radio problems

[ozzietriker]

Hi all,

 

I'm trying to solve a rather interesting radio problem on my friend's microlight.

 

Radio Transmissions & receptions on the ground while taxiing are fine (as verified by radio checks and traffic hearing me ok 15 miles away on ground), but once airborne - transmissions are still fine however receptions become very weak as if the volume was turned down all of a sudden? I verified with one other pilot on the ground then after that he heard me loud & clear on ground and in the air at all times.

 

He has a Vertex VXA-220 radio interfaced with raptor box intercom and antenna mounted on top of kingpost (similar set up to my trike, I have the VXA-150 - which works perfect). I have replace the antenna coax with good quality single strand core RG58, routed the coax well away from power/ignition cables, crimped new BNC & SMA connectors and still no joy :(

 

Can anyone please help us? :(

 

Dennis

 

 

[Harthy]

Regulator

 

I am no expert, but it sounds to me like there could be some sort of interference from the aircraft, more than likely the engine electrical. Also double check all connections again.

 

There is a very good engine regulator that is advertised in the in the RAAus mag which usually solves those problems. The man that supplies these will also be able to help you with advise on your problem.

 

Harthy

 

 

[Kyle Communications]

Hi Oz

 

Problems like these are usualy electrical noise orientated. On the ground your engine rpm is slower and of course once you takeoff the rpm is much higher and that is where the "pulse" noise is much higher. The AM receiver will be getting hammered and audio will be masked a bit. 2stroke engines are notorious for noise. We fitted radios to jet ski's once for a customer these were FM and not AM which are less susceptable to engine interference and the cdi ignition just wipes these UHF out so that were useless to receive on..they transmitted fine but bascially there was no fix for them. So I would maybe look at your regulator also if the engine has a CDI you will need to try to quieten it down.

 

Mark Kyle

 

 

[Guest davidh10]

Sounds like you could test the engine noise theory by selecting a channel that has plenty of traffic, like Melbourne or Brisbane Centre and then drop the engine back to idle and glide, while listening to the radio traffic.

 

Just a wild thought... I don't suppose your are flying near a ground based VHF radio station... they usually pump out a few kilowatts and if you fly near them (within a few kilometres) you may get issues with intermodulation distortion and the automatic gain control in the RF part of the radio will reduce its amplification due to the strong signal, thus making on-channel communications weaker. Even if neither of these factors are the cause, a very strong signal in the same spectrum, but not necessarily on the channel will swamp (saturate) the RF front-end of the receiver making it less sensitive. These effects are greater with receivers that are required to have reception over a broad frequency range (like airband radios) and don't have a tuned front end amplifier, as distinct from those that are designed for narrow bandwidth coverage. Such radios have "low Q" (broad bandwidth) tuned circuits in the front end and thus pick up all signals within the operational band, all the time, and to some extent signals outside the target band as well. Having a signal that is 1,000 times stronger than you are expecting does not need to be within the band of operation to still have a significant effect.

 

 

[ausgee]

Enough theory allready.

 

Time to apply moron math to the problem. He is the guy that cuts lumber by holding the piece up to the gap and marking it instead of doing mental gymnastics.

 

The first step is to connect the radio directly to a head set with the headset adapter that comes with the radio. If that fixes the problem it is the intercom not the radio. A known good headset is another thing to try.

 

The second step is to take the known good setup, the vxa 150 and try it in place of the vxa 220. If it works the antenna, powerful radio stations, engine electrical are not the problem.

 

Third step is optional. Move the VXA 220 to the plane where the vxa 150 is. If it doesn't work then start troubleshooting the vxa 220 and intercom interface.

 

Always try the easiest cheapest thing first. And be methodical.

 

Hope this helps.

 

 

[ausgee]

On re reading the post I noticed that you said you used single strand core RG 58.

 

You want multi strand for the center conductor. Aircraft vibration often snaps a solid center conductor at the connector.

 

Most coax faults occur right at the connectors.

 

 

[ozzietriker]

thankyou gents for your thoughts & advice, you've given me some good leads for now :)

 

Ausgee: I forgot to mention before champ that I have tested the radio as a stand alone handheld and reception volume is perfect. Unfortunately we have no other connectors/adapters as it was bought with the trike 2nd had and the previous owner knows nothing. The Radio is interfaced via a patch lead to the intercom "Raptor Box" (made by Aerial Persuits) the headset connections are RJ12. I'll be also trying to reposition the intercom, filtering techniques and perhaps to run it from a stand alone battery as a trial.

 

thanks again guys!

 

 

[Guest Dick Gower]

All of the above are very valid ideas and worth checking out but this reminds me of a similar problem that I discovered in a Pitts Special years ago.

 

Everything was fine on the ground but reception in the air was hopeless. It turned out that we had all underestimated the significant competition from ambient noise when airbourne on climb/cruise power compared with idling on the ground i.e. the receive volume just was not loud enough.

 

The headphone output on this radio (at the time a state-of-the-art COM 11A) had a source impedence that was far too high to match 600 ohm headsets (or worse: two of them in parallel) and the solution was two-part namely:

 

(a) install an impedence matching transformer on the speaker output to match the 3 ohms output to 600 headsets and therefore get sufficient audio output.

 

(b) use better noise-cancelling helmets rather than looser fitting headsets so that we were not competing with such high ambient noise.

 

After many nights of work in a freezing hangar and getting a special transformer wound I put it all together and it worked magnificently - just in time for somebody to drown it all (and the rest of the aircraft) in a large lake in central Victoria.

 

Had a similar problem in a Wirraway restoration more recently. Same solution but without the lake.

 

Regards,

 

Dick

 

 

[ozzietriker]

Hi again, well after some deeper troubleshooting last weekend I found the problem - two wires on the patch lead were connected around the wrong way. All fixed now and comms system works perrrrfectly :)

 

tailwinds!