, then turn it on and try transmitting. Likewise, if the radio acts up and you turn it off for awhile, then back on, does it get better?

 

[WDD]

Sounds like a voltage issue, either with the radio itself or the aircraft. Would be interested to know how well the radio works if you take off and fly around for >30 min. with it turned off, then turn it on and try transmitting. Likewise, if the radio acts up and you turn it off for awhile, then back on, does it get better?

Keep in mind he swapped in a different radio, same results.

 

[dmspilot]

Keep in mind he swapped in a different radio, same results.

I would still try the experiment, but also a plug in voltmeter might yield some insight if he doesn't already have a digital one.

Ship electrical issue miht not be noticeable in his #1 radio if it's a newer model with lower current draw.

 

[Albany Tom]

Like WDD, my bet is on the cable or connectors. For around $100, you can get a "nano VNA" from Ali or Amazon, vector network analyzer, and sort out if the antenna and cable electrically looks like an antenna. Or probably for the price of a cheeseburger you can get a local ham who has such things to check same as well. Not an A&P, do have some radio experience. I'll add that it's also possible for an antenna to look good on a VNA but not work under transmit. That's because the power output of a VNA is almost nothing, while the radio is 10W or more, and that can make a difference.

Another way to test, not as conclusive, is to go up and just listen, don't talk on that radio. See if it goes past 30 minutes. If the cable is bad, what could be happening is that the radio is overloading/overheating because of reflected power from transmitting into a bad antenna. Modern radios, at least commercial and ham radios, will automatically fold power back to keep from damaging themselves. I don't know if old aircraft radios will do that, but I kinda doubt it. So there may be a risk of running a radio into an unknown cable and antenna. With old but still solid state ham radios it was possible to burn the finals out by transmitting into a bad load.

 

[Domenick]

Obviously, since you swapped radios with the same result, the issue is not the radio. Previous suggestions are good.
One simple thing is the tray. There is a connector or two at the back. Clean all of them.

 

[GeorgeC]

Like WDD, my bet is on the cable or connectors. For around $100, you can get a "nano VNA" from Ali or Amazon, vector network analyzer, and sort out if the antenna and cable electrically looks like an antenna. Or probably for the price of a cheeseburger you can get a local ham who has such things to check same as well. Not an A&P, do have some radio experience. I'll add that it's also possible for an antenna to look good on a VNA but not work under transmit. That's because the power output of a VNA is almost nothing, while the radio is 10W or more, and that can make a difference.

Measure? Before replacing parts?

Can you hook the plane up to ground power and see if the radio still fails after 30 minutes? That would take thermal expansion (of something) due to engine heating out of the equation.