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Sharing ship's COMM antenna with the hand-held

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nuckollsr(at)cox.net
Guest

Posted: Sun Feb 04, 2007 12:21 am Post subject: Sharing ship's COMM antenna with the hand-held

At 07:43 PM 2/3/2007 -0800, you wrote:



 	  Quote: 	
		  Bob,



Re website with discussion on IC-ANT-SB

<http://www.flight.org/forums/showthread.php?t 74288>www.flight.org/forums/showthread.php?t 74288 




	


   Hmmmmm . . . the read I got on use of the

   word "crude" was that he didn't think that a

   3.5mm audio connector was well suited for

   radio frequency coax transmission lines.

   I note that Jim Weir weighed in on the topic

   and rebutted someone assertions that use

   of the wrong connector would introduce

   significant loss. I didn't see anyone's

   teardown analysis that went in to look at

   the construction or robustness of the jack.



   THIS is where the device will become troublesome.



   Thanks for the link!



   Bob . . .



        ----------------------------------------

        ( IF one aspires to be "world class",  )

        ( what ever you do must be exercised   )

        ( EVERY day . . .                      )

        (                  R. L. Nuckolls III  )

        ----------------------------------------

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nuckollsr(at)cox.net
Guest

Posted: Sun Feb 04, 2007 12:01 pm Post subject: Sharing ship's COMM antenna with the hand-held

At 01:12 PM 2/4/2007 -0600, you wrote:



 	  Quote: 	
		  



John, I don't think it's as simple as that.



If you put a 50 ohm resistor at the end of a piece of coax you are 

transmitting into, the resistor becomes a "dummy load", and will heat up 

to the extent that it is rated near the power level of the 

transmitter.  You cannot then simply add a T connector to another device 

and assume that the dummy load will be invisible to the system.



If you connect a handheld radio with a 2W output into the receiver input 

of a panel mounted radio, you'd better hope that receiver's input section 

is capable of handling that level of power.  I'm not sure any of them are.


	


    I'm unaware of any aviation equipment manufacturer

    that rates their products for ability to withstand

    large doses of energy fed directly into the receiver's

    input terminals. Unless you know that some radio you've

    installed is so featured, be VERY CAREFUL how you hook

    up any antenna changeover feature in your airplane.

    A simple swap of coax cables on the ICOM hand-held

    adapter would feed your hand-held's transmitter directly

    to the antenna input of your panel mounted receiver.

    All bets are off as to whether this would produce a

    happy outcome.



    Bob . . .

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N6030X(at)DaveMorris.com
Guest

Posted: Sun Feb 04, 2007 12:11 pm Post subject: Sharing ship's COMM antenna with the hand-held

Thinking outside the box is a good thing.  At least the FAA allows it 

in the experimental aviation world.



Dave

At 01:54 PM 2/4/2007, you wrote:

[quote]



Yes, of course you're right Dave.  I was just thinking out loud, I guess.



-John

Dave N6030X wrote:

>

>

>John, I don't think it's as simple as that.

>

>If you put a 50 ohm resistor at the end of a piece of coax you are 

>transmitting into, the resistor becomes a "dummy load", and will 

>heat up to the extent that it is rated near the power level of the 

>transmitter.  You cannot then simply add a T connector to another 

>device and assume that the dummy load will be invisible to the system.

>

>If you connect a handheld radio with a 2W output into the receiver 

>input of a panel mounted radio, you'd better hope that receiver's 

>input section is capable of handling that level of power.  I'm not 

>sure any of them are.

>

>Dave Morris

>

>At 12:19 PM 2/4/2007, you wrote:

>>

>><john(at)ballofshame.com>

>>

>>If you short a 50 ohm resistor across the end of a 50 ohm coax, 

>>you will effectively remove that "leg" of the cable from the 

>>circuit so far as back reflection is concerned.  This is called terminating.

>>Basically, the 50ohm resistor acts the same as an infinitely long 

>>coax, if that makes sense?

>>

>>So if radio manufacturers where clever, they would setup their 

>>antenna inputs to short across a 50ohm load when the unit is off, 

>>effectively terminating their end of the cable.  On the panel, you 

>>use a T with a 50 ohm terminator in the panel side.  If you need 

>>to use a handheld, you can turn off the panel mount and replace 

>>the 50 ohm terminator with your handheld input.  The downside is 

>>that you will loose range.

>>

>>Don't take any of this as a suggestion.  It was just something 

>>that flashed into my brain this morning when I thought about how 

>>to make every compatible.  I think this would work but you'd 

>>sacrifice performance, so it's probably not viable.

>>

>>-John

>>

>>Doug Windhorn wrote:

>>>

>>><N1DeltaWhiskey(at)comcast.net>

>>>

>>>John,

>>>

>>>Not sure what you are getting at here, but simply measuring the 

>>>in "resistance" of the antenna input will, I think, tell you little.

>>>Or, maybe I am missing what you are suggesting.

>>>

>>>Did a quick look at "characteristic impedance" on 

>>>Wikipedia.  When one talks about transmission cables (and 

>>>matching connections), impedance is the "apparent resistance" to 

>>>AC signals that the line is rated in, not simple resistance.  I 

>>>believe you are talking about measuring the impedance, not solely 

>>>the resistance, of the input.

>>>

>>>I could surmise some thing about this, but probably make myself 

>>>look a little (maybe a lot) unknowledgeable, so will leave 

>>>further clarification to those with a better understanding of the subject.

>>>But a good starting place for discussion is to have an 

>>>understanding of the relevant terms

>>>

>>>Regards,

>>>

>>>Doug Windhorn.

>>>

>>>

>>>

>>>

>>>---

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