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tomcostanza

Joined: 19 Oct 2008
Posts: 49

Posted: Tue Jan 24, 2012 4:26 pm Post subject: Antenna Installation Question

Hi,



Comant instructions for a comm antenna installation says to mount the antenna base directly to the skin for proper electrical bonding.  Makes sense to me.  Then they include a cork gasket with the antenna.  Any suggestions?  



I was planning to alodine the skin under the antenna base.  Should I put dielectric grease or anything else between the base and the skin?

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_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project

ceengland(at)bellsouth.ne
Guest

Posted: Tue Jan 24, 2012 5:01 pm Post subject: Antenna Installation Question

On 01/24/2012 06:26 PM, tomcostanza wrote:

 	  Quote: 	
		   



 Hi,



 Comant instructions for a comm antenna installation says to mount the antenna base directly to the skin for proper electrical bonding.  Makes sense to me.  Then they include a cork gasket with the antenna.  Any suggestions?



 I was planning to alodine the skin under the antenna base.  Should I put dielectric grease or anything else between the base and the skin?



 --------

 Clear Skies,

 Tom Costanza

There should be contact between the base & the heads of the mounting 
	


screws, and contact inside the fuselage from the screws through the nuts 

and lock washers to the inside of the skin. the teeth of the lock 

washers should bite through any paint/primer into the surface of the skin.



Charlie

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Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Tue Jan 24, 2012 6:33 pm Post subject: Antenna Installation Question

The cork gasket goes directly under the antenna, between the antenna and the

skin to prevent water from penetrating the skin under the base.  The antenna

grounds through the washer and mounting nut inside the skin.  This is

important as this is a ground plane for the antenna.  Clean off the alodine,

or zinc chromate from inside the skin around where the mounting nut and

washer grip the inside of the skin.



 	  Quote: 	
		  From here I'm assuming that you have a cloth plane...

Cloth planes usually have a frame structure under the cloth physically
	


strong enough to hold a com. ant. but you may need to install you own foil

ground plane.  If this is your case then make sure your ground plane is well

grounded through the outside of the antenna connector or preferably the

mounting nut.



Small planes generally have their com antenna on top of the plane... away

from landing gear and other parasitic elements like landing gear.  Being on

top is adequate to talk with towers and a good location to communicate with

other aircraft.  If you expect to fly above FL2.0 you may need a com Ant on

the bottom of your plane to contact ATC.

Noel



--

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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

tomcostanza

Joined: 19 Oct 2008
Posts: 49

Posted: Tue Jan 24, 2012 6:36 pm Post subject: Re: Antenna Installation Question

 	  Quote: 	
		  the teeth of the lock washers should bite through any paint/primer into the surface of the skin. 	




Thanks Charlie.  I know this makes a good D.C. ground, but I'm not sure how well it grounds at 100+ MHz.  Maybe Bob could speak to that.

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_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project

nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jan 24, 2012 8:13 pm Post subject: Antenna Installation Question

At 08:36 PM 1/24/2012, you wrote:

 	  Quote: 	
		  

<Tom(at)CostanzaAndAssociates.com>

 > the teeth of the lock washers should bite through any 

 paint/primer into the surface of the skin.

Thanks Charlie.  I know this makes a good D.C. ground, but I'm not 

sure how well it grounds at 100+ MHz.  Maybe Bob could speak to that.


	


It's nigh on to impossible to get the 'ideal' bond

between antenna base and skin without welding it. Further,

adding a cork gasket under the base puts a 'resilient'

material in the stack-up of parts under 'crush'. See:



http://aeroelectric.com/Pictures/Antenna/Comm_Antenna_Installation.gif



Since all electrical connections MUST come though the mounting

hardware, getting the highest practical make up forces

goes directly to gas-tightness of the connections at

interfaces marked with (*). All this happens within a few

tenths of an inch of the center of each screw. Taking all

the paint off skin under the antenna adds nothing.



I'd pitch the cork. Drill out the base holes to take at least

a #10 screw (if not already that large). Clean the mate up

surfaces marked (*) to bright. Run a thin bead of RTV around the

perimeter of the base before pressing it into place on the aircraft.

Tighten the screws to spec limits. Using grade 8 hardware

will let you get some real, lasting pressure in the joints.



Bob . . .





   Bob . . .

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email(at)jaredyates.com
Guest

Posted: Wed Jan 25, 2012 2:32 am Post subject: Antenna Installation Question

I fly airplanes in the high flight levels and even there we use the top antenna primarily. I've thought about it before, and I think a "to scale" diagram would explain why.  Even though 5-6 miles seems high by altitude standards, it's not very far laterally. If the rco or tower is 20 or even 100 miles away, the acute angle in the triangle is small. To visualize more easily, think of the distance in feet instead of miles. The angles are the same regardless of the units of course. If you stood 5 feet below your antenna's base and 100 feet away, would the antenna be visible?  Likewise at 20 and even 5 feet, it is probably visible from most angles. If it is blocked by the wing or some other piece of airplane, then it will likely not be for long in flight. 



On Jan 24, 2012, at 21:29, "Noel Loveys" <noelloveys(at)yahoo.ca> wrote:



[quote] 

 

 The cork gasket goes directly under the antenna, between the antenna and the

 skin to prevent water from penetrating the skin under the base.  The antenna

 grounds through the washer and mounting nut inside the skin.  This is

 important as this is a ground plane for the antenna.  Clean off the alodine,

 or zinc chromate from inside the skin around where the mounting nut and

 washer grip the inside of the skin.

 

> From here I'm assuming that you have a cloth plane...

 Cloth planes usually have a frame structure under the cloth physically

 strong enough to hold a com. ant. but you may need to install you own foil

 ground plane.  If this is your case then make sure your ground plane is well

 grounded through the outside of the antenna connector or preferably the

 mounting nut.

 

 Small planes generally have their com antenna on top of the plane... away

 from landing gear and other parasitic elements like landing gear.  Being on

 top is adequate to talk with towers and a good location to communicate with

 other aircraft.  If you expect to fly above FL2.0 you may need a com Ant on

 the bottom of your plane to contact ATC.

 

 

 Noel

 

 --

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nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jan 25, 2012 9:13 am Post subject: Antenna Installation Question

At 04:29 AM 1/25/2012, you wrote:




I fly airplanes in the high flight levels and even there we use the 

top antenna primarily. I've thought about it before, and I think a 

"to scale" diagram would explain why.  Even though 5-6 miles seems 

high by altitude standards, it's not very far laterally. If the rco 

or tower is 20 or even 100 miles away, the acute angle in the 

triangle is small. To visualize more easily, think of the distance in 

feet instead of miles. The angles are the same regardless of the 

units of course. If you stood 5 feet below your antenna's base and 

100 feet away, would the antenna be visible?  Likewise at 20 and even 

5 feet, it is probably visible from most angles. If it is blocked by 

the wing or some other piece of airplane, then it will likely not be 

for long in flight.



    If the antenna's radiation angle were parallel

    with the ground plane, your analogy would be

    valid. The driving principal for this discussion

    is "radiation angle". See:



http://www.hamradio.in/circuits/radiation_pattern.php



    The short story is that an ideal 1/4 wave radiator's

    angle of greatest sensitivity points up from the

    ground plane by some substantial angle. For many

    decades, ham radio operators have know that increasing

    the mechanical length of an antenna to 5/8 wavelength

    produces a much lower radiation angle. This antenna

    is NOT resonant at the frequency of operation. That

    deficiency is easily offset by a matching network at

    the base of the antenna.



    Antennas that long on airplanes would raise some

    eyebrows if not elicit a few giggles/snickers.

    Nevertheless, radiation angle variations in performance

    between top and bottom mounted antennas can be

    demonstrated. Large aircraft flying at high altitudes

    will always get a stronger measured signal from the

    bottom mounted antenna for stations having approximately

    45 degree depression angle below the plane of flight.

    However, given the line-of-sight performance of VHF/UHF

    communications, the loss of performance from a top

    mounted antenna may not be noticed from the pilot's seat.



    The telling test would be to make contact with

    a station that is barely readable on a top antenna

    and then compare quality of communication with

    a bottom mounted antenna.



    A friend of mine (who is also an electron herder/

    ham radio operator) has flown many of the heavy

    iron birds. He has observed this effect many times.

    At the same time, he has noted that when parked at

    the gate, a top mounted antenna will talk to some

    ground facilities that are degraded for the bottom

    antenna when surrounded by jetways, baggage carts

    and fuel trucks.



   Bob . . .

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