[TowerTalk] Re: Beverage boxes

[Tom Rauch]

> I know, the purists will tell me that I am compromising
> my performance.  Hey, I don't even terminate my Beverages.
> I still hear plenty of DX on the Beverages that I can't copy
> on my transmit antennas.  Results?  Only need A5, BS7, P5
> on 40M,  and only need ~16 countries on 80M.  160 could
> be better (have 240+ countries). 10 or more acres would help.

Hi Tom,

The difference between terminating and not terminating a Beverage 
has progressively less effect as frequency is increased. That's 
because ground losses below the antenna and radiation from the 
antenna tends to "self-terminate" the antenna as it is longer and 
longer in terms of wavelength.

With my 500 foot long antennas on 160 meters here, about 30% of 
the current is lost by the time the far end is reached. That's about 
half the power, or 3 dB one-way loss. That means the antenna has 
almost 4 or 5 dB F/B with no termination!

The longer the antenna in wavelengths, the more the antenna "self-
terminates" along its length.

On 160 meters, if noise is primarily coming from the backfire 
direction, termination can improve S/N ratio by 10 dB or more. If 
noise is evenly distributed around the antenna termination will 
improve S/N by a couple dB. If noise primarily comes from the 
direction of the signal termination will make no difference.

If you use a poor ground system on the Beverage, such as a few 
ground rods, the feedline shield can "pump" the ground rod up and 
down in potential with noise and signals picked up by the feedline 
shield or conducted along the outside of the shield from the house.

That's why I use isolation transformers with a two turn primary and 
five turn secondary on a 73 mix binocular core to feed my 
Beverages. Autotransformers connect the shield directly to the 
Beverage ground system, not always a good idea since the 
feedline can be a big "antenna" when the ground system at the 
Beverage is less-than-perfect.

I also use connectors, primarily so I can service the antenna and 
test the antenna. A shielded box is a waste of time, however. As 
are "Faraday shields" and sloping feedpoints.




73, Tom W8JI
w8ji@contesting.com

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