Antenna too close?

[koda2]

After spending a number of hours scouring the archives and builders sites, I thought I had the placement for comm antennas figured out.

Dual bent whips on the belly, outside bay for separation and under the seat pan.
Since on the 6A the autopilot roll servo is in the same bay, I decided to move the center of the Commant 122 back a bit further. My seat pan is removable so I could get to it if I had to.

I didn't have the flaps mounted when I made the final decision and after the holes in the fuselage were all drilled and the doubler made, I got worried about the proximity of the end of the antenna to the flap.

Sure enough, the tip of the bent whip is about 3" from the edge of the flap when it is lowered.

The pictures are not great but I think they get the point across. I know I won't spend much time flying around with 40 degrees of flap but is the antenna to close? I hate to drill another complete set of holes.

Dave A.
6A build

 

[dpansier]

I believe your antenna position will be fine, I say this because just this week I tested a similar installation with a Comant VHF antenna mounted on a RV-7A.

The antenna was detuned slightly when the flaps were down, the VSWR was higher at both 118 MHZ and 136 MHZ but the curve did dip down to 2.5 to 1 in the center of the frequency range. Not perfect but still very useable.

The above information was gathered with lab grade Anritsu test equipment and traceable standards, I did not save the results in memory so I am unable to post it here.

Under normal operating conditions the flaps will be retracted once the engine is started and before ground communications. Inflight the flaps will only be down near the airport and the effect should be minimal.

 

[koda2]

Antenna testing

Thanks for the reply.
Your comments bring to mind other questions.
I wonder how many builders actually test their antennas once installed.

I did an ohmeter test for ground on the antenna I completed but I don't know where I would find someone with the kind of test equipment you have.
I suppose an avionics shop, or maybe ham radio dealers.

I know the VSWR correlates with the efficiency of the antenna but can you test for null or void areas as well? Do the obstacles like landing gear or steps act as parasitic elements or just absorb or block the radiation?

Dave A.
6A build

 

[dpansier]

Antenna Testing

Hello Dave,
I believe most builders check the antenna installation to insure it is electrically bonded to the airframe and have good continuity with the connectors, co-ax and radios. Use quality components and good construction practices, and in all but a few cases is all that is required to have a reliable installation.

When dealing with 1/4 wavelength antennas, as are most typical Nav, Com, and Transponder antennas, the VSWR is a good indicator of the antenna system efficiency.

Antenna ranges and simulation software are used to record the radiation pattern of antennas, both can be very costly and beyond our needs.
A low cost practical method to test for null and void areas on an aircraft is to perform a 360 degree flat turn keeping the wings nearly level while monitoring a distant ATIS or a VOR in the case of the Nav antenna. This exercise will provide a good indication if any weak areas exist in the antenna pattern. As you mentioned, metal objects such as the steps, landing gear, and other antennas can distort the pattern.

 

[elippse]

Do the obstacles like landing gear or steps act as parasitic elements or just absorb or block the radiation?

Dave A.
6A build

Typically objects in the field don't absorb the radiation. Any metal in the radiated field has a current induced in it and it, in turn, radiates a signal. When this re-radiated signal is received out-of-phase with the main signal, that is where you will have a null. There might be some loss or power absorption due to lower-conductivity material. This also works in the opposite direction with the received signal.
I like the idea of doing the flat turn while monitoring signal level; that is an excellent suggestion! What a way to make a multi-path-free antenna range!