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This explanation of the antenna I'm using on 160 meters may be of
interest to others on the reflector. Using this antenna and running 125
watts, I have worked 43 countries on all continents this DX season,
excluding Asia and Antarctica, mostly on CW. Take care and
73, Thom
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From: kn4lf@webtv.net (Thomas Giella)
To: <topband@contesting.com>
Date: Wed, 24 Mar 1999 13:43:51 -0500 (EST)
To: aa4mm@email.msn.com
Subject: Miscellaneous
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Hi Leo,
Just many lines to further clarify what antenna I'm running hi hi.
Basically I'm using a half wave L. I don't call it an inverted L, as the
highest current point on the antenna is up at the top of the mast at 45
feet, instead of on the ground. By elevating the highest current point
you avoid ground clutter, which can absorb and/or distort the omni-
directional radiation pattern and you reduce local vertically polarized
noise pikup on receive. When I was running the quarter wave inverted L
earlier in the season my noise level was always S9 to +5 but with the
halfwave L my noise level is S2 to 3. In effect the highest current
point get's a better omnidirectional look at the horizon on transmit and
receive.
Also by elevating the highest current point you are reducing
capacitive coupling losses to ground in the near field and therefore can
get by with less radials. My ground system consists of 27 quarter waves
and 4 quarter wave counterpoise wires 10 feet off the ground, as well as
4 10' ground rods and my copper water pipe system in the house and yard.
With a halfwave vertical what becomes more important as far as ground
loss is what's happening in the mid field of the antenna(2-4
wavelengths) and the far field, which extends out to around 100
wavelengths (the Fresnel zone) on 160 meters, of which we have little or
no control over (see section 3.3 on pages 9-30 and 9-31 in ON4UN's Low
Band Dxing book). Fortunately at my QTH, the ground conditions in the
near field are good as I have a deep layer of mucky wet soil and a high
water table with a high iron content. In my mid and far fields I have
numerous lakes and swamps, best case senario!
To visualize my antenna I have a quarter wave horizontal section and
a quarter wave vertical section (total length 258 feet), so the highest
current point is at or near the 90 degree bend at 45 feet up, depending
on the frequency of operation. The quarter wave vertical segment is in
three sections (linear load, see figures 9-27 and 9-29 in ON4UN's Low
Band DXing book) all in the same plane towards the northeast.
As you know, the radiation resistance of a quarter wave vertical (in
theory) is around 36 ohms, a three eights vertical 57 ohms at the high
current point and 300 ohms at the end feed point, a half wave vertical
69-72 ohms at the high current point and 1000 ohms at the end feed
point. So I'm looking at a very high radiation resistance and therefore
a very efficient antenna. I tune out the inductive j with a homebrew
parallel network and then use RG-8 back to the rig. Normally on a
quarterwave inverted L, you are looking at a radiation resistance of
around 15 ohms for a vertical section 45 feet long but when you use the
linear load arrangement you increase the radiation resistance to a more
efficient level, somewhere between 300 and 1000 ohms?
So that's the deal on the antenna and it works pretty darned well,
especially using 125 watts. The halfwave L seems down compared to the
qurterwave inverted L
between 300 and 500 miles out but the halfwave wins easily at 1500-5000
miles out! I hope I don't bore you with the above explaination of the
antenna. Take care and
73, Thom
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