Ian White GM3SEK wrote:
> 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 :-) ]
>
>
OK Ian, I follow you on that, no problem. Was thinking more about the
"load itself". Sure I can see that the " system Z" will change since
feeder will become part of the "radiating system". Maybe in this case
you just can´t discard the feeder from the load.
I was thinking like this, you measure the impedance of the load (Yagi in
this case) right at the feedpoint. Whatever you feed your measuring
device with the impedance of the load object will not change. But as
I said before maybe you just can´t see the load as a isolated object
since the feeder will become an active part.
Now I´m gonna go and play radio, maybe I can upset the impedance of my
Yagi´s.
73 Jim SM2EKM
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