[Antennas] measuring SWR

Sat, 10 May 2003 15:08:02 -0500

I don't claim to be an expert I can only tell you what has worked for me in 
the past. I've always had the best results building jpoles when I treated 
the J as a transmission line and made the ratio of conductor diameter to 
spacing to be around 300 ohms.

Yes, in theory, the top of the J is infinite and the bottom is zero. In 
practice I find that either end comes up a bit short on either end. that's 
my experience.

I just thought I'd offer my experiences with building Jpoles in an attempt 
to help. After reading your response and 6 others off the list (one of 
which was quite abusive) I see now I shouldn't have bothered. Obviously 
your 30 years in amateur radio trumps my 25 years as a paid professional 
communications technician. I will now shut up and unsubscribe from this list.

At 03:31 PM 5/10/2003, Joe Mayenschein wrote:

>"Douglas J. Koehler" wrote:
>
> > Mike,
> >
> > I've built around a dozen j-poles and variants in the past 10 years and
> > what you've got is an impedance mismatch. Your feedline is 50 ohm
> > unbalanced and your antenna is probably around 300 ohms unbalanced.
>
>Now I also have built dozens of "J" Poles in my almost 30 years in this hobby,
>And the above statement is a bit confusing to me,
>
>Yes the balanced to un-balanced thing is correct,, BUT,, I beg to differ 
>on the
>300 ohm statement,,  IIRC the position of the taps determine the antennas
>impedance.
>
>0 ohms at the base of the stub (connected to the short)  and nearly 
>infinite at
>the end of the stub. or at least several thousand ohms,,
>
>and everywhere in-between from 0 to the infinite.  That's why they are soo 
>very
>easy to match..
>
>Joe WB9SBD
>
>
>
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