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King KX155 weak VOR reception

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Jimmyb

Joined: 12 Jul 2014
Posts: 2
Location: Phoenix, AZ

Posted: Sun Jul 13, 2014 12:55 pm Post subject: King KX155 weak VOR reception

Hello all,



New member here!  I'm having an issue that I wanted to share with some of the more experienced members on here hoping to find a resolution.



I have two king KX-155 Nav/Coms one with GS and the other without GS installed in a Glasair III with a single NAV antenna feeding both radios including GS via a multiplexer.  The NAV antenna is embedded in the horizontal stabilizer as the Glasair is a composite aircraft.  The NAV indicators are KI-209 and a KI-208.



Both indicators exhibit erratic needles when turning but only through a portion of the turn (example: sometimes when intercepting a radial the needle will start to move in and nearly be centered but as I roll out expecting to capture the radial the needle suddenly moves three-quarter scale deflection.  Also when flying an ILS the GS flag pops in and out of view well down the approach when the GS signal should be strong.  All needles are not as stable as I think they should be.



I recently had both radios checked out at the local avionics shop and they are performing within specs.  



The thought now is that I either have an antenna or a feedline issue.  The multiplexer currently has one open port that used to service a second glideslope receiver.   I've ordered a 50 ohm BNC terminator to cap the open port thinking that I might be getting some reflections/interference that is causing the issue but that has yet to be proven.



The aircraft was built in 1990 so all of the coax is original RG58.   If the 50 ohm Terminator doesn't solve my problem the next logical step is to replace the RG58 with RG400 but I'm starting to feel like I'm grasping at straws.  With no antenna analyzer/distance to fault meter I have no way to verify the condition of the antenna/coax.



Questions:



1. Do you think it's likely that the open multiplexer port could be causing my issues?



2. How likely is it that the old RG58 cable could be causing my issues?



3. Should I bite the bullet and buy an antenna analyzer such as the rigexpert AA 600?



NOTES: Because the NAV antenna is embedded in the horizontal stabilizer it is physically impossible to access the antenna.  The best that I can do will be to replace the RG58 with RG400 but even at that there's will still be approximately 2 feet of the old RG 58 left that is inaccessible the so I will need to install a BNC connector back near the tail to connect the old RG58 (short pigtail that will still be connected to the antenna) with the new RG400.



Your ideas, tips & suggestions will be most appreciated!



James

Phoenix, AZ

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dlj04(at)josephson.com
Guest

Posted: Mon Jul 14, 2014 9:38 am Post subject: King KX155 weak VOR reception

On 7/13/14 11:58 PM, Avionics-List Digest Server wrote:

 	  Quote: 	
		   *



 1. Do you think it's likely that the open multiplexer port could be causing my

 issues?

It's possible, but not too likely to cause the severity of symptoms you 
	


describe. You could eliminate the nav splitter by taking it out of the 

circuit, connecting the antenna lead to one VOR or the GS only, and 

seeing if the problem remains.

 	  Quote: 	
		  

 2. How likely is it that the old RG58 cable could be causing my issues?

Not likely, unless it has gotten mashed somewhere. More likely that a 
	


connector or the termination between the antenna and the cable has 

failed. I have 50 year old RG-58 that's still fine. There's no practical 

difference between old RG-58 and LMR400 in your application -- there 

will be more loss in the additional coax connectors than there is 

difference in attenuation.



You could try an old vee dipole bolted on to the underside of the tail 

as a test.

 	  Quote: 	
		  

 3. Should I bite the bullet and buy an antenna analyzer such as the rigexpert AA

 600?

Since you can't get to the antenna, no. An "antenna analyzer" will not 
	


tell you whether you have an effective antenna, which may be very 

different from 50 ohms but still work fine, or an open at the end of the 

cable. A 50 ohm resistor at the end of tiny coax will look great but 

won't be a very good antenna. The best comparative test will be with a 

low power VOR/GS ramp tester.

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Jimmyb

Joined: 12 Jul 2014
Posts: 2
Location: Phoenix, AZ

Posted: Mon Jul 14, 2014 1:39 pm Post subject: Re: King KX155 weak VOR reception

Re my interest in an antenna analyzer:  I've been looking at some of the newer units online (such as the Rigexpert AA-600) and it appears they have the capability of testing the coax for faults (distance to fault) while connected to the antenna.  I wasn't thinking of replacing the RG58 with RG400 simply because of the RG400 is a lower loss cable, rather I wanted to use the analyzer to check the RG58 for faults.   I would replace the RG58 with RG400 ONLY IF the RG58 is questionable (open, shorted, etc).  This BTW may be a real possibility because the existing cable makes some pretty tight bends under the panel.   Also using a tester such as the AA-600 would allow me to check the SWR of the antenna.



Thoughts?

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_________________
Jim
Phoenix, AZ

dlj04(at)josephson.com
Guest

Posted: Tue Jul 15, 2014 11:49 am Post subject: King KX155 weak VOR reception

On 7/14/14 11:59 PM, Avionics-List Digest Server wrote:

 	  Quote: 	
		  

 Re my interest in an antenna analyzer:  I've been looking at some of

 the newer units online (such as the Rigexpert AA-600) and it appears

 they have the capability of testing the coax for faults (distance to

 fault) while connected to the antenna.  I wasn't thinking of

 replacing the RG58 with RG400 simply because of the RG400 is a lower

 loss cable, rather I wanted to use the analyzer to check the RG58

 for faults.   I would replace the RG58 with RG400 ONLY IF the RG58

 is questionable (open, shorted, etc).  This BTW may be a real

 possibility because the existing cable makes some pretty tight bends

 under the panel.   Also using a tester such as the AA-600 would

 allow me to check the SWR of the antenna.



 Thoughts?


	


My thoughts are that you really want to buy a new antenna analyzer.

Great, but only if you're willing to invest in the time needed to

understand what's going on. These things are not magic and can easily 

produce numbers and graphs that can fool you if you don't know what 

you're looking at. You do have the ability to make an S-parameter 

measurement over a range of frequencies, and remote it to the end of a 

characterized transmission line by the use of time gating so you can 

actually measure the antenna separately from the coax -- this is a 

powerful tool. Just prepare for a steep learning curve.



I don't know about the AA-600 specifically, I have always done this with 

traditional lab equipment. I would suggest looking at the VNWA 3E 

(sdr-kits.net) as an alternative to the AA-600. It's about the same 

price, uses a PC for display, works up through DME/ATCRBS frequencies, 

and is supported by lots of hams worldwide who use it for antenna 

development.



Finally, no amount of electrical sleuthing will tell you whether the 

antenna was an effective receive device as built. It may have a good 50 

ohm match which is nearly irrelevant to its performance as a VOR/LOC/GS 

antenna.

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