[Antennas] VSWR on Log Periodic.

[George, W5YR]

No, but to be frank and hopefully a little less sarcastic than you, we *do*
understand how to use a Smith Chart to work this sort of problem.

Consider that people who know how have been doing so for 50-75 years very
successfully. 

Perhaps what you are overlooking is the fact that one must know the
electrical length, loss, and Zo of the feedline in order to work this
*magic.*   There is no restriction placed upon the antenna feedpoint
impedance: short, open, complex, real, etc. They all work. That is what you
also say in your second paragraph, but then you dismiss it as "another
matter entirely" which I do not understand.

Seriously, I think that you are misunderstanding what folks are trying to
say about this process. All it amounts to is that if you know the above
information about the feedline, and this is rather easily obtained from
simple measurements, you can translate the input impedance of the feedline
to obtain the driving-point impedance of the antenna. From that you can
obtain the SWR at the antenna feedpoint and other related information. If
you don't like Smith Charts, then use the Transmission Line Equation with
all the sinh and cosh functions and work it out that way.

In the early 80's I wrote a program that ran on the little Texas
Instruments TI99 Home Computer that did the above with remarkable ease and
accuracy. There are any number of excellent programs available now that can
do this job accurately and rapidly.

Sorry you disagree but engineers have been doing this for a long time and
it really works.  

73/72/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121  Kachina 505 DSP  #91900556  Icom IC-765 #02437


Jesper W Jespersen wrote:
> 
> I have heard this statement mentioned before, that you could calculate the
> SWR at the antenna from knowing the SWR at the end of the feedline and the
> feedline propperties, but I have to disagree.
> 
> If you knew the impedance at the end of the feedline and the feedline
> propperties you would be able to calculate the impedance of the antenna. But
> that is another matter entirely.
> 
> Picture a smith chart, a given SWR say 1.6 is a circle arround Zo. Any given
> feedline will move a point on this circle to another point in the smith
> chart, forming another figure. All the points on this distorted circle are
> possible candidates for the ipedanse of the antenna and will give wildly
> different SWR's.
> 
> What makes you guy's think that there is a way to calculate 1:1 from SWR at
> the feedline to the SWR at the antenna ?
> 
> Do you silently considder the load to be purely ressistive ?