[TowerTalk] Radials, resonance and the MFJ259

[L. B. Cebik]

SWR and Impedance:

Like the reflection coefficient on which it is based, the SWR along a
length of transmission line (lossless) will be everywhere the same.  It
will slightly decrease in accord with line losses per unit measure and
total line length.

Impedance will not be the same everywhere along a transmission line
(unless the line and load happen to be matched precisely).  Impedance is
the vector sum of resistance and reactance, which change continuously
along the line--and with them, the impedance.

Look in a handbook at the equations for SWR and for impedance and you can
see the difference.

So, if you measure Impedance at the antenna and at the rig, you can get
legitimate different numbers.  If you measure SWR at the antenna and at
the rig, then they should be the same--or, for line loss, a little lower
at the rig (except for feeding UHF rigs with WWII surplus RG-8, in which
case, SWR at the rig will be lots lower than at the antenna, while power
at the antenna will be lots lower than at the rig).

You can check the R and X transformations (and calculate Z) along
transmission lines with any input antenna values with one of the HAMCALC
programs or use Dean's TL, etc.  It is a good thing on a dead-band
day/night to run lots of values systematically through such programs to
familiarize yourself with trends.  Makes interpreting measured results a
lot easier with fewer wrong paths.

Hope this is useful.  Happy holiday season and a (sun)spotty New Year to
all.

-73-

LB, W4RNL



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