From: "K9AY"
"The remedy is simple -- use new 2-watt carbon, ceramic composition,
or other
robust, non-inductive resistors. Rule-of -thumb for value is 560 ohms
for a
high (8-foot) Beverage, down to maybe 270 or 330 ohms for a very low
or
on-ground Beverage."
-------
Very low or on ground beverages start to get into velocity factor
issues which change performance considerably and effect what value of
terminating resistor produces the best pattern.
For a 147.5 foot (insulated wire) dipole laying on ground, experiments
have measured center-fed resonance indicating a velocity factor of
anywhere between 65% and 90%, with the insulation itself only
contributing a few percent of that. Significantly, this VF varies with
ground moisture changes at a given site.
In a NEC-4 model over the estimated ground type, the wire height
(rather small) of the dipole was varied until the resonance in the
model matched the measured, thus also matching the velocity factors.
Models of various on-ground beverage (BOG) configurations were run at
that wire height to better estimate real world patterns of BOGs .
Terminating the BOG in its characteristic impedance does not always
produce the best pattern result because the speed of incoming RF is
not close to the velocity in the wire. For regular beverage heights
these speeds are nearly identical and the regular rules apply.
The velocity factor differences and un-beverage-like behavior with
different terminations and varying velocity factor surely does account
for some of the mixed success of beverages on the ground. There really
is not a single best termination resistor for a beverage on the
ground.
Some situations produced better patterns simply grounding the
terminated end. And in that situation or with low (e.g. 50 ohm)
resistance grounds, the great unknown was the actual resistive nature
of the far end grounding which when varied had huge effects on
pattern.
73, Guy.
Scanned for viruses by Blue Coat
http://www.WinProxy.com/
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|