Problem with XCOM radio in autogyro MTO Sport

corrado95

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Hi all,

I have mounted an XCOM 760 radio but I have problem, I can't hear what other flyers say. I don't know if the problem is noice or what.
When I stand on the ground and even have the motor running, no problem.
I have a Rami AV529, rod antenna. I have monted it on a L-profile and riveted to the main frame. So it's mounted between my feet.
I have done a SWR-measurment and figures was: 3.0 at 118MHz, 2,4 at 123, 1,7 at 125 and lowest 1,2 129.
I have seen others that have mounted their antenna at that place with good results.
Have anyone of you experience of XCOM radio in autogyro?
Does anyone have an idea how to find the fault?

Regards

Anders
 

fiveboy

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ckurz7000

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What do you mean by "it works when I stand on the ground"? Do you mean to say that with you sitting in the gyro and the engine running you can receive OK but once you are up flying you can't hear the others?

There are a couple of questions I'd like to ask:

1) Is it a problem of reception or also of transmission?

2) Does the antenna have a ground plane?

3) What happens if you attach another radio (i.e. a handheld one) to your antenna?

4) Why don't you mount the antenna on your MTOSport like they are mounted in the factory? That would be near the rudder.

-- Chris.
 

jcarleto

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For one...3.0:1 SWR is AWFUL. 1.2:1 is pretty good. I find it odd you could get such a large differential. Typical might be .8 difference between high and low frequencies. Are you using the right kind of cable? (RG58). How long is the cable? I wonder what the high side would be if you adjusted the low side below 2.0:1?

*JC*
 

ckurz7000

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Yeah, 3:1 suggests to me that there is no ground plane as the most likely (and most frequently encountered) reason.

-- Chris.
 

corrado95

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corrado95

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I am not 100% sure but I think people have problem to hear me when I'm sending, when I'm flying

When I wrote that I have riveted the L-profile to the main frame, main frame of stainless steel is the ground plane. I have measured that the antenna's base is well grounded to the l-profile of aluminum.

Haven't tried another radio, not easy to find one.

I don't use the antenna, which is inbuilt in the stabililisator because I didn't order my gyro with a radio. I liked to save my money. The antenna is built in an all HTC's gyros, but you can't get the wire out. I have made a SWR on an other gyro with that antenna, it showed 2.4. The same as mine!

What do you mean by "it works when I stand on the ground"? Do you mean to say that with you sitting in the gyro and the engine running you can receive OK but once you are up flying you can't hear the others?

There are a couple of questions I'd like to ask:

1) Is it a problem of reception or also of transmission?

2) Does the antenna have a ground plane?

3) What happens if you attach another radio (i.e. a handheld one) to your antenna?

4) Why don't you mount the antenna on your MTOSport like they are mounted in the factory? That would be near the rudder.

-- Chris.
 

corrado95

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I am using RG58. Cable lenght is about 1m. How can I "adjust" the low side below 2.0? radio antenna is more than 1m from transponder antenna, transponder is not on.

For one...3.0:1 SWR is AWFUL. 1.2:1 is pretty good. I find it odd you could get such a large differential. Typical might be .8 difference between high and low frequencies. Are you using the right kind of cable? (RG58). How long is the cable? I wonder what the high side would be if you adjusted the low side below 2.0:1?

*JC*
 

corrado95

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Another thing I have noticed is that the tip of the antenna is very near the encoder for the transponder. When I'm starting the engine on choke, the vibrations make the antenna vibrate against the encoder. Could the metal box about 2 cm from the tip of the antenna have bad effect on the antenna?
 

PW_Plack

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Anders,

The SWR readings suggest your antenna has been cut much too short. If you graph the readings you took, you'll note that the point of resonance appears to lie above 129 MHz. To meet the factory spec of 1.8-to-1 SWR max between 118 and 136 MHz, you'd need to tune it for minimum SWR at about 128 MHz. Then, the SWR graph would look like the shape of a smile, with neither end higher than 1.8.

Since there's no way provided to lengthen the radiating element, it's likely an inadequate ground plane is causing detuning. I don't know what the frame of that gyro looks like inside the fiberglass, but you may need to bond some sheetmetal to the frame making good electrical contact, inside the body, at the base of the antenna. The sheetmetal can be thin and light, but should be an alloy as close as possible to matching the frame members. (Stainless steel, right?)

Having the antenna located that close to the transponder is VERY risky for the XCOM. Transponders usually run 150 watts or more in pulses, risking not only intense interference, but also the destruction of the sensitive circuits in the XCOM's receiver. And yes, that metal box can cause tuning issues, although normally it would pull the resonant frequency of the com antenna lower, not higher.

In general, you need to get as much separation between those antennas as practical.
 

corrado95

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As I understand, Rami AV-529 is a very good antenna. I would say it's impossible "it's cut too short". I haven't cut it. It's a standard factory made antenna.
Could it not be the groundplane that makes the SWR figures looks like they do?
I don't know how to make another kind of groundplane in this area.
The radio and transponder antennas are about 1,5 meter distanced from each other. I have read that 1 m should be enough
An interesting point do I think is: Why does it works good on the ground, standing still and engine running?

Anders,

The SWR readings suggest your antenna has been cut much too short. If you graph the readings you took, you'll note that the point of resonance appears to lie above 129 MHz. To meet the factory spec of 1.8-to-1 SWR max between 118 and 136 MHz, you'd need to tune it for minimum SWR at about 128 MHz. Then, the SWR graph would look like the shape of a smile, with neither end higher than 1.8.

Since there's no way provided to lengthen the radiating element, it's likely an inadequate ground plane is causing detuning. I don't know what the frame of that gyro looks like inside the fiberglass, but you may need to bond some sheetmetal to the frame making good electrical contact, inside the body, at the base of the antenna. The sheetmetal can be thin and light, but should be an alloy as close as possible to matching the frame members. (Stainless steel, right?)

Having the antenna located that close to the transponder is VERY risky for the XCOM. Transponders usually run 150 watts or more in pulses, risking not only intense interference, but also the destruction of the sensitive circuits in the XCOM's receiver. And yes, that metal box can cause tuning issues, although normally it would pull the resonant frequency of the com antenna lower, not higher.

In general, you need to get as much separation between those antennas as practical.
 

jcarleto

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At 3:1 SWR, you can barely (if at all) transmit or receive. Factory spec for that antenna is a 1.8:1. Much more than that and something is wrong with the installation.

Placement may be in question. That comment, "vibrations cause the antenna to vibrate against the encoder," bothers me a little. But, I think you either have a cable problem or grounding problem.

That antenna is basically a wire whip encased in fiberglass. It has a rubber "weatherproof" mount. That may mean it is necessary to connect your cable shield to the frame of the gyro at the antenna end. The screws with which you attached the antenna may not be enough. There is no adjustment for SWR on that antenna. The only way to modify it is to shorten the cable in increments and re-attach the connector to test. It should NOT be necessary to do that.

I would start by taking your cable loose at both ends and checking with a Ohm Meter for a short in the cable. Then attach the antenna and see if you get a reading between center lead and ground/shield. I do not believe that antenna has a matching transformer, so there should be no connection. Now test for connectivity between the shield/ground lead and the gyro frame. That should be a good connection. Check that there is no connection from the center lead to the gyro frame.

Attach the radio and retest SWR. A couple of questions...are you using the cable that came with the antenna or did you make your own? Did you modify the cable length significantly?

*JC*
 

corrado95

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If you have SWR of 1.2 at 129, can it be a contact problem? I don't think so. Antenna is mounted with 3 screws, under each screwhead there is a washer with teeth that grip the metal good. I also checked when I mounted, that contact to frame was good.
I have made the cable.
My guess is that the frame is not a good groundplane for the antenna, the sign for this is that the SWR is varying much.
My thought now is to make a groundplane to test with. a plate at about 0.5x0.5m and to mount it at the back seat. If it works then, I now where the problem is but I don't know how I can solve it permanently

At 3:1 SWR, you can barely (if at all) transmit or receive. Factory spec for that antenna is a 1.8:1. Much more than that and something is wrong with the installation.

Placement may be in question. That comment, "vibrations cause the antenna to vibrate against the encoder," bothers me a little. But, I think you either have a cable problem or grounding problem.

That antenna is basically a wire whip encased in fiberglass. It has a rubber "weatherproof" mount. That may mean it is necessary to connect your cable shield to the frame of the gyro at the antenna end. The screws with which you attached the antenna may not be enough. There is no adjustment for SWR on that antenna. The only way to modify it is to shorten the cable in increments and re-attach the connector to test. It should NOT be necessary to do that.

I would start by taking your cable loose at both ends and checking with a Ohm Meter for a short in the cable. Then attach the antenna and see if you get a reading between center lead and ground/shield. I do not believe that antenna has a matching transformer, so there should be no connection. Now test for connectivity between the shield/ground lead and the gyro frame. That should be a good connection. Check that there is no connection from the center lead to the gyro frame.

Attach the radio and retest SWR. A couple of questions...are you using the cable that came with the antenna or did you make your own? Did you modify the cable length significantly?

*JC*
 

jcarleto

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It really doesn't take much of a ground plane. I'd be surprised if even the metal components for a fiberglass gyro would not be enough. I'd still put a VOM on those antenna leads.

That said...putting a ground plane in a fiberglass body is fairly simple. I've flown many an all-glass FW with a few strips of aluminum tape stuck to the inside of the body for a ground plane that worked well.

*JC*
 

corrado95

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I just talked to a radio specialist. He think the problem is encoder. This case is ground for the antenna and is situated at the tip of the antenna. He think that is what makes the funny SWR reading. It should not be any ground near the antenna rod he says. I will now move the encoder and measure again
 

PW_Plack

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The radio and transponder antennas are about 1,5 meter distanced from each other...Why does it works good on the ground, standing still and engine running?

Anders,

Sorry for any misunderstanding. When you indicated the antenna was contacting the "encoder," I thought you meant the transponder antenna.

On length, if you were starting from scratch with a custom antenna, you'd begin with a whip which was longer than needed, and trim until the lowest SWR occurred just below the center of the desired operating band. Since yours is pre-cut and not adjustable, yes...ground plane issues may be affecting tuning, as I said.

If the antenna is very close to the ground when parked, the tuning could vary after you get airborne, especially if the ground plane is inadequate.
 

ckurz7000

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All right, the picture becomes a bit clearer now. Here are my suggestions:

1) Ideally, a quarter wavelength antenna should not "see" any metal (except its ground plain) within at least a quarter wavelength. So the metal case of the transponder is definitely too close and causing some detuning. If you've mounted the antenna somewhere inside the nose cone of the MTO Sport, the entire instrument panel is made of metal! That would be a very bad spot, indeed.

2) The ground plain should ideally measure about a quarter wavelength around (square or round would be best). A single L-profile, though it probably works to some extent, is certainly not optimal. What I've done in fiberglass planes is to glue some aluminum foil to the fiberglass and use it as a ground plain. Don't forget that the direction the antenna is pointing with respect to the ground plain sets the direction of preferable emission(reception. If the antenna points UP from the ground plain, the emission/reception pattern downwards will be bad.

3) If you've made up your own cable, go back and check that the connections between the cable and the BNC connectors is absolutely perfect. No strands of loose shielding, no sloppy contacts, no kinks in the cable, etc. There's many ways a BNC connector/cable connection can look right but actually be the source of endless grief. If you've never done it before, get someone experienced to put on the connector for you or buy a prefabricated cable to test.

Once you've revisited those points, I think you'll find that your problem has imporved considerably.

-- Chris.
 
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