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  #1  
Old 09-13-2018, 12:57 PM
ngxflyer213 ngxflyer213 is offline
 
Join Date: Nov 2016
Location: Tewksbury
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Default Comm Antenna Problem

I've installed an VAL COM 2000 as #1VHF with an ICOM IC A200 as #2VHF, with a PMA500EX audio panel in a RV6A.

My problem is I can not recieve even the local ATIS on either VAL and ICOM, unless the squelch is tuned off, and they are connected to the Comant CI-122 mount on the left hand side of the aircraft bottom just aft of the firewall. Both radio recieve normal when their coax's are connected to the Comant antenna on the right side.

Replaced the left antenna and same problem.

I mounted the antenna to 10x18 peice of aluminum and both radio's recieve ok when they are connected this way, but if I touch the aluminum to any part of the aircraft skin the reception is lost

Both antenna coax's to skin of acft is .1 ohms

A VWSR check shows the forward power about 7-10 watts and the relected power less than 1.

What would cause the antenna to loose reception when I touch the aluminum plate the the aircraft ? If I need to install a filter why is the antenna on the right side not affected?
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  #2  
Old 09-13-2018, 01:03 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is online now
 
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Id guess the center pin on one end our your coax is broken.

Replace the entire feedline and connectors and start over.

Carl
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  #3  
Old 09-13-2018, 01:10 PM
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Check for continuity between the center pin and the outer shell and to ground. If you have continuity, you have a short and will need to repair or replace one or both connectors.
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  #4  
Old 09-14-2018, 01:11 PM
ngxflyer213 ngxflyer213 is offline
 
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Location: Tewksbury
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Default

thx for your quick replies.

I will ohm out the coax again

If either coax is bad, then why am i able receive on either radio when the coax is connected to the right antenna? if I connect either radio to the left antenna i loose reception. (I have to add a 3 ft coax to the existing coax for the opposite side to reach the opposite antenna)

Why do both radio's using existing coax (with 3 ft added) receive a good signal when the antenna is attached to a scrap of sheet metal, but if sheet metal is touched to any part of fuselage the reception is lost? When the antenna is held away from the airframe I would think it recieving ok because the coax outer shield is a path to ground.
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  #5  
Old 09-14-2018, 02:06 PM
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Raymo Raymo is offline
 
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Assuming your scrap metal antenna is connected to only the core conductor of the RG-400, touching that to ground would, essentially, short out the antenna. (I'm not an expert in this field, consider talking to Don Pansier of Delta Pop Aviation - who is an expert)

You make a good point with your testing; if it both radios work fine on one antenna with existing coax but not at the other antenna, it logically points to a problem with the other antenna. Maybe a poor ground plane or internal short?
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RV-7A - Slider - N495KL - First flt 27 Jan 17
O-360-A4M w/ Catto 3 blade NLE, AFP FM-150 FI, 1 PMag, Vetterman Trombone Exh, SkyTech starter,
PlanePower Alt, FlightLines Interior, James cowl & plenum
All lines by TSFlightLines
NSDQ

"The object of the game, gentlemen, is not to cheat death: the object is not to let him play."
Patrick Poteen, Sgt. U.S. Army
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  #6  
Old 09-14-2018, 03:08 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is online now
 
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Quote:
Originally Posted by ngxflyer213 View Post
SNIP
Why do both radio's using existing coax (with 3 ft added) receive a good signal when the antenna is attached to a scrap of sheet metal, but if sheet metal is touched to any part of fuselage the reception is lost? When the antenna is held away from the airframe I would think it recieving ok because the coax outer shield is a path to ground.

Because it can receive does not mean it receives well, or that you can transmit on it.

I can make a radio receive with a piece of safety wire - or my finger on the center conductor of the radio coax feedline.

Go find a ham that understands antennas and feedlines.

Carl
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  #7  
Old 09-14-2018, 04:12 PM
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N941WR N941WR is offline
 
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Comment antennas ground through the four screws that hold them in place.

Try removing one screw, give the hole a couple of turns with your deburri g tool and put the screew back in. Repeat for the other three holes and see how it works.
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  #8  
Old 09-17-2018, 11:41 AM
ngxflyer213 ngxflyer213 is offline
 
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The plan now is to order a new coax cable and making sure the antenna is grounded to the using a deburring tool as suggested
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