[Top] [All Lists]

[TowerTalk] 160m shunt feed followup

To: <>
Subject: [TowerTalk] 160m shunt feed followup
From: (alsopb)
Date: Tue, 02 Oct 2001 11:28:05 +0000
I'll take Tom's word on this.  

I haven't used this for antennas with feedlines built in --e.g. the
GAP verticals(?).  I think plotting the value of X vs frequency as
suggested, will tip you off.  If it's shape differs considerably
compared to a simple antenna (e.g. dipole), that's a tip-off that the
results should not be trusted. 

73 de Brian/K3KO 

Tom Rauch wrote:
> > 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

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!!

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>