Strange radio problems
 

Has anybody seen weird radio issues in today's high pressure conditions?

I have

com1: kx155a
com2: kx165a

Com1 is fine.

Com2 (an 8.33 channel spaced radio) suffers for a lot of interference (crackling) on all frequencies when in flight, but not on the ground, even if listening to distant transmitters (e.g. airliners calling up Gatwick Director). Tower radio checks are fine. The crackling is present only while there is an incoming carrier i.e. between transmissions the noise is not there.

Obviously the next stage is to swap the two radios over and do a flight test and see if the problem moves, then check the coax to the aerial, etc. I just have extremely limited facilities for checking RF cabling and radio performance where I am based.

Teaching you to suck eggs I know but between transmissions you won't hear the crackling because it's below the squelch level... but you knew that.

OK so you've turned the squelch off and still note the crackling when carrier present? I know a radio that does that. I think it must still have an element of AGC regardless of the squelch, so when it detects carrier it turns the gain up. With no carrier the background is actually noisier, so it turns the gain down.

Is the crackling consistent with 'atmospheric' background noise? Or static? Or distant thunderstorms? Or is it a soldered joint inside the radio? A good whack to the front panel might teach it a lesson.

I wish I knew a radio engineer who could fix this stuff. Good luck, David.

Turning off auto squelch does not change anything. The noise appears the same way.

It could be atmospheric. It is not any kind of "machine" noise.

I was thinking I have a bad RF (antenna) connection but the really weird thing is that listening to distant transmissions when on the ground (with the engine not running - that is something I need to test) works perfectly. And the NAV channels both work equally, with the flag disappearing more or less together, which indicates it is not an RF signal loss.

Nobody I know of in the UK can test a KX165A from first principles. If it is that, I would have to buy an overhauled exchange unit from the USA.

Before you do all of that is it foggy and moist where the plane is sitting?

Is it in a hanger?

MJ, it is in the UK!

I know but there a degrees on moisture and types in the UK.

It can be slashing it down with rain and you won't get any problems but 3 days of fog and you will get all manner of issues with avionics and the like.

The fact there is no problem when its on battery says to me that there is a leak from the earthing side allowing noise from the alt to effect the radio.

This can be for numerous reasons, one of which is that all the connectors are soaked and have condersate on them. A waz round with a can of spray and rerack them and everything will be grand.

Or it could be the back emf thing on the radio DC supply has a water bridge and noise is getting in that way.

If the aircraft is sitting dry in the hanger it would lead me to suspect the radio is fecked internally

If that was the case you stand a very good chance of damaging the radio when you transmit. Mad Jock is right on his pointers.

Check the obvious first. Pull the radio out of the rack and fit it back. Is the problem any better? If not start to drill down Mad Jocks suggestions until the problem is resolved.

I once got fed up with the oft repeated line about the Eskimos having lots and lots of words for snow. So, I started trying to work out how many words the British have for rain.

I got as far as 27, and didn't try all that hard.

G

Even if some of the Caledonian vocabulary is very exotic to this poor stupid foreigner, I share MJ's suspicions on the earthing side.

Oh dear....

If it's a ground fault then it's more likely to exhibit problems on Tx when the current draw is higher. Same with a bad antenna - high VSWR will always make the transmitter worse before you notice much effect on rx. Typical VSWR on a general aviation VHF antenna should be better than 2.5:1 at end of the band and ideally less than 1.5:1 at around 127Mhz. The fault could be a duff screen or filter on a magneto P lead. In any case, if the fault can be reproduced on the ground with engine running, it should be isolatable by turning each item of electrical equipment off during a ground run whilst having the squelch open and listening to the noise on a weak distant station ( a distant volmet transmitter is good for this). If the fault is only apparent in flight, look for poor bonding of a flying control or undercarriage door or poor static dischargers.

I know it's obvious but are all the static wicks in good shape?

Many thanks, wigglyamp :ok: and all.

I will do more tests.

Presumably the SWR can be tested by pulling the radio out and sticking a tester into the now exposed RF connector in the back of the tray? Otherwise, the cable runs from the back of the tray to a bulkhead BNC on the centre stack module (that part of the cable, about 3ft long, is inaccessible without pulling the whole centre stack out ;) ) and from the bulkhead it runs to the antenna directly. I know it is not RG400 and should be, but I have not had this problem before.

There seems no evidence that TX is affected.

Also I wonder why a bad static wick or bonding or a bad ignition lead would have no effect on the #1 radio.

I will swap over the two radios, but also do a ground test with distant stations with the engine running (should have thought of that).

The #2 radio is only ~ 2 years old and was bought brand new. Normally the KX1x5A are very reliable radios.

The ideal way to do the VSWR check is to pull the box and use a tester with an integral signal generator that can be plugged intomthe backplate co-ax socket - HR Smith 12-602 or similar. If this isn't available, then a Thruline watt meter can be installed into the cable behind the rack and you use the radio to generate the test signals. On the TB, Ithink there are disconnect plugs that allow the radio stack to be removed so this is where the in-line wattmeter wil go.

Defective bonding of flying controls or poor static wicks can lead to a build up of static on parts of the airframe when airflow is present, and the static build-up can vary with humidity. When it gets really bad, the squelch on VHF can open on it's own just on the electrical noise! it can affect different systems depending on the relative position of the associated VHF antenna to the noise source.

What puzzles me is that the static wicks are fine, the plane itself is "brand new" (10 years old :) ) and the #1 radio is fine.

I can just about detect a little bit of crackle in the #1 radio, with the auto squelch off.

How can one check bonding, other than by inspection?

Also it has been very humid, which should play against static during flight - or maybe the opposite?

There is 0.5kg of silica gel in the cockpit the whole time, and it is hangared.

The other already mentioned thing is that the noise appears only when somebody is transmitting. There is no trace of it at other times.

You mention that the Coax is the wrong spec, thats a good clue. You also mention that its TX related, checking VSWR is therefore essential before you do damage to the RF front end section of your radio.

I should have made it clearer that this only just started - 2 days ago in fact.

But I did clearly say it is RX related. TX works fine.

Bonding
 

Bonding is checked using a milliOhm meter. The results between each component under test (flying control, gear door, radio unit etc) should be less than 0.05ohm back to main airframe ground (battery earth). When you say the static wicks are good - have you tested them ? Again, the base should be in bond as above. Between the base and the tip should be a resistive value specified in the AMM - it can typically be 2-4Mohm.

It's much harder for you to monitor the quality of the TX signal sitting in the aircraft. If its a coax / connector problem then it will be there during TX, and its a lot easier to check for problems by checking VSWR of the installation. My starting point would be to remove the radios from the rack and plug them back in, you may be surprised, often operating the connectors is enough to "cure" the problem. As is a spray of good quality contact cleaner.

That's interesting... all the wicks I have ever seen were just stranded wire, with no resistor inside.