Topband: Beverage Improvements?

[Tom Rauch]

Hi John,

The vertical end-coupling of Beverages is a mostly non-problem 
that has been over rated, and the "cures" are mostly non-cures for 
a non-problem.

One way to look at it is six feet of vertical drop is "six feet" of 
vertical drop, no matter how the "six foot drop" is spread around, 
and that isn't any big deal when the antenna is several hundreds of 
feet long.

If that isn't reason enough not to worry, the entire antenna 
responds to vertical signals anyway...especially on groundwave!!

Second, it is physically and electrically impossible to "shield" the 
vertical end lead no matter what scheme you employ. The antenna 
MUST always have the same net common mode current flowing to 
earth over the vertical lead distance to ground.

You and I could be millionaires if we found a way to violate that rule.

The only way possible to prevent that effect is to move the entire 
ground system up to the element height, and that means work with 
a bulldozer making a large mound or installing a large counterpoise 
hanging in the air at antenna height. Only those options can 
prevent common mode current from flowing to earth! 

> as possible. I made a 500 ohm transmission line out of scrap 3 inch boom
> material cut to be as long as the beverage is high. I put an so-239 near
> the bottom and a plastic pet food cover as the end insulator on the top
> and ran a #28 wire down the middle (I think the math says #30, but #28 was
> what I had). Using a few inches of larger wire on the ends makes it easier
> to work with and has little effect on the impedence. 

Some books will give you the idea that scheme does something to 
"shield" the vertical end wire, but it does not.

On the outside of the tubing, we would measure exactly the same 
common-mode current as the end-current in the antenna. The 
vertical wire carrying current in the center of the tube induces an 
exactly equal opposing flow on the wall, that spills over at the open 
end and flows down the outside of the tube. That's the same way a 
"shielded loop" receives a signal. (If the shield worked as a shield, 
the shielded loop antenna would be stone dead for all signals!)

If you measure the voltage drop from the top of that vertical tube to 
earth, it will be the same as the voltage drop in a single wire of the 
same diameter connected directly from the antenna to earth! Even 
the overall electric field response does not change with this 
scheme.  

Just like running a vertical piece of coax up to a horizontal wire in 
an inverted L (coaxial inverted L's), the vertical transmission line on 
the Beverage will radiate exactly as a single wire alone would have 
radiated.

Not that it matters, since that radiation is generally insignificant.
  

73, Tom W8JI
w8ji@contesting.com



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