How to tell which comm antenna supplies which radio?

[justincarter]

Dim question here folks.

 

Ive a GNS340 and KX155 in my '67 F.

 

The reception (and transmission) on the 430 has been getting steadily worse.

 

The two comm antennae are in different locations on the fuselage roof, one above the rear seats and the other in the baggage bay roof. Obviously I need to remove interior panels to get to them. I don't know which antenna feeds which radio.

 

I suspect a coax or antenna / grounding issue (rather than the 430).

 

Ill slide the 430 out, inspect the terminals, clean them with contact cleaner and have a look up behind the panel to see if I can see any obvious poor connection of the coax to the back of the 430 mount tray for a start.

 

At the other end (the antenna end) whats the easiest way to determine which antenna is connected to which radio (without removing the headliner to both and disconnecting the antenna one by one- which is the obvious answer). Can I shield each antenna somehow and when I prevent the KX155 from receiving (thereby proving which is the 155 antenna) - meaning the other now is the 430 antenna ?

 

ie how to target the right antenna without taking both antennae down?

 

thanks for your help /any other tips on the matter!

 

Justin

[Guest]

Generally there are short sections of coax cable from the tray to the cable leading to the antenna. These are between the radio racks and the firewall, locate these and switch them.

Clarence

[RocketAviator]

Somewhat of an educated guess. Cover one of the antennas completely with not one but two layers of heavy duty aluminum foil then tape down the edges with a good duct tape. Warning the rape can damage paint I'm told. Go fly if necessary or i believe it will also test on the ground. One should work as before the other will have none or very bad reception. Let me know how it works.

[Piloto]

  On 6/9/2015 at 2:57 AM, RocketAviator said:

Somewhat of an educated guess. Cover one of the antennas completely with not one but two layers of heavy duty aluminum foil then tape down the edges with a good duct tape. Warning the rape can damage paint I'm told. Go fly if necessary or i believe it will also test on the ground. One should work as before the other will have none or very bad reception. Let me know how it works.

Good idea but the aluminum foil will need to be electrically grounded to the skin surface to make a difference. Another suggestion is to hold the antenna with both hands and watch for signal change.

José

[jetdriven]

Get a handheld. Wrap one antenna with aluminum foil and see if it changes the strength received by the handheld com radio.

[PTK]

Typically the most forward antenna is comm1. 

 

Keyword is "typically."

[mooniac15u]

If the antenna was installed with the 430 then the location might be indicated in the airframe log or 337.

[JaredDavis]

Do you know anyone with a signal tracer? Just make sure the coax cables are disconnected from the radios.

 

Something like http://amzn.com/B00APD16D2would isolate it fairly quickly.

[justincarter]

Thanks Guys - thats very helpful.

 

Can't find it in the (German) maint logs

 

Ill start with a check to see if there is a joint behind the radio stack to see if switching will give the answer then ill take the forward most antenna, try the foil wrap and handheld trick on that one and failing that - the signal tracer looks a great idea!

 

 

what a resource this forum is ! thanks all.

 

Justin

[cujet]

If you have a small neon bulb, sometimes they can be excited by RF. Simply wrap one leg of the neon bulb (not both) around the antenna near the top. It will illuminate on transmit. 

[Yetti]

Or a florescent bulb can be excited by enough watts.  Maybe a smaller one from a kitchen under counter lamp.   It could all be on a combiner and going through one antenna.  One of the coax could run up the pilot side pillar

[Guest]

  On 6/10/2015 at 12:06 AM, Yetti said:

Or a florescent bulb can be excited by enough watts. Maybe a smaller one from a kitchen under counter lamp. It could all be on a combiner and going through one antenna. One of the coax could run up the pilot side pillar

Highly unlikely that both comms use the same antenna, now nav radios on the other hand have a splitter to provide signal to the navs and G/S. Take a n

Look at most planes and count and figure out what antennas are for what radio.

Clarence

[Andy95W]

An old, cheap transistor AM radio held close to the antenna(s) and then keying the transmit button can overwhelm the cheap radio and produce noise. The "problem" with modern consumer electronics is that they are too well shielded internally so this may not work. Worth a shot, anyways, before removing your interior.