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In the past I have had instructors suggest that I use COM1 in the air and COM2 on the ground. The rationale was that most aircraft have the COM1 antenna on the belly making it favorable for air to ground when in the air and COM2 on the roof making it favorable for ground to ground when on the ground.

It can definitely be helpful to use the top antenna for ground communications and the belly antenna for air.

I frequently fly a late model PA-32 and I haven't found documentation on which communications antenna is which.

I also frequently fly Cessna aircraft which usually have all comms antennas on the roof.

I sometimes fly an SR-22 and my instructor suggested using COM1 on the ground and COM2 in the air. This is reverse from what I thought was usual, but indeed the SR-22 AMM shows that the COM1 antenna is on the roof with COM2 on the belly.

Is there really a rule of thumb here or does it vary so much from one aircraft to another that no rule of thumb is possible to say?

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  • $\begingroup$ Your best bet would be to follow the guidance found in the POH. Any A&P mechanic should be able to answer your question on the PA-32. $\endgroup$
    – wbeard52
    Commented Sep 30, 2015 at 13:20
  • $\begingroup$ On the PA-32 I haven't found an answer in the POH. As far as asking the mechanic, I want documented answers in this case. $\endgroup$
    – ryan1618
    Commented Sep 30, 2015 at 13:22
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    $\begingroup$ @RyanBurnette, if all else fails you can always ask your mechanic and then post the result (answering your own questions is OK and even encouraged here). $\endgroup$
    – Jan Hudec
    Commented Sep 30, 2015 at 13:33
  • $\begingroup$ Sure thing, if my mechanic gives me some documentation I'll post it. $\endgroup$
    – ryan1618
    Commented Sep 30, 2015 at 14:05
  • $\begingroup$ @RyanBurnette "documented answers" would mean "Call your mechanic or avionics tech and have them trace the wires and find out": Your aircraft has almost certainly had its avionics modified over the years, so unless you examine the aircraft and determine this empirically you have about a 50/50 chance of guessing right. $\endgroup$
    – voretaq7
    Commented Sep 30, 2015 at 14:39

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Honestly? That AOPA article overstates the issue of airframe shadowing (at least for light GA singles): Both antennas will usually work fine on the ground, and will probably work without any issues in the air as well.

Airframe shadowing can cause issues, but if it were as big an issue as people like to make of it there would be no aircraft with single-radio installations (because they would either not be able to talk to people on the ground, or not be able to talk to people in the air).
There are plenty of aircraft flying around with only one radio.

If you're concerned about whether your transmissions are readable pick a nice quiet time of the day and ask the tower for a radio check on Com #1, then repeat the process on Com #2. (Because we're talking about AM radio the results of this test are valid for your aircraft's position and heading when you did the test, and in weather conditions similar to those the test was conducted.)

The general rule of thumb is that each radio will be connected to a separate antenna (this is necessary if you want them to work independently). Some avionics shops have a preference of which way they wire things, and I'm sure manufacturers have a standard they use in their shops (either the POH or the maintenance manual for your aircraft MAY tell you what the manufacturer's standard is; it may also be noted on the FAA Form 337 for the avionics installation if one was filed).

I've seen varying combinations of which radio is connected to which antenna ("Single radio - Antenna on top" ; "Single radio - Antenna on bottom" ; "#1 Left Wing, #2 Right Wing" ; "#1 Top-Front, #2 Top-Rear", "#1 Top/Ground, #2 Bottom/Air" ; "#1 Bottom/Air, #2 Top/Ground").

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  • $\begingroup$ Great answer. My routine involves using COM1 for tower/appr, COM2 for atis/ground. The only time I vary that is when I only have 1 good radio. Bottom line, it probably doesn't matter enough to worry about it or enforce a particular procedure on a student. As long as they have something that works. $\endgroup$
    – ryan1618
    Commented Sep 30, 2015 at 15:23
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In general, while choosing the antenna locations the following considerations are taken into account:

  • Obstruction of the signal by the aircraft (components).
  • Interference
  • Structural issues like vibration and flutter.

In general, if the aircraft being operated from a towered airport is fitted with two radios, it is advisable to have the antenna for COM1 on top of the aircraft and COM2 on the bottom, with a separation of at-least one meter.

Also, usually, the top antenna is used in ground and the bottom antenna is used for air to ground communication as VHF (or UHF, for that matter) is line of sight. Note that it does not matter whether the COM1 antenna is on top or bottom; the top antenna is usually used for on-ground communication (sometimes, this doesn't work, as the tail plane might obstruct the signal). Another reason for using top antenna is that the bottom antenna could be damaged in case of hard landing.

As long as the antennae don't suffer from 'shadowing' of the aircraft, it can be located anywhere. A good example is the location of antennae in case of homebuilt aircraft from kits, where the both the antenna could be on top or bottom or even have a single antenna for the same model.

In the other extreme, the (avionics) manufacturer may prescribe a particular installation location for the antenna (usually for COM1 on top). Also, in some aircraft, the VHF radios will use both the upper and lower antenna. The signal is switched between the upper and lower antennas with coaxial relays, with the top antenna used while on ground, while the bottom one is used in air.

Also, the HF antenna is usually on the vertical tail and GPS antenna on top of the fuselage (for satellite signal reception). The antenna are mounted on the aircraft plane of symmetry or symmetrically to the either side of it (like Cessna 150).

For a short answer to your question, in general aviation aircraft, the location of com antenna is immaterial as long as there are no communication issues.

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