Antenna Sharing

MBDiagMan

MBDiagMan

Final Approach
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Mount Pleasant,Texas
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Doc
I have two identical slide in radios connected to an audio panel. Com 2 has audible static when receiving and I have not been able to test to see if transmissions have static.

I am going to swap the radios between their positions to see if the static moves with the radio or stays with the position. If it stays with the position I would like to know what is specific to the position besides, of course, the connectors and wiring.

Would it be that these radios share a common antenna or would there be a separate antenna for each?

Thanks for your help and comments.

Doc
 
MBDiagMan said:
I have two identical slide in radios connected to an audio panel. Com 2 has audible static when receiving and I have not been able to test to see if transmissions have static.

I am going to swap the radios between their positions to see if the static moves with the radio or stays with the position. If it stays with the position I would like to know what is specific to the position besides, of course, the connectors and wiring.

Would it be that these radios share a common antenna or would there be a separate antenna for each?

Thanks for your help and comments.

Doc
Click to expand...

Not being a real radio guy I can't give you a good answer but I'm sure many here can. My best guess is that if you split the antenna feed you are changing the impedance each radio will see and thus changing the VSWR for each radio not to mention what happens when you transmit on one radio and that energy is directed into the second radio without any type of isolation.
 
Usually comm antenna are not shared. It should be easy enough to count the number of VHF antenna on the aircraft. If'n there's only one then maybe it's shared, or maybe there's an installation problem. ;)
 
Com antennas are almost never shared because the transmitter power will "blow out" the front end of the other receiver. There are some configurations where an antenna may be shared with a portable device, but the portable device disconnects the other com tranceiver when plugged in.

Nav antennas are often shared with a splitter to feed both receivers. Since they don't transmit, this is not a big deal.

DME, transponder, and (almost all) GPS antennas are not shared.
 
Thanks very much for the information. I thought the antenna might be shared by means of the audio panel switching the antenna from one radio to the other, but I understand that even by it's name the audio panel probably only deals with the audio signal from the radio and possibly the headset input to the radio.

If the problem does not follow the radio, I will try to trace to see which antenna is involved with the bad position. In my experience static is usually due to a flaky connection somewhere, so I'll be trying to clean and tighten all the connections that are accessible.

Thanks again,
Doc
 
As you have figured out, the Audio panel deals with which noises (com, nav, adf, dme, whatever) go to what output device (headset, speaker) and what mic input goes to which com.

Good luck tracking down the noise source. Radios can be a real pain.
 
They could be shared but through a relay switching the antenna to the active radio. The antenna wouldn't be on both radios at the same time. TKM will provide an interconnection between radios that kills the input to the receiver of the non transmitting radio. It is much more likely that each radio has its own antenna.
 
Clark1961 said:
Usually comm antenna are not shared. It should be easy enough to count the number of VHF antenna on the aircraft. If'n there's only one then maybe it's shared, or maybe there's an installation problem. ;)
Click to expand...

Here ya go:wink2:
 
MBDiagMan said:
Thanks very much for the information. I thought the antenna might be shared by means of the audio panel switching the antenna from one radio to the other, but I understand that even by it's name the audio panel probably only deals with the audio signal from the radio and possibly the headset input to the radio.

If the problem does not follow the radio, I will try to trace to see which antenna is involved with the bad position. In my experience static is usually due to a flaky connection somewhere, so I'll be trying to clean and tighten all the connections that are accessible.
Click to expand...
.

Switching the antenna is typically never done on transmitters because you could transmit into an "open" and damage the RF power transistor during the switch. Plus, you still want to receive on the other comm and they're the same antenna.

Continuous static can also be a sign that one receiver simply can't "hear" as well as the other. That'd be in the "problem follows the radio" column in your analysis.

Likely culprits:

- crusted up tray connectors
- loose fit in the tray
- bent/loose connectors
- improperly installed BNC or other RF connectors that have an intermittent open in the center conductor
- same thing, but vibration-induced inside a cable that's not properly tied down
- corrosion around antenna base keeping the antenna from grounding to the aircraft frame properly.
- failed stage in the radio in the receiver RF pre-amplifier

You're headed the right direction. Swapping identical radios between trays helps figure out which system has the problem.

It's also a good time to look at all the contacts and they'll get a little "wiping action" moving from one tray to another.

And it's legal for the owner to do.

Keep in mind that there's test gear available for both the receiver sensitivity and the antenna at your local avionics shop. Even bad cables can be swept to see their loss and know for sure. They can nail it down to a component easily.

I, of course, would never tell you to do something a properly licensed Avionics tech should rightly do... But...

If you can find an RF guy who has a "radio service monitor" who knows how to use it (!) along with an iso-T and other various tools, the tests are pretty simple to do. ;)

Checking the antennas with an MFJ is even easier.

But I wouldn't know anything about it. The FAA says so. My IFR 1500 has never been in AM mode, and I've only ever used the MFJ on my Amateur gear. ;)
 
Thanks DenverPilot. If I had read your response first, I would not have started my Tray Connector? thread.

Now that I know both radios are good, my thinking is to get a new tray and take it with me to the avionics shop so that they have it available if they need it. I need to go there anyway because my transponder certification is due.

When I swapped the radios, I used a pencil eraser to clean the circuit board edge connectors on the radios, but had no way to clean the receptacle connectors. I was disappointed to see that the circuit board edge connector was not a gold plated contact. I expect that the problem is the BNC connector.

I have considered buying a plug in replacement flip flop radio and simply plug it into the good position. If I did that, however, it would not give me any functionality that I do not already have if both radios are working properly.

Doc
 
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MBDiagMan said:
Now that I know both radios are good, my thinking is to get a new tray and take it with me to the avionics shop so that they have it available if they need it.
Doc
Click to expand...

The tray is just sheet metal with a connector or two stuck on the back.

Connectors (at least the one's I have dealt with) have replaceable pins. I assume the avionics shop will be able to work with the existing tray.

My uneducated guess is that you have a problem with either the RF connector at the back of the tray, the antenna cable, or the antenna itself.
 
Light & Sporty,

Yes, I believe that the problem is the BNC connector or the antenna. I checked the antenna for solid mounting and good connection at the base of the antenna.

It is good to know that the shop can work with my existing tray. It sounds like something they do on a regular basis. Even as hard as it is to get to the back of it, it appears easier than replacing the whole tray.

Thanks.
Doc
 
Usually the tray is just screwed into rails on either side (imagine a "trayless" radio in the same hole) and just pulls out toward the cabin like a radio would.

Piece of cake for your shop to pull it out and give it the evil eye.
 
denverpilot said:
Usually the tray is just screwed into rails on either side (imagine a "trayless" radio in the same hole) and just pulls out toward the cabin like a radio would.

Piece of cake for your shop to pull it out
Click to expand...

You haven't seen the trays in my ride...


and give it the evil eye.
Click to expand...

Then again, perhaps you have.
 
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