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Microair transmit issue in new Jabiru


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Hi all.

 

Today, when taxiing out at Bankstown, Tower advised almost unreadable on ground freq 119.9.

 

A check on Tower freq 132.8 OK initially, then unreadable 5 min later.

 

Radio Techs removed Radio and checks perfectly on the Bench.

 

Reception fine and Sidetone fine. Just transmit probs.

 

Later on ground checks, transmit distorted on lower freq but twr reading loud and clear at higher freq 132.8

 

Ground test of Antenna OK.

 

Problem just appeared today and no issues since Radio installed new in late March this year.

 

Am using latest David Clark DC-PROX ANR Headset but problem still there on old Headset and no improvement on copilot side.

 

Appreciate comments guys.

 

Cheers Roscoe

 

 

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Roscoe

 

Ignoring any voice aspects what is the signal strength, or to put it another way a 5W airborne radio close to a receiver when you key the transmitter and do nothing/say nothing should immediately trigger the receiver squelch circuit and you should hear virtually no hiss. That shows that the transmitter circuit carrier is working fine. Try it high and low frequency.

 

When it comes to the voice aspect what does unreadable actually mean, is it:-

 

1) The receiver can barely hear any modulation (voice) against a strong carrier

 

2) The receiver can hear plenty of voice but its distorted (describe distortion()

 

3) The receiver can hear something but it doesn't sound like voice (describe or record what it does sound like)

 

Now, ignoring the receiver what can you hear in your headset (sidetone) when you transmit and does it sound Ok?

 

Lastly, has anything new from an avionics perspective been added to the aircraft? doesn't matter what it is and whether its related to the radio, if anything added let us know what and when

 

also try all the above when wiggling the mic/headset leads, could be a dodgy connector (we are only looking for problems, not trying to create them so be gentle)

 

Andy

 

 

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Andy tn

 

RoscoeIgnoring any voice aspects what is the signal strength, or to put it another way a 5W airborne radio close to a receiver when you key the transmitter and do nothing/say nothing should immediately trigger the receiver squelch circuit and you should hear virtually no hiss. That shows that the transmitter circuit carrier is working fine. Try it high and low frequency.

 

When it comes to the voice aspect what does unreadable actually mean, is it:-

 

1) The receiver can barely hear any modulation (voice) against a strong carrier

 

2) The receiver can hear plenty of voice but its distorted (describe distortion()

 

3) The receiver can hear something but it doesn't sound like voice (describe or record what it does sound like)

 

Now, ignoring the receiver what can you hear in your headset (sidetone) when you transmit and does it sound Ok?

 

Lastly, has anything new from an avionics perspective been added to the aircraft? doesn't matter what it is and whether its related to the radio, if anything added let us know what and when

 

also try all the above when wiggling the mic/headset leads, could be a dodgy connector (we are only looking for problems, not trying to create them so be gentle)

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Andy tn

Andy thanks for quick reply.

Tower said distorted and unreadable 0n lower freq 119.9.

 

Sidetone clear and transmit clear thru headset but unreadable by tower.

 

Have jiggled plugs in Jacks and tried different headsets in both sides.

 

Radio guy will speak to Microair tmrow & report back.

 

Very frustrating however.

 

Cheers RoxieRoxie

 

 

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Andy thanks for quick reply.Tower said distorted and unreadable 0n lower freq 119.9.

Sidetone clear and transmit clear thru headset but unreadable by tower.

 

Have jiggled plugs in Jacks and tried different headsets in both sides.

 

Radio guy will speak to Microair tmrow & report back.

 

Very frustrating however.

 

Cheers RoxieRoxie

Andy tn

Signoff Roscoe........not Roxie...hate predictive text!!!

 

 

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RoscoeIgnoring any voice aspects what is the signal strength, or to put it another way a 5W airborne radio close to a receiver when you key the transmitter and do nothing/say nothing should immediately trigger the receiver squelch circuit and you should hear virtually no hiss. That shows that the transmitter circuit carrier is working fine. Try it high and low frequency.

 

When it comes to the voice aspect what does unreadable actually mean, is it:-

 

1) The receiver can barely hear any modulation (voice) against a strong carrier

 

2) The receiver can hear plenty of voice but its distorted (describe distortion()

 

3) The receiver can hear something but it doesn't sound like voice (describe or record what it does sound like)

 

Now, ignoring the receiver what can you hear in your headset (sidetone) when you transmit and does it sound Ok?

 

Lastly, has anything new from an avionics perspective been added to the aircraft? doesn't matter what it is and whether its related to the radio, if anything added let us know what and when

 

also try all the above when wiggling the mic/headset leads, could be a dodgy connector (we are only looking for problems, not trying to create them so be gentle)

 

Andy

Andy, yes nothing added to Avionics since new.

 

 

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Sorry. Also forgot to ask what your radio tech did to bench test the set. He should have run the set into a dummy load to test the transmit at lowest and highest frequency and monitored the transmission with a monitor receiver to test for modulation and carrier strength.

 

Paul

 

 

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I started my career as a radio Tradesman in the early 70s. An AM transmitter is as simple as one can hope for and I am often astonished at the problems an aircraft radio can produce. To be honest, a lot of problems I have seen (heard) have been folks yelling into the mic or low voltage at the transmitter during transmit. I hope your problems is solved quickly. I jumped ship and got into I.T. when electronics got smaller than what my eyeballs could handle.

 

 

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Sorry. Also forgot to ask what your radio tech did to bench test the set. He should have run the set into a dummy load to test the transmit at lowest and highest frequency and monitored the transmission with a monitor receiver to test for modulation and carrier strength.Paul

Hi Paul, not exactly sure what he did on the Bench, but stated that Radio checked out fine, then reinstalled today in Aircraft but still horribly distorted on transmit on all freq therefore assumed problem is with Antenna installation.

Jab Antenna is encased in fin, so Radio tech not able to go any further and still no closer to solution.

 

Am seeking 2nd opinion and may have to opt for another Antenna installation on top canopy or on belly but will see when i get another opinion. Will let you know and thanks for your interest.

 

Cheers Roscoe

 

 

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I started my career as a radio Tradesman in the early 70s. An AM transmitter is as simple as one can hope for and I am often astonished at the problems an aircraft radio can produce. To be honest, a lot of problems I have seen (heard) have been folks yelling into the mic or low voltage at the transmitter during transmit. I hope your problems is solved quickly. I jumped ship and got into I.T. when electronics got smaller than what my eyeballs could handle.

Yeah i know what you mean. At this stage getting another Bench test done but ive been advised Radio fineon the Bench. Now looking if it might be Antenna issue. Frustrating.

Will keep you in the loop

 

Cheers Roscoe

 

 

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I started my career as a radio Tradesman in the early 70s. An AM transmitter is as simple as one can hope for and I am often astonished at the problems an aircraft radio can produce. To be honest, a lot of problems I have seen (heard) have been folks yelling into the mic or low voltage at the transmitter during transmit. I hope your problems is solved quickly. I jumped ship and got into I.T. when electronics got smaller than what my eyeballs could handle.

What he said, exactly the same for me.....

 

Id also agree that DC sag under Tx is something I'd be quickly testing for. With the Aircraft running see if you can measure with a DC voltmeter the supply voltage to the transceiver (at the 25 pin D connector) on Rx and also again during Tx........an older analogue voltmeter may be a better tester than an LCD voltmeter for this one...... I personally would be surprised if the jab antenna could cause distortion on Tx especially if the SWR shows good fwd to reflected power ratios so IMHO changing antennas would be the last thing I looked at not one of the first and only if I can prove a fault but cant fix because I cant get to it.....

 

Have you changed headsets recently? I asked previously about avionics changes and you said nope, but if you've added ANR headsets or changed brands etc then that's a change I would want to know about... All headsets should offer the same interface characteristics but in reality they don't and that's why there is a mic gain adjustment on the side of the radio (not really an end user adjustable device rather a tech adjustable with test equipment)

 

BTW is the problem only in the aircraft when its running? or is it present on the ground with avionics on but motor off....

 

Andy

 

 

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What he said, exactly the same for me.....Id also agree that DC sag under Tx is something I'd be quickly testing for. With the Aircraft running see if you can measure with a DC voltmeter the supply voltage to the transceiver (at the 25 pin D connector) on Rx and also again during Tx........an older analogue voltmeter may be a better tester than an LCD voltmeter for this one...... I personally would be surprised if the jab antenna could cause distortion on Tx especially if the SWR shows good fwd to reflected power ratios so IMHO changing antennas would be the last thing I looked at not one of the first and only if I can prove a fault but cant fix because I cant get to it.....

 

Have you changed headsets recently? I asked previously about avionics changes and you said nope, but if you've added ANR headsets or changed brands etc then that's a change I would want to know about... All headsets should offer the same interface characteristics but in reality they don't and that's why there is a mic gain adjustment on the side of the radio (not really an end user adjustable device rather a tech adjustable with test equipment)

 

BTW is the problem only in the aircraft when its running? or is it present on the ground with avionics on but motor off.

 

Andy

Andy thnks again for your interest and comprehensive reply. Will pass on your comments to the shop where i am getting Radio tested again.

No change in Headset. Am using David Clark ProX and tested other Headsets in both sides with no difference.

 

The problem is big distortion on transmit with engine both running and stopped. The problem arose a week ago when taxiing at Bankstown without any warning.

 

Today the first Radio shop i tried said that it worked fine on the Bench so it MUST be the Antenna, and because they couldnt get to it, i was told, sorry mate, cant help you.

 

Andy, have you heard of any instances where Jab antennas have been replaced?

 

Anyway, lets see what happens on the second opinion scenario. Will keep you all posted

 

Cheers Roscoe

 

 

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Roscoe, is this an ongoing problem? I can remember times when at Parafield the tower would say 'unreadable' when replying to my taxi call but as I taxied to a different location things would come good. It's not something as simple as that is it?

 

 

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I'm told jab antenna is two pieces of aluminum with coax attachéd and is. not can't be tuned to the full range and is usually only good at top or bottom. you should buy a comant 121 aerial

Deb coincidentally i was looking at a US website ast night where Comant make Antennas that fit to the Belly. Is is difficult to remove the existing Antnna in the Fin if we decide thats the problem?

 

 

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Roscoe, is this an ongoing problem? I can remember times when at Parafield the tower would say 'unreadable' when replying to my taxi call but as I taxied to a different location things would come good. It's not something as simple as that is it?

Roscoe, is this an ongoing problem? I can remember times when at Parafield the tower would say 'unreadable' when replying to my taxi call but as I taxied to a different location things would come good. It's not something as simple as that is it?

No, problem tested the same right through a long taxi with turns etc. I was told of someone that had this issue at Archerfield once where distorted facing one way and clear another way.

 

 

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If as stated before, you have a good SWR I can't see another antenna making any difference. I would test the operation with the original micro air headsets to eliminate that option - can't see the aerial causing distortion.

 

Have had one installation giving "broken/distorted/blocking " transmissions - radio checked OK on bench, SWR checked mid range OK, put back in aircraft and worked fine. My conclusion, although unable to prove it, a bad connection on one of the coax connectors. Might be worth checking or reterminating them.(as referred to previously by AVOSET)

 

 

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Yep, Radio is out at present

 

The directional issue is common. I "suspect" shielding of transmission by close proximity of the rudder hinges in the fin mounted aerial - from my experience it is when the "receiver" is behind the aircraft.[/Q

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Reality is an antenna doesn't know if a signal has modulation or not and the only way it can cause modulation issues is if the reflected power (being that power the antenna cant turn into radiated power) is enough to cause distortion in the Tx circuits as it reenters. In a modern semiconductor transmitter with a bunch of protection circuits built in that to me is highly unlikely, especially if the SWR is within limits. Directionality is a function of the antenna's radiation pattern where the practical aspects of the antenna butt up against that technical requirements. As I've previously suggested a radio operating at the frequencies we use doesn't need anywhere near 5W of radiated power to give the necessary line of sight coverage in fact we could drop an order of magnitude without any issue....except that antenna's on aircraft are far from perfect but add a magnitude of power to the Tx'd signal and weak spots within the radiation pattern can generally be overcome.

 

So again, modulation issues without any corresponding drop in perceived carrier strength is very unlikely to be antenna related.

 

In a previous thread there was great discussion (which I originally got wrong) on cigarette plug chargers for mobile phones/ tablets. The correct conclusion was that as a switchmode device (and not the linear regulator I thought they were) the cheaper of these (which are designed with cost as THE driver) often don't include the necessary circuitry to keep the high frequency switching signals away from other devices where the sharp square waves create lots of harmonic distortion.......Have you recently added a charger to your cigarette lighter power socket? if so try removing it?

 

Andy

 

 

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