Comm Receive and Transmit Issue

[robcaldwell]

RE: Avidyne IFD-540

I also posted this problem on the Avidyne Forum. Not much traffic there, so I thought I would try this non-Bearhawk build question here since our community has always been very helpful. And I appreciate that very much!

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I am almost ready for the first flight of my Bearhawk Model B. I've been performing some taxi testing and calling for random radio checks occasionally. Those that can hear me are relatively close. Near the runway the AWOS antenna is about 2500' away and in my line of sight. I cannot hear the AWOS unless I have the squelch on. Friday we did a high speed taxi test down the runway. The plane behind me on the taxiway heard our call to take the runway. However when I turned off at about 4,000' he could not hear when we called clear of the runway.

A friend of mine called his avionics buddy. He suggested that I install a ground strap to one of the mounting bolts to my belly mounted Comant VHF CI-122 antenna. Effectively grounding the antenna to the airframe. He says they do that on all of their antenna installations. I did. You can see the #10 screw and washer just 4" to the right of the antenna base where I connected a 10ga copper wire. This did not change anything. I am still having trouble receiving and transmitting.

I am using RG400 coaxial cable and RG400 BNC fittings.

Today Amazon is delivering a SWR meter and I will test the signal at the antenna BNC connection.

Any thoughts on how I can improve my reception and transmissions? I REALLY do not want to move the antenna if I can help it. Other than GPS pucks, I do not care for top mounted whip antennas. If you've seen my head, you know my aversion to hair!

Thanks for any suggestions!

IMG_3096.JPG

[Pete Campbell]

Rob, chasing antenna problems can be frustrating, I have a 540 in my rv 10 with the com mounted on top a get signal fine. My dynon back up com has the same antenna you have mounted under back seat and if using it on ground I have mixed results. U may have a connection problem but I bet once you’re airborne all will be fine. Been a lot of discussions over time on the vans forum about belly mount versus top mount, many have noted problems with reception on bottom at different airports on the ground only. Best of luck chasing problem. Pete

[robcaldwell]

I'm kinda thinking the same... As soon as I am 100' off the ground, I'm sure the thing will work. It seems I can't raise a Murphy Rebel 4,000' behind me on the runway, but a King Air a mile out in the left downwind gives me a "Loud and Clear" radio check.

[whee]

Avionics issues are a pain. I don’t have any problems with my radio and I used the bent whip antenna in the same location. I’d try flying with it and see what happens.

[Nev]

I can’t contribute anything unfortunately Rob, but I do have the same aerial.
Do you know if it makes a difference if I mount it on top ?

[robcaldwell]

I'm going to guess and say that you will probably have fantastic results with a top mounted comm antenna! But don't know for certain. Comant VHF antennas are well know for their performance and reliability. Probably not so much on the bottom of an aircraft in ground operations...

[gregc]

You are on the right track. You have a good electrical ground via the ground strap/antenna mount. There is an adequate ground plane in that location provided by the aluminum skin/stainless steel tunnel. RG400 coax is fine. So my next steps would be:

- double check all the coax connections
- insert an SWR meter in the line and measure the SWR. You are looking for less than 3:1. A good reference is https://www.arrl.org/files/file/Tech...f/q1106037.pdf
- If everything checks out and you still have poor results, you could borrow an RF signal strength meter and measure the antenna radiation pattern by walking around and mapping levels while someone keys the transmitter. 1/4 wave vertical anntenas should have a roughly omnidirectional pattern.

Also I would experiment a bit on the ground with a handheld radio. You should receive the AWOS signal in the airplane at least as well as you do with a handheld.

[Bdflies]

Being "Folically Challenged" myself, I understand Rob's resistance to top mounted appliances... Seeing the Comm antenna mounted under the fuselage, right between the gear legs, kinda raises concerns though. I'm pretty handy with electrical stuff, but electronics remain mysterious. I understand that metallic structures (especially grounded ones) are pretty good at blocking RF signals. It appears that your installation probably is blocking (partially at least) signals in nearly every direction but directly in front. The fact that a plane on the runway, behind you, couldn't hear you doesn't surprise me. The fuselage and tail feathers could be acting as a shield. Was the King Air in front of you, when you transmitted? Try directly facing the AWOS transmitter, note the reception and then turn 180 degrees. Any difference?
It'll probably work fine in the air, but it's nice to be able to communicate with aircraft on the ground, not to mention ATC.
One last mention about the bottom mount; If you plan to do much Off-airport ops, you might find a lot of grass stains on that whip.

I've been watching your videos, Rob. Great stuff!

Bill

[jcowgar]

I have often wondered about belly mounted antennas. I'm no expert by far but being an Amateur Radio Operator I know that proximity to the ground has a profound effect on radiation angle. I also wonder about the rest of your aircraft being in the way in regards to line of sight for aircraft (and other radios, AWOS, tower, etc...).

Proximity to the ground aside, I think that Bill has a great suggestion to try facing the AWOS transmitter and see if that has any effect. My guess is it will, and if so, I would say that the belly of your aircraft is blocking a large path behind you. If you can hear the AWOS when facing it, I wonder if you were to point your tail to the AWOS transmitter where you can only hear it with the squelch being off, and then lift the tail to a flying attitude to get the tail a bit out of the way. If that makes a difference as well, I'd venture to say your only option is to move the location of it. It seems the antenna is mounted in such a way that it is on an upward slope in the front of the aircraft. Other belly mounted antennas I have seen that were effective were on a part of the belly that has as much visibility around it as possible.

You certainly want to be able to hear AWOS, other aircraft in the traffic pattern, etc... and you want them to hear you as well!

[noema]

Very curious if your antenna setup works out. I also would rather not have top mounted antennas to make dealing with wing/fuselage covers easier.

[whee]

I took a closer look at your picture and note that your antenna is mounted further aft than mine. I essentially copied Battson's installation where the antenna is mounted as far forward on the tunnel as possible. This keeps the antenna in front of the gear legs vs between them like yours. Maybe this makes a difference?

So far I've had zero trouble with ground communications; I transmit and receive ATC ground crystal clear. When taxiing to my hangar the tower is 2500' behind me.

At first, I had transmitting issues that I thought were antenna related which is why I installed this bent whip and stopped using my homemade internal antenna. Turned out it was settings in the radio and intercom that needed adjustment so they would play nice with each other. Since yours are both TX and RX it sure seems like an antenna issue which is why I think you should fly it and see. If coms clear up when flying then you know it's likely an antenna issue. If they don't clear up then it still could be anything.

[jaredyates]

The ham radio guys other than me can get really serious about antennas. I do have a little SWR meter if you need to borrow it Rob. I only run the top-mounted vhf antennas on the Bearhawk, but almost all of the transport category planes that I have experience with use one antenna on the belly for com 2, and one on the top for com 1. On the ground, it's not uncommon at all to have to switch to com 1 to hear around buildings, hills, or other obstructions.

[robcaldwell]

Yesterday I taxied right up to the AWOS transmitter with the nose facing it. I did not get receive a signal until I was about 200' from it. I was also 1 taxiway over from a Cherokee that was taking the runway for departure. I never heard a single transmission from him, and I'm pretty sure he wasn't NORDO.

I think whee is right. I will move the antenna further forward to get it away from the gear legs as much as possible.

Thanks jaredyates I just received my SWR meter from Amazon yesterday and will be testing the signal soon.

You guys have been a great help!

[davzLSA]

Hi Rob, if that swr meter has a power function as well check the power output. I have worked with radio for over 25 years and it seems you lack a connection between the radio and the antenna. The swr and the power readings will confirm this.

[jcowgar]

If you are not getting the AWOS signal until 200' from it, I do not believe it has to do with antenna placement or even SWR. While a good RX antenna has a good SWR, you should be able to receive the AWOS with a paper clip stuck in the antenna jack from more than 200'. Something else has to be wrong :-/

[robcaldwell]

I agree. Later today I will disconnect the antenna cable at the back of the radio and connect it to a handheld and see what happens. That should tell me if it is the radio, or the antenna.

[gregc]

I agree 100% with jcowgar. At 200' and line of site you would receive the AWOS with any kind of antenna at all, located anywhere on the plane. Either the antenna connection is bad or the radio has a problem.

[jcowgar]

I was thinking about this more, if you have a Multimeter, set it to Resistance and w/the antenna and radio unplugged and see if your coax is shorted. Also, if your leads will reach make sure the shield and center conductor are actually making contact at each ends of the cable. Jiggle the wire around as well and try again.

Is this a commercially made antenna? If not, it is easy to get a short when installing the connectors.

P.S. I really have enjoyed your videos, thank you for taking the time to produce/publish them.

[whee]

I agree with jcowgar I think. I’d do some more diagnostics before poking new holes to move the antenna. Testing the antenna with a handheld, hooking the radio to a temporary antenna, and I’m not sure what else but I’d try some more things before moving the antenna.

[jaredyates]

If you end up needing to crimp on a new end to your coax, I can help with that. Although I don't have any ends on hand, but I have the tools. I also have a factory-terminated 50' rg58 cable with BNC connectors if that helps with troubleshooting. I'm not sure if the radio uses bnc?

[schu]

Rob, divide and conquer. Swap the radio with a handheld, see if that works, if it does it's the radio, if not swap the cable, if that works, it's the cable. If neither work, it's the antenna, which could be defective or because it's in a poor location for ground ops. I doubt it's poor enough to warrant your results, so if you find it's the antenna, then it could be internally shorted or something. You could also key the portable under the airplane to test the location. I'm betting on cable at this point.

schu

[predragvasic]

In other words, isolate the problem by eliminating components one by one until you hit the culprit.