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[TowerTalk] TA-33 feed

To: <towertalk@contesting.com>
Subject: [TowerTalk] TA-33 feed
From: n7ml@imt.net (Michael Lamb)
Date: Mon, 27 Jul 1998 20:22:17 -0600


----------
From:   T A RUSSELL[SMTP:n4kg@juno.com]
Sent:   Sunday, July 26, 1998 5:22 PM
To:     w7ni@teleport.com; TOWERTALK@contesting.com
Subject:        Re: [TowerTalk] TA-33 feed

Hello Tom:  There is an interesting article on the topic of RF spillover onto 
coax feedlines that was written by the late Dr. Donald K. Reynolds.  It is on 
the Internet at:   www.aea-wireless.com     Click on articles at the bottom of 
the home page (I believe) and I think you will find some very interesting 
reading.  It explains in pretty straight forward language how you can really 
get radiation from the outside surface of a feedline when all the RF energy 
from the transmitter source started out between the center conductor and the 
INSIDE surface of the shield.  The amount of leakage through the shield is 
usually very small compared to the spillover effect when the antenna is not 
properly decoupled.

73/Mike, N7ML



On 26 Jul 1998 20:19:41 -0000 w7ni@teleport.com (Stan Griffiths) writes:
>
>>The advertisements indicting feedline radiation as the cause of
>>TVI and BCI are a joke.  Does anyone really believe that feedline
>>radiation will cause TVI but radiation from the antenna does not?
>>
>>de  Tom  N4KG
>
>Hi Tom,
>
>I always figured that the radiation coming from a feedline was 
>physically closer to the TV antenna than that coming from an antenna 
>so it was much stronger and more likely to cause overload in the TV set.
 
>I never saw it explained tht way, but it is the only explanation that
makes 
>any sense  to me!

        That would only be true if your feedline has VERY poor
        shielding.  This is true for ladderline in the near field.
        Ideal COAXIAL cable keeps all of the field between 
        the center conductor and the INSIDE of the braid.
        Only a small amount of leakage should radiate from
        the line.  (BTW, I was only thinking of a neighbor's
        TVI....when I am operating, MY  TV  is OFF :-)   N4KG

>I have also heard that a high VSWR will render a low pass filter 
>somewhat
>less effective and this could be the actual cause of TVI coming from a
>tranmitting system with a relatively high VSWR.  That makes some sense 
>to me as well.
>
>So it seems to me there COULD be a relationship between TVI and VSWR 
>but not
>because feedline radiation is fundamentally any different than antenna
>radiation.
>It could be because with feedline radiation, you are simply stuffing 
>more
>power (and more harmonics and parasitics) into the TV or BC set than 
>you
>would if the feedline did not radiate.
>
>What do you think?
>
>Stan  w7ni@teleport.com
>
        SWR at the antenna WILL cause higher currents to flow
        in the feedline and so will tend to exacerbate whatever
        leakage there is from the coax.   I don't know the attenuation  
        of a signal that leaks from a cable vs. what is radiated from
        the antenna, but I expect it is a LOT.  I would expect poor
        cable might be as bad as 10 dB down and good cable 
         (like hardline) could reach 50 dB or more.    N4KG


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