> 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.
That is similar to what I have measured Rick. I measured in the
range of 60-80% of current remaining at the far end of a properly
terminated Beverage 1WL long.
The correct way to terminate a Beverage is for a smooth current (or
voltage) decrease along the length, rather than ripples in current
(voltage) or equal current (voltage) at each end. Lowest SWR does
not work, but what will work is adjusting the antenna for minimum
SWR change (or SWR ripple) with frequency.
We can also measure the highest and lowest resonant resistances
of an intentionally misterminated Beverage as we sweep frequency,
and set the termination at Rt= sqr root of Rmax times Rmin.
Obviously the Beverage MUST have noticeable current reduction
along the length, because the feedpoint resistance indicates only
an ohm or so of radiation resistance with the remainder related to
loss resistances. It is a very lossy system, with ground coupled
loss the bulk of the losses along with termination losses.
Models of the beverage do not seem to accurately account for
current loss. I think this also has something to do with why
elevated radials have less efficiency than predicted by models in
many cases.
The effect you describe is well known in longwire arrays, even
those spaced high above ground. In actual experiments with
antennas at General Dynamics, wires 1/3 of their length above
ground gain limited at about 3-1/2 wavelengths. That is the point
where V Beams and Rhombics failed to show a gain increase with
increased side length. Of course that distance would be smaller if
the wire is closer to lossy earth.
I find my Beverage systems seem to "performance limit" at around
750 feet or so on 160 meters...and actually become poorer if made
significantly longer. For example, I have a new Beverage array of
two ~1000 foot Beverages to the west that is a real dog compared
to an array with two 780 foot wires.
I believe this effect is rooted in three problems. The inability to have
uniform current with properly terminated long antennas near earth,
destructive phase errors along the length, and the obvious random
variations in phase and level with incoming skywave signals in
areas separated by more than a wavelength or two.
It looks like your observations are reasonable.
73, Tom W8JI
W8JI@contesting.com
|