How to Check Your Com Antenna to find Weak Areas of Signal

[xavierm]

as I was compiling the submitted articles for my EAA chapters article, I discovered Bob Archer submitted an article. I thought it might be interesting to others.

How to Check Your Com Antenna Radiation Pattern to find Weak Areas of Signal Reception.


I strongly suggest that on any antenna installation a flight test to check the radiation pattern should be considered. In the particular instance of the Com antenna in front of the vertical stabilizer I believe there would be a significant reduction of signal in the aft direction because of the energy being reflected forward and upwards by the vertical stabilizer. On a belly mounted antenna the landing gear legs can have a significant effect on the pattern. On a wing tip mounted COM antenna I would guess that reception would be reduced on the opposite side. I do not have an aircraft of this type so I have not done the test myself. If someone would do the test and let me know the results I would be eternally grateful.

One way to accomplish this type of test follows: Climb to an altitude of about 5000 ft. and tune in to an airport ATIS signal at some range at which the signal is readable but sort of scratchy and fly flat circles and listen for the signal fading out and back in and record the directions off the nose of the signal loss. Don?t attempt this test using the volume control to set the audio level because you have to get far enough away from the station to prevent the automatic gain control of the radio from working. It would be best to have a flat surface between the transmitter and the airplane because hills and valleys and such would have an effect. Water would be good. I tried this a while back with my belly mounted antenna installed next to the access panel on my Bellanca and I had equal signal all the way around. I was about 70 miles from Palomar airport in Southern California tuned to their ATIS and over the ocean south of the Palos Verdes peninsula.

A similar test could be performed on VOR antennas by using the flag indicators on the instruments or the audio level but the audio on the VORs is pretty weak.

Good luck on our drag free and better performing antennas.

[LAMPSguy]

I would imagine there is someone here who is smart enough to figure out how to map signal strength to time, correlate that with heading and position from other in-aircraft devices. Sounds like a fairly simple data logging project...for someone who can make it happen. I'm just an idea man

[Pepe]

Interesting experiment but not much can be done to correct for radiation pattern anomalies. You have to live with what you got. Belly mounted antennas works very well for air to ground communications. While top mounted antennas works very well for when on the ground. If you look at air liners closely you will notice a COM antenna back on the belly and another forward on top. This antenna separation allows for simultaneous use of both COMs and ACARS.

Pepe