[TowerTalk] loss resistance

[Tom Rauch]

> >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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