> 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
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 96 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
|