Com radio issues...

[Lopez]

I have a pair of KX-170Bs and a KA-134 audio panel. When I transmit on #1, it bleeds over and transmits on whatever frequency is selected on com #2. The only way to use #1 to transmit is to shut off #2. I've tried 2 audio panels with no change. I've swapped the #1 and #2 radios between trays with no change. I've narrowed it down to the wiring/antennas, but I'm not sure where.

Any ideas???

[Bill Hart]

Is this a new sqwak, if so what was the last maintenance you performed? If this started recently with no other maintenance performed recently I would start by pulling both of your antennas and look for corrosion on the mounting surfaces. I would also check and clean the terminations of you annteas, I would also check and possibly re route the antenna cables.

[Lopez]

No recent maintenance, but not a new squawk either. I haven't had time to work on this bird (my 76 185) other than oil changes, for about 18 months. (too much time getting the 170 flying)

[Metal Master]

I have 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 Gauge and the radio. It was interesting that when I put an RF choke over the lead from the sender at the fuel gauge 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 gauge 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 gauge and the poor transmission. I have since seen this in several other aircraft. 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 zero 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.
I have posted this before.
Regards,
Jim

[FredMa]

What happens when you transmit on comm 2, does it also tramsmit on comm1? Shielding problems can cause the RF signal to induce a voltage into other circuits as metal master described, especially sensor circuits that convert small voltages into gauge readings. I don't think that is your problem however. Even if you had a shielding problem with the antennae or coax the transmission would still be on the same frequency. I think you have a problem with your PTT circuits. Take a look at how they are wired to each radio. I think what is happening is that your mic circuit is inducing a voltage into the mic circuit of the other radio causing you to transmit on both at the same time. Are these circuits Shielded? make sure there is no coiled wiring and the different mic circuits are not bundled together.

[GAHorn]

I hope this is not taken as contradictory, but as for the use of gaskets supplied by antenna mfr's... It is my belief that is exactly what to use. Some folks substitute other gaskets than those originally supplied and then later-on, a defect is traced to the gasket or ant. not making proper bonding to the airframe. Unfortunately, the technician making that diagnosis may be unaware of the previous substitution and blame the "mfr's gasket".

In actual fact, antenna mfr's know their product. Those ant. designs which rely upon secondary grounding/bonding are supplied with gaskets which are impregnated with conducting components. What may appear to be an ordinary cork gasket may actually be a conductive carbon-impregnated gasket. If the shipping box is unpacked and the un-informed installer sees a teardrop shaped gasket "just like the one" still stuck to the airframe, he might be tempted to save the new gasket and re-use the old.... or he may accidentally mix that new gasket with others on the dis-organized workbench.
In either case, the gasket intended by the mfr'r for installation with his antenna, doesn't make the install. Months or years later, when the owner has radio troubles, the problem is difficult to diagnose.

Follow instructions! Why buy an expensive antenna and not use the hardware/mat'ls supplied with it? (And avoid cheap and/or used antennas. Why? I personally recall a particular Bendix/King antenna sold to me by an avionics shop who sold me a used radio in accompaniment. That radio had P-static problems, eventually ($$) traced to the counter-sunk areas of the mounting screw-holes in that antenna. When new, the antenna had a carbon-base-paint inside that countersink area intended to bond with the mounting screws, and thru them the airframe. Removal (from the donor aircraft) and re-mounting the antenna on my airplane did not re-new that surface, resulting in poor bonding. The money "saved" by purchasing used equipment was partially spent diagnosing the problem.

[Bill Hart]

George you are correct with what you said. My experience has been even if you have the correct gasket and it is well sealed there is always a way for some trapped water to get between the airframe and the antenna. This leads to corrosion and poor bonding to the airframe. Anytime you have an antenna off you should clean up the mating surface, re-allodine (sp) and used a good gasket.

[FredMa]

I would also suggest sealing the hole in the fuselage through which the coax passes. It doesn't do much good to seal the outside and leave the inside open for moisture to seep right in. I would even fill the cut out completely as it will otherwise leave a small depression for water to collect in. This is what happened to this ADF antenna I found a while back. notice the brown mud around the fuselage cut-out. Water collected and formed a small puddle of standing water. You can see what the result was, the antenna part of the connector is still inside the cannon plug. I barely touched the wiring harness and it just fell off.