Jan Erik Holm wrote:
>Ian White GM3SEK wrote:
>>
>> RF currents on the outside of the feedline will also change the
>> impedance at the antenna feedpoint,
>
>Ian,
>
>I´m not sure I buy this. It might introduce measurement
>errors but thats another thing.
>
>I might be tired this morning, can you explain further.
(Pause to reload coffee pot...)
Let's assume that the yagi would have a low VSWR if the coax feedline is
installed correctly.
If we somehow install the yagi and its feedline in a way that makes
current flow on the outside of the coax shield, what happens then? The
shield has now become an unwanted "long wire antenna" connected to one
side of the yagi's driven element.
The simple point is: THIS IS NOT THE SAME ANTENNA ANY MORE!
We now have some weird kind of "yagi terminated long wire" antenna,
which is sure to have a different radiation pattern and a different
feedpoint impedance from what the yagi had. Whatever the VSWR was
before, it will now be different... and almost certainly worse.
If we install a choke balun at the feedpoint (also known as a feedline
choke, 1:1 current balun or line isolator - all different names for the
same device), we are placing a large impedance in the path of any
current being launched onto the outside of the feedline. This should
mostly prevent the unwanted "long wire" behavior, and the VSWR should
return to whatever is normal for the yagi on its own.
[There are many more details, exceptions and unusual possibilities. That
was the short version for amplifier users :-) ]
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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