Terrible Transmission above Idle

[Griphos]

I'm still trying to figure out my radio problems and could use a little more help.

Here's my system: Icom 24a connected to a Flightcom IIsx intercom. The handheld is connected by coax cable to a Advanced Aircraft Electronics 5I dipole antenna (http://www.advancedaircraft.com/index.htm#ourproducts) mounted inside the frame behind the baggage area in my 11AC Chief. It is mounted on a balsa strip with the bottom of the strip about mid-plane on the bottom cross tube right behind the baggage area and the top mounted to a corner of a longitudinal and cross tube at the top of the frame further back. So, it tilts a bit to one side, and about 45 degrees from vertical or horizontal, but mostly in line with the longitudinal axis of the frame, rather than the plane of rotation of the propeller. My understanding is that it would be mounted better if the bottom were on one corner of the frame and the top on the opposite top corner more in plane with the propeller rotation.

My problem is that although I have great reception, often hearing traffic from planes in patterns at airports up to 100 miles away, transmission is terrible, and I mean terrible, if my engine rpm is anything more than idle. People frequently complain! Once I'm in the pattern and at idle, I can call base and final and I appear to be clear as a bell (and there is no noise for me as well). At cruise rpm, I can barely hear myself in my own headset for the noise.

I have Slick mags, and I believe these come with shielded cables, right? I'm pretty new to all of this.

I'm happy to try moving the AAE antenna to be more crosswise the frame, or even to install an external whip antenna either in the wing-root fairing or from the belly, but I'm thinking the problem isn't antenna reception, it's noise, and I don't know how to get rid of it. There's no other electrical system in my plane. I have a handheld GPS that I never use. Reception is great! I typically fly low and slow, so I often can't pick up AWOS until I'm about 5 or 10 miles from the airport, but then I'm often 1000 agl or less when this is the case. And, as I said, I can hear planes calling distance from airports that are very far away, even flying low (because they are higher than me, of course). Reception is always clear. Talking on the intercom is no problem. But when I key the mike button, and the rpm is anything above 1000rpm, then the static is terrible.

I've searched and searched the internet to figure out about shielding and engine noise. I've asked a few local guys. I haven't really been able to find out squat.

I just don't want to buy and mount an external antenna and still have the same problem.

Any ideas/suggestions?

[Lostdutchman]

You might try grounding the antenna. I really hate electrical stuff but that might help. shielded p-leads and/or noise filters may help. Its all black magic to me but these things have helped me in the past. hope this helps

[Griphos]

Because of the kind of antenna this is, I'm not sure how to ground it. In fact, I think it's not supposed to be grounded, if you mean provided with a ground plane.

You'll have to tell me what a shielded p-lead is. And where would I put a noise filter?

[Lostdutchman]

The shielded p-lead has a wire braid around the center wire. The shielding is usually grounded to the magneto. The noise fliters hook inline with the p-lead at the magneto. I will have to look into the antenna a little more. I tend to stay out of the avionics side of things. Spruce carries the shielded wire as well as the magneto filters.

[Griphos]

I'll take a look at Spruce for them and then look to see whether I have them already. I thought Slick magnetos come with a shielded harness but maybe I'm wrong about that.

EDIT: hmmm...I couldn't find shielded p-leads at Spruce. I found p-leads. And the filters I found seem to be for alternators or generators. I don't have an electrical system.

My confusion is mostly about why the problem is transmission. I had thought that RF noise, such as from the ignition mostly affects reception.

[colie85857]

do you have a muff of some kind on your headset mike, if not put one on it to cutback or eliminate background noise while transmitting. and use the squelch on the radio and the intercom. you should then have no problem.

[Griphos]

No, that's not the problem. I have a muff, and I have the squelch set well. As I said, there's no noise when transmitting on the intercom. It's not squelch noise, and it's not noise inside the cockpit that I'm picking up, it's noise caused by something interfering electronically, I think, when rpm's are high. It happens as soon as I press the PTT button, and whether I say anything or not. And others hear it as well, complaining that my radio is terrible. But the noise is not there when I cut the throttle.

[CptKelly]

I think I would do two things: 1) mount the antenna outside on a ground plate, and 2) install noise suppressors on the mags. Gail's Champ uses the same radio, and we have none of your issues.

Mike

[Griphos]

If I were to do an external antenna, then would you recommend the wing root, or the belly?

As I said, though, I worry that the issue will remain, since I receive well, just can't transmit when rpm's are high. If it were the antenna, wouldn't reception suffer? Clearly, the antenna is picking up signals well. It's just sending signals with lots of noise. And I'd rather not drill holes in the wing root or belly if I will have the same problem.

[RickH]

Do you have the ability to plug the headset directly into the radio instead going through the intercom? That would be a way to see if something the intercom circuitry is picking up the engine noise.

[Griphos]

Yeah, I can do that. I haven't tried it, because the intercom doesn't have the noise when in use, but I'll give it a try and see.

[Sgt. NCSHP]

I had the same issue and was using an external antena. I could communicate just fine talking to passenger through the intercom at cruise but nobody could understand me transmitting. I unplugged the intercom and PTT switch and put the rubber ducky antena on. Transmitted without inercom or headset. No problems. I plugged everything back up and borrowed a set of Dave Clark headsets and at full throttle the FBO could here me clearly. I switched to SoftComm headset and have not had anymore problems.

[Griphos]

Hmmm...this is interesting. I will definitely try the straight to radio hookup, and then will put the rubber antenna back on and try that without headset and see. I have a pretty new Lightspeed headset that I'm using. It's weird to think it could be headset specific. But I've got a DC headset around somewhere so I'll try that as well.

[jc pacquin]

My antenna is mounted outside over the cockpit, on top. I use a DC headset with a muff,connected directly to the radio. There is a slight clicking but other wise it works very well both ways. I also have the same model radio which has an "AN" button that you push to take out static. Without this pushed to on (it shows on the radio dial as being on) its not so good. More static. I don not have an intercom as I fly alone most of the time. JP

[MikeB]

One of the Champs on our field with a A65 and new Slick mags also had a noise problem. He wound up putting filters on it. I think from Wag Aero which I notice are only PMA'd for Bendix mags (for whatever reason). I guess that solved his problem but looking at your posting, think I'd try to eliminate the intercom and maybe the push to talk first.

MikeB