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07-11-2017, 05:42 AM
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Join Date: Jun 2015
Location: Boulder, CO
Posts: 130
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How to mount transponder antenna for proper grounding?
Hey folks,
Simple little question here. I'm going to mount my transponder antenna today and noticed that, like in this picture, there's a rubber gasket that will fit between the antenna and the skin of the airplane when installed:
Do I need to remove this to get a solid connection between the antenna and the skin for a grounding plane? Or is the idea that the lock washer and nut against the doubler INSIDE the plane provide enough contact for the grounding plane?
My intuition's failing me here. Thanks all!
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07-11-2017, 05:49 AM
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Join Date: May 2005
Location: X35 - Ocala, FL
Posts: 3,679
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The lock washer and nut give an adequate ground.
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Jesse Saint
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07-11-2017, 08:50 AM
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Join Date: Oct 2005
Location: Estes Park, CO
Posts: 3,947
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Ground
Quote:
Originally Posted by Jesse
The lock washer and nut give an adequate ground.
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Yes, but you may need to clean the area down to bare metal. If the washer can't cut through the paint, it won't make contact to establish ground. Check with an ohmeter to be certain.
__________________
Larry Larson
Estes Park, CO
http://wirejockrv7a.blogspot.com
wirejock at yahoo dot com
Donated 12/03/2019, plus a little extra.
RV-7A #73391, N511RV reserved (2,000+ hours)
HS SB, empennage, tanks, wings, fuse, working finishing kit
Disclaimer
I cannot be, nor will I be, held responsible if you try to do the same things I do and it does not work and/or causes you loss, injury, or even death in the process.
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07-11-2017, 09:26 AM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,597
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Quote:
Originally Posted by wirejock
Yes, but you may need to clean the area down to bare metal. If the washer can't cut through the paint, it won't make contact to establish ground. Check with an ohmeter to be certain.
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Not required or desired. This just creates a future corrosion area.
While I suspect the star washer will provide a DC ground, the antenna does not need or care if there is a DC ground or not. It wants an RF ground. Metal that is clamped together like this provides the coupling needed for an RF ground. Think of the plates in a capacitor. They are not touching but RF is conducted though it. While a simplistic example (and yes I do know how this works and the limitations) perhaps it will help understand.
Nomex suit on for the anticipated flames.
Carl
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07-11-2017, 10:01 AM
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Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 5,687
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From the "Bible" (43-13, 2B) and what most vendors recommend.
307. ANTENNA BONDING.
a. The electrical bonding of the antenna to the aircraft surface is extremely important. The conductive skin of an aircraft is an electrical part of the antenna system. If an antenna is not properly bonded to the aircraft, its pattern may be distorted and nulls in coverage may appear.
b. The electrical bonding of the antennas to the aircraft skin of a metal aircraft is best accomplished by direct metal-to-metal contact of the antenna base to the skin. A resistance of no more than 0.003 ohms between the antenna base plate and skin should be achieved.
NOTE: To achieve this electrical bonding, the aircraft paint in the mounting area will need to be removed and the surface covered with an oxide film (i.e., aluminum conversion coat) to protect aluminum against corrosion in accordance with MIL-C-5541B.
c. An alternate method for providing electrical bonding to metal aircraft skin is through the antenna mounting screws, which attach to a backing plate inside the aircraft, making electrical contact with the backside of the skin. To ensure good contact, remove any interior paint in the area where the backing plate is placed and coat this area in accordance with MIL-C-5541B to minimize corrosion.
__________________
Walt Aronow, DFW, TX (52F)
EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 1700+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
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07-11-2017, 10:22 AM
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Join Date: Mar 2005
Location: Dallas area
Posts: 10,769
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I agree 100% with Walt. This is exactly how we were taught in "Antennas and Transmission Lines" in the USAF.
Granted this was 53 years ago, but these "basic rules" haven't changed.
__________________
Mel Asberry, DAR since the last century.
EAA Flight Advisor/Tech Counselor, Friend of the RV-1
Recipient of Tony Bingelis Award and Wright Brothers Master Pilot Award
USAF Vet, High School E-LSA Project Mentor.
RV-6 Flying since 1993 (sold)
<rvmel(at)icloud.com>
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07-11-2017, 10:27 AM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,597
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Yep - that is what is says
Quote:
Originally Posted by Mel
I agree 100% with Walt. This is exactly how we were taught in "Antennas and Transmission Lines" in the USAF.
Granted this was 53 years ago, but these "basic rules" haven't changed.
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And if I was going to write a sailor proof set of instructions, it would sound just about the same.
Carl
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07-11-2017, 12:13 PM
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Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
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Carl, while the impedance to ground may be low, capacitive coupling will throw in an undesirable phase shift.
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07-11-2017, 12:18 PM
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Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,436
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It's possible that the gasket will compress, allowing good electrical contact between the rim of the antenna and the skin of the airplane, while still sealing the hole. If so, then the skin should be prepped locally as Walt described.
Dave
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07-11-2017, 12:22 PM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,597
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Quote:
Originally Posted by BobTurner
Carl, while the impedance to ground may be low, capacitive coupling will throw in an undesirable phase shift.
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No. Does not work that way at all.
Antennas with no DC connection to ground have been around for decades and I have used many of them HF and up. Here is a simple example of an antenna that is magnetally held to your metal car - so there is paint and a protective plastic cover between the antenna base and the car metal: https://www.amazon.com/dp/B00CJ37IQK...a-312080067105
Carl
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