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Engine interference through the radio

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Engine interference through the radio

Postby Gleee » Sun Mar 15, 2015 7:26 pm

I am getting a strange symptom of degrading radio performance in proportion to engine rpm.

When the engine is on tick over the radio tx is received ok but as I increase rpm the speech becomes distorted and ultimately becomes unreadable as I reach high rpm.

It also became apparent yesterday that when on downwind leg the radio was reported as readability 3 but when on finals became unreadable again.

Any advice would be gratefully received. I am thinking ignition related?
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Re: Engine interference through the radio

Postby Steevo » Sun Mar 15, 2015 7:41 pm

Are ATC reporting noise on the radio or just a weak/quiet signal?

Both these problems have completely different answers. If they are receiving a weak signal, then it's going to be a cable/antenna/SWR fault. If it's due to noise, then it's the age old Jabiru noisy engine electrics issue. You will find tons of articles around for Jabiru alternator/Mag interference and it really can be a chore to get rid of it. In-fact, there are a few threads on here regarding the problem.

Try to find out which of the above it is and also, please list the radio make/model and if you have an intercom then the make and model of that as well. For radio interference, it really can be difficult to get rid of all the interference in a Jab and sometimes you have to resort to expensive electronics to completely cure it. I battled with mine for ages and now have a DC to DC converter on there. I have not completely tested it yet due to time and weather but I should know how successful it is in the next few days.
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Re: Engine interference through the radio

Postby Gleee » Sun Mar 15, 2015 8:16 pm

Thanks for the reply

Atc are reporting noise and transmission unreadable at high rpm and on final. When talking to them whilst taxiing to the hold it was full 5.

The radio is a Garmin GNC 250 XL. The intercom is a DRE 244e.

Another strange thing is that I get a tone through the headsets when I key the Mike. The best way to describe it is to sing the word "boo" and hold that note. I don't know if they receive this tone though.

As I increase rpm my voice through the headset becomes distorted like I am shouting into the microphone although my voice is constant.

Cheers
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Re: Engine interference through the radio

Postby Steevo » Sun Mar 15, 2015 10:07 pm

It sounds very much like you are getting interference in to the radio circuit. There are a number of things you will have to try and find out about how the system has been installed before you can begin to work out how to cure it.

First of all, you need to find out if the wiring for the radio/intercom system is using shielded cable. This is important in any radio/intercom installation and especially important in a Jab. Some people do get away without using shielded cabling but on normal cable you have a very high chance of interference coming from anywhere including strobes, electric fuel pumps and even instruments nearby. The 'boo' sound could be feedback although that is usually more of a squeal and this is usually the result of the microphone cables running very close to the phone cables and not using shielded cable.

The 'boo' could also be a hum which is consistent with a ground loop in the system. In any radio installation all earths from the system should terminate at exactly the same place and there should be no duplicate routes back to ground.

The interference could be alternator/mag noise which will increase with RPM. You need to check all the grounding of the system. Another aid are ferrites which can help remove frequencies that will mix with the VHF range of frequencies and also using shielded cable on the mag wires to your mag switches. I have also seen installations where wires were running close to choke cables and under certain conditions would contact the metal casing of the cables causing a ground loop.

I am afraid there is no easy solution to radio noise and what works for one person doesn't always work for another. There is usually a lot of trial and error involved.

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Re: Engine interference through the radio

Postby Gleee » Sun Mar 15, 2015 10:34 pm

Thanks for your thoughts Steevo.

I have also read the threads in the archive and it would appear that trial and error is the way to go.

I am based at otherton airfield, Staffordshire

Regards

Lee
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Re: Engine interference through the radio

Postby NigelW » Mon Mar 23, 2015 8:46 pm

Please post any results. I have issues with radio noise and not sure where to start. I bought the Jab last year and other than this radio issue I feel quite happy with it. Like many the little spare time I have my preference would be to go flying so I when I can tackle this problem it would be good to have as many hints as possible. I have just fitted a Trig radio with internal intercom so I think I can eliminate that possibility.
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Re: Engine interference through the radio

Postby Gleee » Mon Mar 23, 2015 9:54 pm

I contacted the previous owner, Peter, and he said that after a period of storage such as winter layup the radio would play up for a flight or two then would sort itself out. We did another couple of hours short field landing practice this weekend and the radio was much better. I suspect that we have an issue of damp in the electrics which disappears with use.
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Re: Engine interference through the radio

Postby PKnight » Tue Mar 24, 2015 10:12 pm

Lee
From your description I would suspect that there is a poor/damp connection somewhere in the circuit. As the interference (there is always some) from the engine will increase with revs the squelch circuit, that is in either the radio or intercom, is overcome and you get the noise that you have described. I am not familiar with the GNC250XL but have you tried adjusting the squelch setting to see if you can isolate out the noise when it is at its greatest level? If you can and are still able to transmit and receive satisfactorily I would not fiddle with it any more particularly as the radio works ok at taxi speed. One place to look for a poor connection is often the jack plugs on the headset. A gentle rub with a bit of wire wool will often materially improve the connection.
Hope this helps, if not warm weather will solve a lot of electrical problems.
Peter
P.S. I am quite close to Otherton but I keep my Jab at Halfpenny Green.
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Re: Engine interference through the radio

Postby Gleee » Wed Mar 25, 2015 8:58 pm

Thanks Peter,

The radio has fully automatic squelch as such can only be turned on or off. Interestingly I don't get any engine interference when receiving.

Good heads up regarding the jack plugs, i will have a look next time.

Regarding halfpenny green; we used them for unlimited touch and go's the weekend before last to get the feel of landing a jab. Great, friendly airport. Next time your going up give me a shout and drop into Otherton for a coffee & a chat.

Lee
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Re: Engine interference through the radio

Postby Gleee » Sun Apr 26, 2015 9:30 pm

Fixed the interference today.

If, like us, you have the standard mag charging system with a rectifier/regulator, check the path of the 12v output.
I had noticed that the wiring from the output (red wire) disappeared into the aircraft and terminated at the rear of the dashboard where the various avionics and consumers got there feed.

I disconnected this cable from the rectifier and did a ground run.....all interference was gone.

Obviously with the wire disconnected the battery would be getting no charge current so we made a new cable and connected the rectifier output directly to the battery. The outcome was a steady 14.4v at the dashboard at cruise speed and zero interference.

The way the aircraft was previously wired was allowing the bus voltage to creep up dangerously high, sometimes over 16 volts but by rerouting the wire the battery will act as a buffer and supply a lower voltage.

I hope this helps someone out there having similar problems.

Regards

Lee
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