Topband: Beverage current distribution

[k6se@juno.com]

Richard, N6RK wrote:

"Yesterday, I measured the current distribution on my 550 ft beverages. 
The near end/far end current ratio
was 5 dB on 160m gradually increasing to 10 dB on 40m."
==========
If properly teminated, The current distribution along a Beverage should
be almost equal along the full length of the wire, with no peaks and
valleys along the wire.

The way H.H. Beverage determined the correct termination resistance was
to measure the current and/or voltage along the wire while varying the
termination value until he found the most flat dustribution along the
wire.  I do this by transmitting about one watt of power into the wire
and walking down the full length of the wire with a voltage-detecting
field strength meter.

One way to visualize what is happening is to assume that the termination
resistance is zero ohms, in which case the voltage at the terminated end
would be zero (assuming a perfect ground connection).  If the terminated
end is open (infinite termination resistance), the voltage would be at a
maximum (the same as it is at the open end of any antenna).

The correct termination value is where there are no peaks and valleys
along the full length of the wire.  The field strength reading should
gradually decrease slightly (due to wire loss) as you walk from the fed
end to the terminated end of the wire.

The correct termination will probably be different on 40 meters than it
is on 160 meters, mainly because of the fixed height of the wire above
ground.

Because the velocity factor of the wire is less than the velocity factor
in air, the optimum Beverage length is approximately 2 wavelengths as far
as gain is concerned, although narrower patterns can be acheived (better
S/N) with Beverages longer than that.

73, de Earl, K6SE
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