Crazy Radio Issue, Help Needed.
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Join Date: Mar 2009
Location: Perth.AU
Age: 52
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Ok, it seems that I was a little premature in thinking the problem was beaten.
The ADF sense antenna was replaced with copper cable and within a couple of flights the issue re-appeared.
Through a process of elimination I now suspect that it might be the stand on which the lower end of the antenna terminates. I am guessing that this is shorting internally and causing the issue. Given that the existing ADF sense antenna has been there in one form or another for the best part of 40 years without issue, it simply has to be some failing component that is causing the problem.
My LAME wants to re-locate the ADF antenna to the belly of the aircraft, but I think it is worth replacing the post on top first.
I'll let you know how it goes!
The ADF sense antenna was replaced with copper cable and within a couple of flights the issue re-appeared.
Through a process of elimination I now suspect that it might be the stand on which the lower end of the antenna terminates. I am guessing that this is shorting internally and causing the issue. Given that the existing ADF sense antenna has been there in one form or another for the best part of 40 years without issue, it simply has to be some failing component that is causing the problem.
My LAME wants to re-locate the ADF antenna to the belly of the aircraft, but I think it is worth replacing the post on top first.
I'll let you know how it goes!
Join Date: May 2010
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HF suspects
If I read the picture correctly that post could be just about the right
length to cause issues if it is alternalty shorted/open with vibration.
The clean flights can be explained by tensions changing a bit after the antenna was swapped.
When you do replace the post keep a sharp eye out for where something might be making intermitant contact, does not matter if it is supposeded to be grounded and coming loose or meant to be isolated getting grounded, either one could do it.
Thanks for keeping us posted, this is indeed one of the more fascinating issues I have followed in a long time.
BTW: Suspect the issue was just waiting for you to claim victory before re-appearing
Through a process of elimination I now suspect that it might be the stand on which the lower end of the antenna terminates. I am guessing that this is shorting internally and causing the issue.
length to cause issues if it is alternalty shorted/open with vibration.
The clean flights can be explained by tensions changing a bit after the antenna was swapped.
When you do replace the post keep a sharp eye out for where something might be making intermitant contact, does not matter if it is supposeded to be grounded and coming loose or meant to be isolated getting grounded, either one could do it.
Thanks for keeping us posted, this is indeed one of the more fascinating issues I have followed in a long time.
BTW: Suspect the issue was just waiting for you to claim victory before re-appearing
Last edited by MurphyWasRight; 11th Mar 2011 at 13:57. Reason: Added note on post to really test.
Well, glad we seemed to get it right - ADF antenna subsystem. Obviously the wire itself is not the issue, but the fact the wire is connected, so, yes, the lower termination would seem a likely culprit.
Keep us updated!
Keep us updated!
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Dunno anything about actual installations but in theory at least I would expect to see a vhf choke in series with each end of the adf sense aerial. Does the installation kit come with chokes (coils) and are they shown on the installation schematic?
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Final Post. Problem found!!
After the new copper ADF sense antenna did the same thing as the old steel wire it rules out the antenna itself.
Given that the problem occurred with the Coax disconnected from the antenna itself it seemed to leave only the coax attachment point as the cause of the issue.
I removed the ceramic post that the antenna terminated onto and found that the coax was supposed to join onto the bottom with a nut and washer, the nut and washer were sitting loose in the bottom of the unit. The connection to the coax was suspended just touching the bottom of the antenna mast, but would have vibrated in time with either engine RPMs or antenna movement to create the issue.
The worst vibration was caused by the engine harmonic and hence the RPM dependant part. The directionalility was indeed due to the directional nature of the sense antenna being aligned with the longitudinal axis of the aircraft.
Interestingly it was just a plain nut and washer, no locking mechanism to prevent it from vibrating loose over time.
Given that the problem occurred with the Coax disconnected from the antenna itself it seemed to leave only the coax attachment point as the cause of the issue.
I removed the ceramic post that the antenna terminated onto and found that the coax was supposed to join onto the bottom with a nut and washer, the nut and washer were sitting loose in the bottom of the unit. The connection to the coax was suspended just touching the bottom of the antenna mast, but would have vibrated in time with either engine RPMs or antenna movement to create the issue.
The worst vibration was caused by the engine harmonic and hence the RPM dependant part. The directionalility was indeed due to the directional nature of the sense antenna being aligned with the longitudinal axis of the aircraft.
Interestingly it was just a plain nut and washer, no locking mechanism to prevent it from vibrating loose over time.
Thanks for the final update! This has been a rather wild ride...
Are you sure there's nothing related to spark plugs... shhheeshh!
Good work from a few here - you know who you are.
Certainly one or two lessons learned I think.
Cheers, GY
Are you sure there's nothing related to spark plugs... shhheeshh!
Good work from a few here - you know who you are.
Certainly one or two lessons learned I think.
Cheers, GY
Delighted that it's finally fixed. This has provided the weirdest set of symptoms I've ever read about - much less encountered in real life. I've learned a lot from it all, so we both win . Thanks for keeping us informed and involved.
Regards
FOR
. Thanks for keeping us informed and involved.Regards
FOR
