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Comm Antenna Radiation Pattern

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dreel(at)cox.net
Guest

Posted: Sat Apr 01, 2006 3:36 am Post subject: Comm Antenna Radiation Pattern

On my second test flight yesterday, The control tower's transmission during my initial climb was so weak I could just barely tell someone had transmitted.  I was climbing with the tower at my back and at a pretty steep angle due to other problems.  Except during this climb phase, I heard the tower loud and clear.  Refering to the radiation pattern in figure 13-7 of the AeroElectric Connection book, I'm theorizing that my comm antenna, which is mounted beneath the left wing/fuselage joint so that about a half foot of the bent antenna protrudes behind the left flap on my RV8A, was blocked by the wing/fuselage from radiating towards the tower.  So, I have two questions. 



First, is the null on the ground plane side of this type of installation deep enough to attenuate the tower transmission so thoroughly?



Second, would moving the antenna further aft reduce this problem?



Dave Reel - RV8A

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brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Sat Apr 01, 2006 8:24 am Post subject: Comm Antenna Radiation Pattern

DAVID REEL wrote:

 	  Quote: 	
		   

 

 On my second test flight yesterday, The control tower's transmission during my initial climb was so weak I could just barely tell someone had transmitted.  

 > ...
	


 	  Quote: 	
		   First, is the null on the ground plane side of this type of installation deep enough to attenuate the tower transmission so thoroughly?


	


It could be. There really is no way to tell without testing with 

something like a far-field field-strength meter. You should do some 

flyover tests with someone on the ground who has a receiver with a 

signal strength meter so you can plot signal strength vs. position. It 

is tough to do on the ground because of ground proximity distorting the 

radiation pattern.



 	  Quote: 	
		   Second, would moving the antenna further aft reduce this problem?


	


Any time you can put the antenna on an uncluttered ground plane with 

clear line-of-sight is going to be better but there are so many 

variables in this picture that I just can't advise you.



Again, if you can mount it on an uncluttered ground plane with a clear 

LoS it will work as good as it can. The gear legs very well could be 

distorting the radiation pattern.



-- 

Brian Lloyd                         361 Catterline Way

brian-yak at lloyd dot com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

- Antoine de Saint-Exupery

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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery

nuckollsr(at)cox.net
Guest

Posted: Sat Apr 01, 2006 4:11 pm Post subject: Comm Antenna Radiation Pattern

At 06:30 AM 4/1/2006 -0500, you wrote:



 	  Quote: 	
		  



On my second test flight yesterday, The control tower's transmission 

during my initial climb was so weak I could just barely tell someone had 

transmitted.  I was climbing with the tower at my back and at a pretty 

steep angle due to other problems.  Except during this climb phase, I 

heard the tower loud and clear.  Refering to the radiation pattern in 

figure 13-7 of the AeroElectric Connection book, I'm theorizing that my 

comm antenna, which is mounted beneath the left wing/fuselage joint so 

that about a half foot of the bent antenna protrudes behind the left flap 

on my RV8A, was blocked by the wing/fuselage from radiating towards the 

tower.  So, I have two questions.



First, is the null on the ground plane side of this type of installation 

deep enough to attenuate the tower transmission so thoroughly?



Second, would moving the antenna further aft reduce this problem?


	


    The variables here are huge. First, we know that the radiation/reception

    pattern of antennas mounted on aircraft seldom have lab-perfect

    shapes. Further, depending on operating frequency, antenna location

    and aircraft geometry, the variability from minimum to maximum

    performance can be large . . . notice "large" is not quantified.



    If you just departed an airport and had trouble communicating from

    less than 5 miles away, I'm skeptical that ordinary distortions

    of the antenna's pattern could cause the tower to drop away markedly

    in strength. They use 20-50 watt transmitters and very omni-directional

    antennas.



    You'll need to conduct some experiments to see if you can duplicate the

    condition. Then vary your heading by 20 degrees or so either side of

    the "null" without changing deck angle. If the signal strength comes

    up markedly by changing directions, then pattern is certainly a component

    of your difficulty. How's your performance at altitude for stations out

    at the horizon?



    One of the problems for testing comm antenna patterns is that there

    are no steady-on ground based signal sources like we have with

    VOR, LOC, VOT, etc.



    At RAC, we have a steady-on beacon we can use to radiate for flight

    testing antennas. The aircraft flies a zero-bank 360 out 100 miles

    or so and we plot the received signal strength from the ground

    beacon. Gives very nice pattern data.



    You may be suffering from both pattern effects and efficiency problems.

    A weakly performing antenna will display pattern effects that are

    otherwise quite tolerable.  I've had a friend stand in a clear space

    with a hand held and give me several minutes of long "aaaaaahhhhhhh" while

    I did the a merry-go-'round maneuver to check for deep nulls.



    Be sure the hand-held will tolerate this. MANY comm transmitters are not

    rated for long transmissions like this. If the hand-held has a high-low

    power setting, you can certainly do it in the low power setting.



    Bob . . .

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