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From: "Ashlock,William" <William.Ashlock@edwards.boc.com>
To: rsgb_lf_group@blacksheep.org
Date: Thu, 14 Nov 2002 22:17:57 -0500
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Subject: LF: RE: Re: RE: Loop vs Marconi
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Hi Mal,

Sort of figured you would be popping in on this.

>A loop only 6db down is a BIG DOWN ie signal 4  times weaker than a
vertical

6db is 6db! That's 1/2 the reading in uv/m on my SVM. You can try to over
emphasize this by stating the power multiplier. So far I'm NOT impressed.
<G>

>A canopy of trees will also effect a loop antenna, try a 50 ft vertical and
>gain 6dbs and you will probably still be on the plus side even with some
>attenuation from the trees.

You sir, are absolutely WRONG!

A spent 100s of hours on my vertical antennas before changing over to loops.
I carefully set up receiving sites at which to measure the far field
radiation with an accurate SVM. The field strength closely matched the
mathematics for a vertical radiator of my size, and current distribution.
The problem was high system resistance (as it appears to be in most of the
poor vertical installations). The system resistance plots from the chart
recorder made an excellent report of the sap content in the trees totally
surrounding the top hat. The only time the resistance was a worthy number,
say 50 ohms, was when the temperature dropped below 10deg F. This when most
of the sap exited from the branches are was stored in the roots. In the
summer the system resistance could be as high as 150 ohms. I even designed a
tree sensing probe that measured the effective series resistance of the
tress.

When I set up my first 50' x 50' loop I couldn't believe the increase in
signal at all my monitoring locations! About 6db using a #12 loop wire. At
that point I knew I had something worth investment of more time and began a
series of tests to determine the effect of varying everything: the number of
loop conductors (parallel and series connected), the effect of the spacing
to the ground, the effect of placing the loop right over the top of the
trees, the effect of ground moisture content, the effect of varying the
conductor diameter, use of Litz wire. I then spend many hours analyzing this
data and applied the math to it. In summary: LF loops can be placed on the
top of trees with lees than a 1db loss in loop current and the signal
strength in the far field is very close to the numbers from the text books.

My present loop is constructed from .63" OD copper pipe. The figure of merit
for 50/50 loop antennas is the total AC resistance (Rac), including ground
losses, and this one measures under 0.4 ohms. An RG-8 loop of the same
dimensions will have a dry Rac of 0.60 ohms and is one that has more appeal
to the average Lowfer. Signal is down just 1.7db from the pipe loop.

I wish you well my friend with your monster vertical, but just remember
there are 'mortals' like myself that are perfectly content with a 'second
rate' loop that installs in a couple of hours, doesn't need a ground sytem
or a chain saw, is nearly invisible to friends and neighbors...... and gets
out dam well, to boot!

Perhaps next year we can learn the real truth about how these antennas
compare by looking at each others signal strength readings? 

Bill Ashlock

 

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