>>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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