Hi Brent,
A few additions to what Tom said, that may help clarify the situation:
In a two wire, selectable direction Beverage, differential mode or
transmission line mode is only used to send the signal reflected off the
far end of the antenna back to the near end where the feed points are. (
I am calling the end of the Beverage with the feed lines and feed
transformers the "near end". ) Transmission line mode of signal
propagation on a pair of wires, in which the currents in the two wires
are equal and opposite, cannot radiate or or pick up signals. That is
what defines a transmission line. In a two wire Beverage, when receiving
from either direction, the two wires are working in common mode to
receive the signal. That is the currents induced in the two wires by the
incoming signal are in phase with each other. When receiving from the
direction that the near end points towards, these currents are then
reflected from the far end of the Beverage. They are reflected in
opposing phase, so that they propagate back to the near end in
differential mode, transmission line mode, using the two wires as a
transmission line to send the signal to the differential mode feed
transformer.
There are a number of possible ways to produce this phase reversal. The
simplest being to have one wire open circuit and the other grounded.
That method works, although the method using transformers at the far end
probably results in better current balance and 180 degree phase relation.
The dielectric constant of the insulating material between the two wires
determines the velocity factor of the differential mode, or transmission
line mode of propagation on the wires. The dielectric constant of the
material between the wires and the ground, and the ground
characteristics, determine the velocity factor of the common mode of
propagation on the wires. The common mode velocity is the one that
counts for receiving the signal, and determines the optimum length.
There is a book titled Beverage Antenna Handbook, if I remember
correctly, by Misek W1CSK, wherein both directional feeds from the two
wire Beverage are combined with adjustable phase. This is purported to
facilitate better side lobe reduction, and it may actually work. If you
were using this method, the velocity factor of the transmission line
mode would have some effect on the relative phase the two signals
arriving at the combining network. So the network would have to be
adjusted slightly differently.
DE N6KB
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