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King KR-86 Loop Cable

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Hopperdhh(at)aol.com
Guest

Posted: Wed Feb 08, 2012 1:28 pm Post subject: King KR-86 Loop Cable

I have a KR-86 ADF receiver and KA42 loop antenna, but no cable.  I'd  like to fabricate a cable.

  

 Apparently the capacitance of the cable is critical.  Can anyone tell  me what this capacitance should be and how to measure it?  Also, is the  cable 2 separately shielded twisted pairs, or can the twisted pairs both be  in the same shield, like telephone cable?

  

 Thanks, Dan K9WEK

 RV-7A 766DH flying since 2001

 [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Wed Feb 08, 2012 4:44 pm Post subject: King KR-86 Loop Cable

At 03:15 PM 2/8/2012, you wrote:

  	  Quote: 	
		    

 I have a KR-86 ADF receiver and KA42 loop antenna, but no cable.  I'd like to fabricate a cable.

  

 Apparently the capacitance of the cable is critical.  Can anyone tell me what this capacitance should be and how to measure it?  Also, is the cable 2 separately shielded twisted pairs, or can the twisted pairs both be in the same shield, like telephone cable? 	


    Not sure without schematics and a service

    manual. There have been numerous service bulletins

    over the years that called for confirmation

    of a sense or loop cable assembly's capacitance.

    For many radios of the era, coax capacitance

    was an integral component of resonant or reactive

    circuits within the receiver.



    I recall writing accessory kits for ADF installations

    wherein the installer fabricated a sense antenna

    coax from some weird coax with a very tiny center

    conductor in a relatively large hollow insulator

    sleeve. Car radios used to use the same stuff . . .

    very LOW capacitance/foot was key. Here's a tid-bit

    I found on the 'net about low cap receiving coax:



 The real reason car radios used ~90 ohm coax is 90-95 ohms works out to be the very highest impedance practical in a coaxial line having minimum possible shunt capacitance and reasonable outer and workable conductor size. This is entirely because of the short unloaded whip antenna on the car, and the AM broadcast bands low frequency that makes the antenna the tiniest fraction of a wave tall. The small car antenna is a voltage probe on AM broadcast. The short car antenna is an extremely high impedance source on the AM band. The AM receiver has a very high input impedance, ideally hundreds or thousands of ohms. Any shunt capacitance to ground in the connection between the receiver and antenna forms the lower leg of a voltage divider that reduces the RF voltage available at the AM radio input.



 The car system is not operating in a normal transmission line mode.



 This type of cable was also used for some old data lines, and it can work as a transmission line, but generally it is pretty lossy compared to other cables. After all, nearly all HF and VHF loss is caused by conductor resistance and this cable has a pretty thin center conductor. 



    As the article states, coax used in this application

    is not behaving like a matched transmission line but

    simply a shielded wire chosen to minimize the upset

    for very high impedance receiver front ends.



    RG62 is the only modern coax I can find with a low

    capacitance (13.0 pf/ft). You could probably make

    a new cable from this stuff if you knew what the total

    allowable capacitance was . . . this sets cable

    length.



 



    These folks offer cables for KR-76 in various

    lengths. Perhaps they fabricate them locally.



     http://www.bennettavionics.com/kr86system.html



 



 

   Bob . . .    [quote][b]

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Hopperdhh(at)aol.com
Guest

Posted: Thu Feb 09, 2012 6:03 am Post subject: King KR-86 Loop Cable

Thanks Bob, 

  

I have the service  manual, but there is no hint of what the capacitance or impedance is of the loop  cables. It does show the cable as 2 separately shielded 2 conductor cables. The  install manual gives 2 part numbers of cables (different lengths) and also says  to not alter the length. One is 15 ft. and one is 24 ft. 

  

I bought this radio  and antenna at OSH for, I think, $25. I repaired the radio -- found a leaky  capacitor in the synthesizer loop filter which caused a loud 1 KHz  noise that covered up all signals. 

  

This is just a hobby  project, so I don't want to spend lots of money on it. I've already spent twice  as much on manuals as the radio cost! 

  

I am a EE, and have  been a ham for over 50 years, built a lot of equipment, etc. There should be a  way to solve this problem, but so far I haven't been able to!  I'll try  some things -- like measure the antenna inductance to get an idea of the  impedance we're dealing with. I can also make some measurement on the  goniometer, and front end coils that it feeds. That gets a little complicated!   

  

Thanks for the help.  If anything else comes to mind please let me know. 

  

Dan 

 

  

  In a message dated 2/8/2012 7:47:36 P.M. Eastern Standard Time,  nuckolls.bob(at)aeroelectric.com writes:

  	  Quote: 	
		  At 03:15    PM 2/8/2012, you wrote:

    	  Quote: 	
		  

I have a KR-86 ADF receiver and KA42 loop      antenna, but no cable.  I'd like to fabricate a      cable.

 

Apparently the capacitance of the cable is      critical.  Can anyone tell me what this capacitance should be and how      to measure it?  Also, is the cable 2 separately shielded twisted pairs,      or can the twisted pairs both be in the same shield, like telephone      cable? 	


   Not sure without schematics and a    service

   manual. There have been numerous service    bulletins

   over the years that called for    confirmation

   of a sense or loop cable assembly's    capacitance.

   For many radios of the era, coax    capacitance

   was an integral component of resonant or    reactive

   circuits within the receiver.



   I    recall writing accessory kits for ADF installations

   wherein    the installer fabricated a sense antenna

   coax from some weird    coax with a very tiny center

   conductor in a relatively large    hollow insulator

   sleeve. Car radios used to use the same stuff    . . .

   very LOW capacitance/foot was key. Here's a    tid-bit

   I found on the 'net about low cap receiving    coax:



The real reason car radios used ~90 ohm coax is 90-95 ohms works    out to be the very highest impedance practical in a coaxial line having    minimum possible shunt capacitance and reasonable outer and workable conductor    size. This is entirely because of the short unloaded whip antenna on the car,    and the AM broadcast bands low frequency that makes the antenna the tiniest    fraction of a wave tall. The small car antenna is a voltage probe on AM    broadcast. The short car antenna is an extremely high impedance source on the    AM band. The AM receiver has a very high input impedance, ideally hundreds or    thousands of ohms. Any shunt capacitance to ground in the connection between    the receiver and antenna forms the lower leg of a voltage divider that reduces    the RF voltage available at the AM radio input.



The car system is not    operating in a normal transmission line mode.



This type of cable was    also used for some old data lines, and it can work as a transmission line, but    generally it is pretty lossy compared to other cables. After all, nearly all    HF and VHF loss is caused by conductor resistance and this cable has a pretty    thin center conductor. 



   As the article states, coax used    in this application

   is not behaving like a matched    transmission line but

   simply a shielded wire chosen to    minimize the upset

   for very high impedance receiver front    ends.



   RG62 is the only modern coax I can find with a    low

   capacitance (13.0 pf/ft). You could probably    make

   a new cable from this stuff if you knew what the    total

   allowable capacitance was . . . this sets    cable

   length.



   These folks offer    cables for KR-76 in various

   lengths. Perhaps they fabricate    them locally.



   http://www.bennettavionics.com/kr86system.html



   

  Bob . . .  	  Quote: 	
		  



ist href="http://www.matronics.com/Navigator?AeroElectric-List">http://www.matronics.com/Navigator?AeroElectric-List

s.matronics.com/">http://forums.matronics.com

p://www.matronics.com/contribution">http://www.matronics.com/contribution



	
	


 [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Feb 09, 2012 6:57 am Post subject: King KR-86 Loop Cable

At 08:02 AM 2/9/2012, you wrote:



  	  Quote: 	
		    



 Thanks Bob,



  



 I have the service manual, but there is no hint of what the capacitance or impedance is of the loop cables. It does show the cable as 2 separately shielded 2 conductor cables. The install manual gives 2 part numbers of cables (different lengths) and also says to not alter the length. One is 15 ft. and one is 24 ft. 	


   Could you scan the radio's front end schematic

   and share it?



 

 

   Bob . . .    [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Feb 09, 2012 6:57 am Post subject: King KR-86 Loop Cable

At 08:02 AM 2/9/2012, you wrote:



  	  Quote: 	
		    



 Thanks Bob,



  



 I have the service manual, but there is no hint of what the capacitance or impedance is of the loop cables. It does show the cable as 2 separately shielded 2 conductor cables. The install manual gives 2 part numbers of cables (different lengths) and also says to not alter the length. One is 15 ft. and one is 24 ft. 	


    Yeah. I seem to recall a tid bit from the dark recesses

    of history that the longer cable was the 'golden' device

    and the shorter one had padding capacitors in the connector

    to make up the difference between the two.



    I don't think there were any off the shelf, low capacitance

    cables made as twisted pairs. Those would be comparatively

    huge bundles. When they went to an internal goniometer,

    the loop became less critical . . . impedance matching

    could be accomplished in the gonio-transformer.



    Do you intend to fly this radio? What's the expected

    distance from radio to loop?  If it's relatively

    short, say on the order of 6-10 feet, I think I'd

    try plain vanilla shielded pairs and see what happens.

    I don't know if I ever knew . . . is this radio among

    the ranks of those where the long-wire sense antenna was

    eliminated?



    Given the wide frequency coverage (250-1700 KHz) 

    1:7 ratio, it seem unlikely that antenna features

    and feedlines will be in any way resonant. It seems

    more likely that special attention be paid to ratiometric

    values of impedance and phase. So I'm guessing that

    the feedlines for this radio need only be identical

    as opposed to trimmed for resonance.



 

   Bob . . .    [quote][b]

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Hopperdhh(at)aol.com
Guest

Posted: Thu Feb 09, 2012 8:15 am Post subject: King KR-86 Loop Cable

Hi Bob,



Mainly I just want to learn  more about ADF.  I also have a 1946 Stinson 

(besides the RV-7A) that I may  install it in once its working just to see 

what ADF navigation is like, but that  is some time away.



Yes, I'll try some plain vanilla cable to start  with.  I think that 6 to 

10 ft. would work in the Stinson.  Two ft.  would work on the bench here.



If one knew the value of the padding  capacitor and the length of cable it 

replaced, that would answer the question,  wouldn't it?



There is a bench setup in the service manual that mentions a  cable (pn 

200-0440-00) with capacitance of 144 pF.  (I found this after  sending the 

email below.)  It also shows a sense cable (pn 200-0447-00)  with the same 

capacitance.  The loop cable part number is not the same as  in the installation 

manual, though!!



There is mention of a King Service  Memo -- SM75 -- in The Technicians 

Notebook by Jerry  Gordon.



www.aerostandard.com/images/TecnNote.pdf



He says, "Its a  long drawn out procedure to determine the proper ADF sense 

antenna  configuration.  What it boils down to is you need a capacitance 

meter in  your shop."  I know we're talking about loop antennas here, but I'd 

be  interested to see SM75 if you have access to it.



I can scan the test  setup page and the receiver front end.  I don't think 

attachments will work  on this list.  How do I share the scans.  I can send 

them as .pdf  files.



Dan

In a message dated 2/9/2012 9:57:56 A.M. Eastern  Standard Time, 

nuckolls.bob(at)aeroelectric.com writes:

At 08:02 AM 2/9/2012,  you wrote:





Thanks Bob,



I have the  service manual, but there is no hint of what the capacitance or 

impedance is of  the loop cables. It does show the cable as 2 separately 

shielded 2 conductor  cables. The install manual gives 2 part numbers of 

cables (different lengths)  and also says to not alter the length. One is 15 ft. 

and one is 24  ft.



Yeah. I seem to recall a tid bit from the dark  recesses

of history that the longer cable was the 'golden'  device

and the shorter one had padding capacitors in the  connector

to make up the difference between the  two.



I don't think there were any off the shelf, low  capacitance

cables made as twisted pairs. Those would be  comparatively

huge bundles. When they went to an internal  goniometer,

the loop became less critical . . . impedance  matching

could be accomplished in the  gonio-transformer.



Do you intend to fly this radio? What's  the expected

distance from radio to loop?  If it's  relatively

short, say on the order of 6-10 feet, I think  I'd

try plain vanilla shielded pairs and see what  happens.

I don't know if I ever knew . . . is this radio  among

the ranks of those where the long-wire sense antenna  was

eliminated?



Given the wide frequency  coverage (250-1700 KHz) 

1:7 ratio, it seem unlikely that  antenna features

and feedlines will be in any way resonant. It  seems

more likely that special attention be paid to  ratiometric

values of impedance and phase. So I'm guessing  that

the feedlines for this radio need only be  identical

as opposed to trimmed for resonance.

Bob . . .

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Hopperdhh(at)aol.com
Guest

Posted: Thu Feb 09, 2012 10:14 am Post subject: King KR-86 Loop Cable

Bob,

  

 I forgot to answer this question.

  

 Bob asked:

   	  Quote: 	
		     I don't know if I ever knew . . . is this radio among

the ranks    of those where the long-wire sense antenna was

eliminated?

   

 

	


 Yes, sort of, but not in my case:

  

 The loop antenna I have -- KA42 -- requires a separate "50 pF" sense  antenna.  I see in the install manual that the radio is designed for 194 pF  sense antenna system -- 144 pF cable plus 50 pF antenna.  It also  refers to Field Service Memo #75 which I would like to find.

  

 The loop antenna KA42B has a built in sense antenna.  It requires  a matching assembly (pn 200-02104-0000).  The schematic for this assembly  is shown in the install manual.  It feeds 8V to the antenna unit and uses  50 ohm coax -- length not critical.  Apparently there is an amplifier in  the antenna unit.

  

 Dan

  

 [quote][b]

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