[TowerTalk] Determining "True" Resonance of Verticals

[Tom Rauch]

> The 6 or 7 ohms is only the *reactance* reading, not the
> resistance reading and at this point I am only recording
> reactance which as you said, should be zero at resonance.

Two things can cause that. 

Improper adjustment of the 259B's internal controls or external 
voltage coming from the antenna that upsets the detectors.

There is a filter sold for HF that cures most external voltage 
problems, although a better long term solution would be if the 259B 
had actual receivers for detectors. Unfortunately that would make it 
a $1000 instrument or more...so none of the popular bridges use 
selective detectors.

Check a 1/4 wl open stub and see if reactance crosses zero. If 
reactance does not cross zero, either the bias in the RF amplifier 
of the 259 is set wrong or the Vs and Vz gain settings are wrong 
and the unit needs calibrating. Keep in mind that as little as 1/2% 
error in detector voltage can result in a few ohms of reactance 
showing when there is none! 

If reactance does cross zero with the stub, you have external 
voltage problems on the antenna.    

Lowest SWR is always lowest reactance when you are looking at a 
load, no matter what load impedance is.

There is no combination of reactance and resistance that results in 
a lower SWR than a pure resistance...even if the resistance is not 
50 ohms. For example, a 20 ohm resistive load has the lowest 
possible SWR (referenced to 50 ohms) when reactance is zero. 
The same is true of a 400 ohm resistance load.   

If you look at a load through any transmission line, it will always 
shift resonant frequency if the load and line are not matched. The 
exception is when the line is an exact multiple of  90-degrees long.

The shorter the line, the less the problem.

 
73, Tom W8JI
W8JI@contesting.com 

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