[TowerTalk] TA-33 feed

[Stan Griffiths]

>>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 agree that is true for a normal matched coax application.  I don't really
know what happens when the VSWR is high.  I am not sure the field is really
confined to the dielectric between the shield and center conductor in that
case.  (I'm not sure it's not either, for that matter!)

>>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

Yep, I agree, but all this assumes there is no radiation from the outside of
the coaxial shield.  I'm out of my league here since I don't really know
what radiation really occurs from coax with high VSWR.

Stan  w7ni@teleport.com


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