Topband: Beverage height issues
ford at cmgate.com
Thu Jan 11 10:45:54 EST 2007
This discussion is somewhat tainted by everybody's personal experience. And
I don't think experience translates well from QTH to QTH.
A discussion of "height above ground" should really be (it seems to me) a
discussion about "height above apparent ground." There is a difference.
Over sea water, ground effectively extends down to what depth? Just a
SWAG--maybe 6" to 12"? I have read of studies where real ground can extend
down 30 feet or more over rocky and otherwise miserable ground.
To make empirical evidence comparable from one QTH to another, it seems to
this novice that a beverage mounted 3 feet over some typical ground
characteristic may need to be mounted 20 feet over salt water to be
comparable. When I built my beverage, I was looking at height as being some
magical equation. Yet empirical evidence is all over the map as far as what
works and what doesn't work. In 100% of the evidence I found, I could
ascertain NO discussion of apparent ground to isolate the height variable in
any meaningful manner. And dozens of other factors may contribute to
During my investigations, I attempted to actually measure my "apparent
ground." As an experiment, I used the transmission line equation to
identify some sort of average ground depth. I did this by using a 500'
spool of wire laying directly on the ground. My study included looking at
the characteristic impedance of a two wire line, using ground as the second
wire. I also spent some time looking at the Vp of that wire. The closer it
was to apparent ground, the slower the line. While I did that study many
years ago, my conclusion was that my apparent ground is (on average along
that 500' length) roughly 30" to 36" below the surface.
If you were to discover that your apparent ground is 72" below the surface,
then it seems to me that a beverage mounted at 8' at my QTH would be roughly
comparable to a beverage mounted at 5' at your QTH. In practice, we design
our beverages based on other circumstances. In my case, I needed to be able
to allow neighborhood teenagers to drive their snowmobiles at 50MPH
underneath that wire. In your case, it may be the availability of 4'
fiberglass electric fence posts that dictates your "height."
The bottom line is that ANY beverage is better than NO beverage. Build it,
use it, enjoy the results. What you find in terms of performance is going
to be dictated by the dozens of variables that make up your experience--and
it likely will not translate to results elsewhere.
ford at cmgate.com
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