> >Think about that Pete. If the loss resistance were in parallel
> >with the feed impedance, then the higher the LOSS resistance,
> >the LESS it's effect would be. NOT logical.
> >
> >The Loss resistance is in SERIES.
>
> Tom's right, of course, but .... if I measure R=7 at the bottom of my
> shunt feed, and the tower is only grounded by three ground rods and 4
> x 125 foot radials, how much of that series combination is loss, and
> how much is the feedpoint? It's hard for me to imagine that the loss
> component is only 7 ohms.
It may not be 7 ohms Pete. It is a mistake to simply assume the
deviation you see from a certain feed impedance directly translates
to loss.
In order to sort loss resistance from radiation resistance you have
to measure the resistance at one point in the system, then enclose
the entire system in a very large conductive enclosure and
measure the change in resistance. While that is sometimes done
at UHF, it is a big task at lower frequencies and impossible on 160.
The ONLY way the FCC accepts data on loss is by measuring field
strength along a long path distance in several directions. You
measure the slope of attenuation to estimate the ground
conductivity (poorer ground has greater attenuation with distance),
and then use that slope to predict what the FS should be for a
perfect radiator over the same ground. Everything is normalized to
that value by factoring in estimated losses by the slope of
attenuation.
While far from ideal, the FCC method is the best thing we have for
low frequencies.
Base impedance is unreliable because of all the other things
around our antenna, and because some ground losses do NOT
show as an increase in base resistance.
73, Tom W8JI
W8JI@contesting.com
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