[TowerTalk] 160m shunt feed followup

[Tom Rauch]

> You can back out the value of X using the SWR and value of Z.

True.
 
> The formula is in the AUTEK manual.  The sign of it is easy to
> determine.

Not true!

> First find the resonant freq.  If your freq of interest is higher then
> the sign of Z is + (inductive) and vice versa.  To get practice, try
> this first on a simple antenna with well known characteristics.  Take
> about 5 points either side of resonance and plot the R and X values.
> It will be pretty obvious when X changes signs.

Unfortunately this is what some analyzers assume. It is reliable 
with lumped components.

It is unreliable when you are measuring a system with transmission 
lines. It is possible to have increasing Z or X with decreasing or 
increasing frequency and the sign to be either capacitive or 
inductive randomly.

Since antennas have transmission lines integrated in them, even 
measuring a simple antenna with that method can prove misleading.

The only reliable way is to measure phase angle, or add a 
reactance of a known sign and see how impedance changes.
73, Tom W8JI
W8JI@contesting.com 

List Sponsored by AN Wireless:  AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!!  http://www.anwireless.com

-----
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com