Light Aircraft Association

Hi all. I recently bought my first aircraft. It's a total dream come true and I've been flying it at every opportunity I can, but there's one issue which is really causing me grief - the radio is really bad.

It's an Icom IC-A21 running through a Lynx intercom. It seems to receive OK, not great, but the transmission is extremely poor. I was up again this evening and Farnborough could barely hear me and made me repeat everything twice over. It's embarrassing.

I'm OK with technical stuff, but I don't know much about radios. Before I replace the whole lot, could anyone offer me up some advice? I've cleaned the contacts at each end of the aerial wire and there's a good clean ground with the airframe. The aerial looks OK (?). I've tried different headsets and its makes no difference. The transceiver itself seems fine, but it's obviously an old unit now. Do they go bad over time? I don't have access to another to check.

Any thoughts on how to get to the bottom of this would be much appreciated...

Also, for what it's worth, I also tried Farnborough North whilst on a flight up to Shotteswell last weekend. All I could hear from the ATCO was intermittent fragments of his transmission, whereas I could clearly hear transmissions from other aircraft.

If this is a handheld, how does it work out of the aircraft? It could be many things affecting transmission quality such as a poor antenna cable, poor antenna installation etc etc. What aircraft is it fitted in; that is metal or wood?
Tony N

Hi,
firstly can u get hold of another handheld and see
if it makes any difference!
What interface are you using? If you have a powered
interface you have to open it up and
adjust the jumpers to match the radio.
That is exactly what I had to do with mine
using a microavionics power interface and lynx
headsets.

What is your power source? Garbled TX with OK RX is often an indication of low battery power.

Looks like the power comes from the Lynx intercom system. I presume it once worked fine because it looks like it's been in there a while!

I should check the output of the power plug. Unfortunately I don't have a second unit to test.

The aircraft is an Avid Speedwing, and the antenna is grounded to the airframe, just aft of the decklid.

Showing my ignorance of the type perhaps but isn't the avid tube and fabric? If so the antenna would need a ground plane, not just earthing to the frame.

I had a cH601 with virtually the same setup (A2 not A21). It was actually capable of better range than my friends certified setup!

But the antenna does need a large ground plane - easy with an aluminium airframe rather harder with tube and fabric. Try and borrow a SWR meter (they just plug into the output lead - this will read what the transmit power is. A problem there and its upstream, good power - then it is the antenna or ground plane. Best of luck.

Hi James,
your problem has been highlighted in a good few postings on the forum over the years, it may be an idea to trawl through and look at the solutions. However, as previously said by Colin you need to find out if it has a groundplane (normally a piece of thin copper or aluminium or radiated metal strips from the antenna base - normally the same length or radius of the antenna) this is probably the most important feature that some builders fluff at the last minute. It is a good idea to check another transceiver, but unless you've got a mate with a similar unit, it's probably not an option. Check your voltage supply to the radio and the intercom check to see if it deteriorates as you transmit and look at how the co-ax is fitted and the routing through the airframe. If you can, try to test just using the radio as a 'standalone' with the whip aerial and see if anyone picks your transmission up from the ground.

It's all common sense really, but as most pilots I know, don't have much of a clue how their radio or antenna actually works in a fixed position, it is a baptism of fire I'm afraid.

regards,

Andy

OK - making some progress on this... I pulled-out the Lynx intercom system and fitted a GA battery unit. It's much clearer and a lot of the hiss has gone. All that's left is a ticking / rotor RF noise originating from the ignition system. It matches the revs of the motor and disappears when the motor is off.

I bought some ferrites and put them on the magneto switch wires, then the tacho pickup, then various random places but I just can't get rid of the interference. Any ideas?

James,

I assume you have screened plugs and HT harness? Eliminating ignition interference can be tricky.

Sometimes using screened wiring for the mag switch leads helps a lot.

Is your engine bonded to the airframe properly?

Do you get equal levels of interference on each mag?

Tony nowak

Hi Tony. I'm not sure if the plugs and leads are screened - I don't know what to look for! It's a Jabiru 2200 if that helps? The mag switch leads are definitely not screened. I put ferrites on them but it made no difference.

I'll check the engine is bonded to the airframe. I presume it's OK because the starter motor cranks fine, and that draws about 600 amps back to the airframe.

I get exactly the same interference whether I'm on left, right or both mags.

Screened mag leads to the switch are an absolute must - they are connected to the mags' points which spark every opening, despite a capacitor fitted across them. Without screens they act as a great transmitter aerial!!

Fix that first.

I have screened leads and no noise but once the earthing connection of the shield came just a bit loose and the radio noise was horrendous - all went quiet with a re-tightened screw.

Steve

I agree with all the points about screening and earthing. I can't help but go back to my earlier comment about the groundplane too as nothing you have said suggests that it is effective.

A good antenna/goundplane will provide a cleaner/stronger input to the RF circuits and improve the signal to noise ratio, allowing the reciever to operate at a lower level of amplification, i.e. more signal = less hunting for something and amplifying the garbage along with the signal.

Just a thought, but it has to be worth trying out?

James,

Simplistically speaking, screened plugs will have a metal, screw on, HT lead connector. Plugs won't look like regular car spark plugs. The HT leads themselves will probably have the outer braiding visible, sometimes through a thin layer of protective coating.
As other posters have pointed out, eliminating engine interference will be a combination of a number factors:
Screened plugs
Screened HT leads
Screened mag switch leads
Route antenna cable away from other wiring (as far as practical)
Improve antenna ground plane
Ensure antenna cable is of correct specification

Regards

Tony