Topband: Beverage current distribution

[Tom Rauch]

> 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