I have a Narco 12E and a KX-170B radio in my plane. They are switched thru a KMA-24. Both have always worked well until my recent camping trip to Oshkosh. During the trip the plane was exposed to several heavy rainstorms. Now I am having problems with the transmission and reception of the Narco. ATC reports that the transmissions from it are hard to understand or of poor quality. However, the KX-170B continues to work well, i.e. when ATC said they could not understand transmissions on the Narco, upon switching to the KX-170B ATC could understand. I have two antennas mounted on the upper wing surface above the cabin. I have always assumed they were for the radios. One for each radio. But I haven't actually traced the cables yet. I do note that one antenna base has old RTV sealant around the base. That sealant has seen better days. Does anyone have any ideas that I could check before taking the plane to an avionics shop?
Thanks,
Pete
Last edited by ptporebski on Thu Sep 15, 2011 11:17 am, edited 1 time in total.
Sitting outside in the rain...and the next flight has poor transmission quality?
Points strongly at a leaky/wet/corroded antenna-base and/or connections....OR... if an old fiberglass antenna (with a foam-filled base-fairing), it may have lost it's gel-coat and be saturated/waterlogged.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention. An originality nut (mostly) for the right reasons.
I don't know how the antenna actually works. I always thought it was just mounted. Is it in fact supposed to be electrically bonded/grounded to the base structure?
Absolutely! That's how it provides a "ground plane" for broadcast.
The interface between the antenna base and the aircraft should be electrically bonded. (This is how modern antennas produce "gain" and why they are superior to the old single-element or "wire element" types.)
A VERY short article (limited actual technical info) is found in Comant Antenna installation instructions, but it will serve the purpose of this discussion:
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention. An originality nut (mostly) for the right reasons.
AFTER CONFIRMING that bought antenna are actually the com antenna. And assuming you can easily get to the back of the antenna through your headliner. And assuming each coax has enough length. You could temporarily switch the antenna for each radio at the BNC connector at the base of the antenna.
Then see it the trouble follows the antenna or stays with the radio. Even if only one coax cable will reach the other you can see if switching it improves the bad radio or makes the good one bad.
The problem could also be in the BNC connector if it's dirty or loose and just fiddling with them could clear it up. I'm thinking the base of the antennna for the radio has lost it's ground. Remove it and clean it and perhaps wear it would be grounded to the aircraft skin. A green scotch brite might be appropriate.
CAUTION - My forum posts may be worth what you paid for them!
Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
I’ve had some strange issues over the years with aircraft radio antennas. The first I bring up was with weak radio transmissions. However when the Mike button was pushed the Fuel Gage would peg. This was in a 172P. Obviously there is no connection between the Fuel Gage and the radio. It was interesting that when I put an RF choke over the lead from the sender at the fuel gage the pegging could be diminished but only on some frequencies. The RF choke helped but did not relieve the problem. Checking the bonding at the base of the antenna it was found to have good connection and ground with the airframe. However the BNC receptacle on the base of the antenna was found to not be making good connection with the metal base of the Antenna. So that even though the antenna was well bonded to the airframe the Connector which is part of the Antenna was not. So why was the Fuel gage pegging. The RF (Radio Frequency) transmissions were being strongly transmitted inside the airframe. The Antenna was internally defective. Replacing the Antenna repaired the problem of both the pegging of the fuel gage and the poor transmission. I have since seen this in several other aircraft since then. In each case one of the Radio antennas was found to be defective. In the other cases of poor transmissions poor bonding of the Antenna to the airframe was almost always traced to be the cause. Having an avionics shop work to determine this by having them troubleshoot the radio first can get expensive for no good reason.
The following is for the swept antennas of any manufacture that have a fiberglass or plastic shell
The way to check for this issue is to remove the Antenna completely. While on the bench check continuity from the BNC connector Barrel on the base of the antenna to the flat metal base on the bottom of the antenna there should be no resistance or “0†ohms. This is not the center electrode of the Antenna but the 5/16 inch barrel that has the lugs on it for the BNC connector of the Coax.
Some Antenna manufacturers send a rubberized cork or rubber gasket with the new Antenna, do not use these. Do not put silicon or any other sealant between the Antenna and the airframe.. It is even better if there is no paint between the Antenna and airframe. Any insulator between the antenna and the airframe can cause problems if the bolts and screws holding the antenna to the aircraft become corroded or loose. Only seal the edge of the antenna to the airframe.
A&P, IA, New owner C170A N1208D, Have rebuilt some 50 aircraft. So many airplanes, So little time!
I recently replaced both com antennas on the 170s successor (1957 172 ) because of weakness both recieving and transmitting. The number two improved only a little; the old antenna was a pre-Cessna ARC from the late '50s or early '60s. The number one had a much newer fiberglass type. Replacing that one resulted in a much greater improvement, go figure. I used two DM-170s so at least now they are both the same type. I made sure the backing plates had a good ground against the inside fuselage skin by removing the goop Cessna used for insulation/oil canning.
) because of weakness both recieving and transmitting. The number two improved only a little; the old antenna was a pre-Cessna ARC from the late '50s or early '60s. The number one had a much newer fiberglass type. Replacing that one resulted in a much greater improvement, go figure. I used two DM-170s so at least now they are both the same type. I made sure the backing plates had a good ground against the inside fuselage skin by removing the goop Cessna used for insulation/oil canning.