Topband: Velocity factor in two wire Beverages

[Ken Brown]

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