I believe there is a large influence of height above ground on VF.
From a theoretical basis, this has been "known" since Wise's classic paper
"Propagation Of High Frequency Currents In Ground Return Circuits" (1934).
This was experimentally verified in the Litva and Rook report from the CRC
(Canada), and compared with theoretical results.
These guys didn't extend their calculations to right-on-the-ground
antennas. The attached (if it gets through the server) is from a
spreadsheet of mine based on the Wise equations. The influence of height
on VF is very very pronounced.
Chuck
Frank is absolutely correct in what he said.
The velocity factor decrease in the Beverage has nothing to do with the
arriving wave velocity that affects the required phasing spacing. The wire
looks longer because the earth slows the wave ***in the wire's transmission
line mode***. The required spacing and stagger is set by the wave, not the
wire. It is the same in a BOG, a normal Beverage, or in a vertical.
The broadside spacing, to increase directivity a useful amount, has to be up
around 1/2 wave or more. The end-fire or echelon spacing has to be the same
as a normal Beverage, or vertical, to have useful directivity increase.
The only thing the earth does is slow the velocity in the transmission line
formed by the wire and earth image. The antenna cannot be a long as a
regular Beverage because of the slowed propagation in that "transmission
line". It is little different than loading the wire with any lossy
dielectric.
While the antenna is limited to less length because of velocity factor in
the wire's transmission line mode, the fact it is a BOG has no bearing on
the wave velocity, or the required spacing or stagger.
73 Tom
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|