Antenna Troubleshooting

[Ragsf15e]

Hello, Like everyone else with a radio problem, I don’t think it’s the radio…

Can someone tell me the proper way to troubleshoot my antenna and antenna cable?  My radio 1 is a 430W and it has significantly more noise and worse reception than the sl30 which is #2.  It also seems to be worse when I’m pointed away from the transmission.  The 430w was rebuilt by garmin in 2015 (waas upgrade).  I have had a few complaints about weak transmission in the past, but generally, nobody says anything, so I’m not really sure about transmission?

I believe the 430w uses the top antenna, but I’m not positive. How would I test this safely?

Thank you!

[KSMooniac]

You'll have to remove the headliner and trace the coax to confirm which antenna goes to which radio, unless you have notes somewhere from the original installation.  It might be corrosion under the antenna base interfering with the ground plane connection, in which case you can remove the antenna, clean everything and alodine the bare aluminum, and re-assemble.  If you really want to improve it, consider replacing the coax with RG400, which is of course more tedious, but a better long-term solution.  50 year old coax can break down and reduce your radio performance...

My J had two top COM antennas when I purchased it, and last summer I deleted the forward one (in pursuit of drag reduction) and installed a bent whip on the belly, aft of the cabin.  I ran RG400 to it, and connected it to COM 1 for airborne primary use.  I connected the remaining upper antenna to COM 2 for backup and ground use.  There was light corrosion under it, so a good cleaning and alodine treatment was done as well.

[Ragsf15e]

2 hours ago, KSMooniac said:

You'll have to remove the headliner and trace the coax to confirm which antenna goes to which radio, unless you have notes somewhere from the original installation.  It might be corrosion under the antenna base interfering with the ground plane connection, in which case you can remove the antenna, clean everything and alodine the bare aluminum, and re-assemble.  If you really want to improve it, consider replacing the coax with RG400, which is of course more tedious, but a better long-term solution.  50 year old coax can break down and reduce your radio performance...

My J had two top COM antennas when I purchased it, and last summer I deleted the forward one (in pursuit of drag reduction) and installed a bent whip on the belly, aft of the cabin.  I ran RG400 to it, and connected it to COM 1 for airborne primary use.  I connected the remaining upper antenna to COM 2 for backup and ground use.  There was light corrosion under it, so a good cleaning and alodine treatment was done as well.

I can access both antenna- one is above and slightly aft of the rear avionics shelf.  One is in the belly - bent whip as you mentioned.  Is there a safe way to just disconnect one and see which radio it’s connected to?  Id like to troubleshoot the correct one…

[KSMooniac]

An avionics shop likely has equipment to do so elegantly, but i suppose you could disconnect one, transmit on a frequency and listen on a hand held radio to see if you got the right one. Honestly, I would just clean both locations since it isn't a huge effort if you don't have to pull the headliner.

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[Ragsf15e]

15 minutes ago, KSMooniac said:

An avionics shop likely has equipment to do so elegantly, but i suppose you could disconnect one, transmit on a frequency and listen on a hand held radio to see if you got the right one. Honestly, I would just clean both locations since it isn't a huge effort if you don't have to pull the headliner.

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I don’t know why this would be, but I thought there was the possibility of hurting a radio by using it with the antenna disconnected?  Probably i was wrong…

[PT20J]

There are a couple of simple things to do. The best test is to switch the coax at the back of the radios and see if the problem follows the antenna, or if the 430W still has a problem with the other antenna. I would avoid running the radios with the antenna disconnected. The transmitters may be protected from running without a load but it's not best for it. If you want to run a test with an antenna disconnected it would be best to get a 50 ohm dummy load to terminate the coax from the disconnected antenna. 

Antennas are essentially mechanical devices and don't fail often unless damaged. I would inspect the antennas for any cracking of the antenna plastic or caulking at the base where water might intrude and cause corrosion. Coax will last essentially forever unless it is damaged by crimping or has a bad termination at the connectors. RG 400 is all the rage because it is required for GPS antennas and a of shops only stock it nowadays as it can be used for everything. However, it doesn't really offer a significant advantage at lower VHF nav/com frequencies. (People will argue with me, but I'm a EE and I didn't replace my nav/com coax when I upgraded my panel and my radios work just fine).

If you really suspect something is amiss with the coax or antennas, before tearing into stuff I would take it to an avionics shop and have them measure VSWR which should tell you the state of things.

Skip

[EricJ]

35 minutes ago, Ragsf15e said:

I don’t know why this would be, but I thought there was the possibility of hurting a radio by using it with the antenna disconnected?  Probably i was wrong…

Yes, it's generally a Bad Idea to transmit without a load (antenna) connected.    All of the transmitted power goes somewhere, and without a load (antenna) it's reflected back into the input of the amplifier where it isn't meant to be.   In many cases the additional heat can damage the amplifier.

One trick to tell which antenna is transmitting, though, is to take a handheld radio and remove the antenna and use it to listen when the airplane is transmitting.   Without the antenna attached you'll likely have to move it very close to the transmitting antenna to pick up anything, and often it is possible to tell which antenna is transmitting using this trick.   As before, DON'T transmit on the handheld with the antenna removed.   It won't hurt the receiver, so as long as you don't transmit it's okay to remove the antenna.

 

[EricJ]

31 minutes ago, PT20J said:

RG 400 is all the rage because it is required for GPS antennas and a of shops only stock it nowadays as it can be used for everything. However, it doesn't really offer a significant advantage at lower VHF nav/com frequencies. (People will argue with me, but I'm a EE and I didn't replace my nav/com coax when I upgraded my panel and my radios work just fine).

The main advantage of RG-400 is that it is very low-loss cable, especially at higher frequencies.   Any application that has long cable runs between the antenna and receiver (like in an airplane) can benefit, especially at higher frequencies like L-band  (e.g., GPS) or above.   It also has a good frequency response, so works well up into S-band, C-band, etc.

For VHF, older cable (RG-58) is fine as long as it's in good shape.   It seems like over the years many get bent, nicked, kinked, etc., or the insulation gets abraded and moisture gets inside and corrodes it or something.    I'm helping a little bit to debug an airborne camera system where the installer wasn't very careful about putting tight bends into some of the video coax and that may be contributing to the problems.

[PT20J]

6 minutes ago, EricJ said:

The main advantage of RG-400 is that it is very low-loss cable, especially at higher frequencies.   Any application that has long cable runs between the antenna and receiver (like in an airplane) can benefit, especially at higher frequencies like L-band  (e.g., GPS) or above.   It also has a good frequency response, so works well up into S-band, C-band, etc.

For VHF, older cable (RG-58) is fine as long as it's in good shape.   It seems like over the years many get bent, nicked, kinked, etc., or the insulation gets abraded and moisture gets inside and corrodes it or something.    I'm helping a little bit to debug an airborne camera system where the installer wasn't very careful about putting tight bends into some of the video coax and that may be contributing to the problems.

I agree. My point is that if the cable hasn’t been mechanically damaged, was terminated properly, hasn’t been exposed to the elements or temperatures greater than 150 deg F it should be fine for the fairly short runs in an airplane at 100 Mhz. I should have mentioned that we inspected all my cable runs before deciding to reuse the cable.

I worked as a maintenance technician at a broadcast television station during my college years where we had literally miles of RG-59 (75 ohm common for video) and we never would have thought of replacing it simply because it was old.

Skip

[Ragsf15e]

1 hour ago, EricJ said:

Yes, it's generally a Bad Idea to transmit without a load (antenna) connected.    All of the transmitted power goes somewhere, and without a load (antenna) it's reflected back into the input of the amplifier where it isn't meant to be.   In many cases the additional heat can damage the amplifier.

One trick to tell which antenna is transmitting, though, is to take a handheld radio and remove the antenna and use it to listen when the airplane is transmitting.   Without the antenna attached you'll likely have to move it very close to the transmitting antenna to pick up anything, and often it is possible to tell which antenna is transmitting using this trick.   As before, DON'T transmit on the handheld with the antenna removed.   It won't hurt the receiver, so as long as you don't transmit it's okay to remove the antenna.

 

Thanks guys that’s awesome!  So it sounds like I could also cue up atis on the 430w and disconnect the top antenna and see if it gets worse or no reception.  I wouldn’t hurt anything and could prove it’s that antenna?

Skip, I’ll try to get the motivation to get behind the radios and switch the antenna wires, but man is that a rats nest.

[glbtrottr]

Yesterday a customer with a mooney asked for help with his 430. Couldn’t really hear it well, some people would talk to him, he would transmit and his reception was intermittent. Peeled back the belly pa el fully intending to replace the antenna cable with a strand of Rg400 as a last resort - was ready to cut off the end and terminate both ends, test continuity, blah blah.

I peeled back the belly pan…

…only to find out the IA that did the last annual disconnected the antenna and didn’t reconnect it. Crisis averted….

Had another customer bring in a plane for paint, similar issue - coax cable improperly crimped peeled back and needed to be reterminated. One good crimp and c’ya.

$6/7 a foot for RG400 plus a lot of panels to remove to replace coax? Not always needed.


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[Pinecone]

9 hours ago, KSMooniac said:

An avionics shop likely has equipment to do so elegantly, but i suppose you could disconnect one, transmit on a frequency and listen on a hand held radio to see if you got the right one. Honestly, I would just clean both locations since it isn't a huge effort if you don't have to pull the headliner.

I HIGHLY recommend doing it the other way around.  Disconnect antenna and then transmit on the HANDHELD and see which one receives.   Or just listen on a busy frequency.

Radios do NOT like being keyed on with no antenna.

[EricJ]

11 hours ago, Ragsf15e said:

Thanks guys that’s awesome!  So it sounds like I could also cue up atis on the 430w and disconnect the top antenna and see if it gets worse or no reception.  I wouldn’t hurt anything and could prove it’s that antenna?

That should work, too!

 

[KSMooniac]

I apologize for the notsogood recommendation! I'm a structures guy, not an EE, even though my dad is... None of that knowledge rubbed off on me. Listening on each COM definitely sounds much better.

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[jamesm]

I know this gets negative/ null response...  I would check Antenna  ground make sure that the antenna ground and antenna bonding good and proper. Another thing I might try is that if the antenna are close if you have short piece coax swap antenna. 

pretty all that I got. hope this helps.

James '67C