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HT interference on radio reception

Postby Stevenbonzo » Wed Aug 28, 2019 8:18 pm

Has anyone found a way to get rid of HT interference. It’s coming through the aerial on reception not transmission. I am using an aerial inside the fin Jab 450UL. I’m using 6 ohm leads and 4 ohm plugs. I have choked the engine kill wires.
Any ideas?


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Re: HT interference on radio reception

Postby Steevo » Fri Aug 30, 2019 8:07 pm

Usually on the Jab, the HT interference gets in to the wiring, especially if the wiring on the intercom/radio are not good quality shielded wire with the shield only terminated at one end.

Possible solutions:

Check the intercom/radio wiring and ensure all the leads relating to headphone, microphone and PTT are using good quality aviation shielded cable with the braids grounded at one end of the connection. The braid should not be used for carrying any signals.

Change spark plug leads. The older Jabiru spark plug leads were known for their HT interference leakage and they were updated with a different type.

Ensure the p-leads use shielded cable with the earth running through the shield. A ferrite on there will not be sufficient if the leads are just standard cable. They really need to be 16AWG shielded cable with the braid connected to the engine and then routing that through the switch.

Shield the HT leads and ground the shield.

Change the antenna cable to RG400 or Aircell 5/7 if only standard RG58 cable is being used.

Try DR9EA type spark plugs.

Also, are you sure it is coming through the antenna? If you have a separate radio and intercom then try turning off the radio and see if it stops. This is usually intercom related. Obviously, if you have a combined radio/intercom it is difficult to se which.
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Re: HT interference on radio reception

Postby PKnight » Fri Aug 30, 2019 9:48 pm

I agree with Steevo.
A useful check to see if it is actually radiated and detected by the aerial is to see if you get similar noise through a hand held radio. If a hand held is ok that usually indicates that the interference is coming from some intermediate wiring such as the intercom connections.
Is the interference the same at all revs of does it increase with engine speed?
If it cuts in at a particular power setting that is usually indicative of the point at which the noise signal overcomes the squelch on the radio.
At one time I had significant interference but only at take-off power so quite difficult to trace!
A chunky ferrite on the intercom cables completely eliminated it.
Good earth connections on all components are vital otherwise the lazy easy (but it often works) solution is to scatter ferrites everywhere until it goes away.
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Re: HT interference on radio reception

Postby PKnight » Fri Aug 30, 2019 9:50 pm

I agree with Steevo.
A useful check to see if it is actually radiated and detected by the aerial is to see if you get similar noise through a hand held radio. If a hand held is ok that usually indicates that the interference is coming from some intermediate wiring such as the intercom connections.
Is the interference the same at all revs of does it increase with engine speed?
If it cuts in at a particular power setting that is usually indicative of the point at which the noise signal overcomes the squelch on the radio.
At one time I had significant interference but only at take-off power so quite difficult to trace!
A chunky ferrite on the intercom cables completely eliminated it.
Good earth connections on all components are vital otherwise the lazy easy (but it often works) solution is to scatter ferrites everywhere until it goes away.
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Re: HT interference on radio reception

Postby Stevenbonzo » Sat Aug 31, 2019 2:10 pm

It’s definitely coming through the aerial. It’s worse at higher RPMs. It make no difference if the coil P wires are disconnected the HT leads are new and are 6K and the plugs are 4 K.
Can you get resistor rotor arms?
Do you know what the Maximum TOTAL resistance is for the ignition HT?



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Re: HT interference on radio reception

Postby PKnight » Sat Aug 31, 2019 7:03 pm

What radio are you using? Is there a separate intercom?
Does turning the squelch up cause the interference to suddenly cut off?
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Re: HT interference on radio reception

Postby Steevo » Sat Aug 31, 2019 9:02 pm

Quick question. How do you definitely know it’s coming through the antenna?

If the interference was getting in to the wiring, you would have the same effect as if the antenna was picking it up as it would be squelched out until you receive something and squelch breaks. If the antenna was picking up the interference, you would very likely get interference on both transmit and receive. The reason Jab put the antenna right at the back was to minimise any chance of it getting interference from the engine. Although interference can travel some distance, that’s quite a way to reach the antenna directly. My guess is that it is getting there through the wiring. I have dealt with a fair few Jabs or Jab engined aircraft that have the issue you describe and I have yet to come across one where the physical antenna was getting the interference directly. But there is always a first time.

Unfortunately, the Jab engine is very noisy electrically. Most of the ignition parts come from a lawnmower so they were never designed with avionics in mind. Coupled with the fact the Jab is made of composite rather than metal, there is nothing to absorb that interference.

Every radio has a different tolerance to rejecting interference, some are a lot better than others. Older Microair radios were very susceptible to interference and even Trig is very hit and miss on a Jab engined composite aircraft. I have had numerous discussions with Trig direct on this topic who admitted the issue. They did do some independent testing just on the Jab, but I have no idea if they ever brought out a firmware that fixed it in every case.

Where I am based at present, we had 2 Jabs with the problem you describe. One was fixed by rewiring the radio (a Funke), the 2nd had a Microair and no matter what we tried, we couldn’t get rid of it. The owner eventually changed the radio to a Dittel KRT2 and that solved the issues. I had the issue on my previous Jab where during reception you would hear HT noise and sometimes it would even break squelch briefly when it was turned right up. That turned out to not be the radio, intercom or antenna. That was a CO monitor that was plugged in to the audio in on the radio. The monitor was picking up the HT interference and sending it along to the radio through the wire.

One thing to test is to turn the squelch right the way down so you hear all the white noise. You should be able to hear the interference amongst the white noise. This should vary with engine RPM.

As Peter said, use a handheld and if the handheld is not getting interfered with when it is at the back of the aircraft, it is very unlikely that the antenna is getting the interference directly.

No easy fix unfortunately and different cases can have different solutions so there isn’t any magic one size fits all solution to interference issues.
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Re: HT interference on radio reception

Postby skyjeep » Sun Sep 01, 2019 8:54 am

ferrites b.jpg
ferrites a.jpg
The HT leads need to be spiral wound inductive type, these are the later black leads they changed to. depending on where you are, you can do better with Magnecor or the BOSCH ferrite core leads. I run around 6k leads and the DR9EA plugs
Your coils might be the main source: add 2 ferrite clip over beads to each distributor centre lead and towards the coil.
each of these changes improved the situation, I do run HF radio as well.
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Re: HT interference on radio reception

Postby Stevenbonzo » Mon Sep 02, 2019 7:36 pm

Which leads are the best?


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Re: HT interference on radio reception

Postby skyjeep » Sat Sep 07, 2019 9:15 pm

radio noise suppression parts.jpg
e suppression parts.jpg[/attachment]In NZ and Australia the Bosch leads are available
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