Not only that, but it defies logic that radials would NOT exhibit the same
current and voltage distribution of any other conductor carrying RF
current. The boundary condition is near zero at the end ( "near" because
of capacitance at the end). Rudy Severns, N6LF, has explored this in his
studies of radial systems. Rudy's work includes extensive modeling to
understand and document what he was seeing in measurements of carefully
constructed experimental systems.
There are only three ways a radial would not exhibit standing waves (waves
that increase and decrease in level with distance):
1.) The radial is too short to have enough space for a part of the
wavelength to stand. An example of this would be trying to measure the
current difference along a 60-foot wire on 160 meters. It would just
smoothly taper.
2.) The radial is terminated in the surge impedance of the radial. This
would be like a transmission line terminated with the correct resistance.
3.) The ground sucks up the current at such a rate that there is not enough
current left to increase.
All the radials I have measured that are long enough to be over 1/4 wave
electrical have shown standing waves. The current is less at the base than
some distance out from the base, and it never seems to have anything
noticeable to do with radiator height.
I can take a 40M 1/4 wave vertical, surface bury 15-20 radials, and find
long lengths (beyond 1/4 wave) that make base impedance go to 50 ohms or
more, and the FS measures the same as other systems that have 35-38 ohm base
impedance.
Feed impedance doesn't even have to track the efficiency.
73 Tom
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|