[TowerTalk] KN4LF/KK4TR 160-10 METER L ANTENNA
Thomas Giella
kn4lf@webtv.net
Thu, 19 Aug 1999 12:30:22 -0400 (EDT)
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KN4LF/KK4TR 160-10 METER ANTENNA
Let me start off by stating that KK4TR and I don't make a claim that
we have invented a brand new concept in vertical antenna design, though
some of the combined design aspects of the antenna may be unique! We
have simply identified the basic inherent weaknesses of your average
city lot 1/4 wave inverted L and have devised methods to overcome these
weaknesses. This antenna design is not a rival to a 4 square vertical
array but will outperform your average backyard 1/4 wave inverted L or
dipole by leaps and bounds.
KK4TR and I are not Electrical Engineers, just two voracious readers
of every book on antenna theory and design that we can get our hands on,
some 50 years old. As avid antenna experimenters, we have put 4 years of
field experimentation into this antenna design,in 1990-91 and 1998-99.
Along the way we have come to the conclusion that antenna theory is just
that theory, concepts not yet completely proven by controlled scientific
experiment and not to be taken as gospel! We have also concluded that
alot of sound basic antenna theory and design has been lost to time
and/or watered down, to the point that many Amateur Radio Operators are
grossly misinformed about the basics.
To be certain, an Electrical Engineer may come along and try to poke
holes in some of the following antenna theory and concepts but one thing
that can't be disputed is that the antenna is a proven performer! The
average city lot backyard 1/4 wave inverted L suffers from several
inherent weaknesses, high vertically polarized local noise pickup,
absorption and pattern distortion of radiated signal due to surrounding
ground clutter, high capacitive coupling signal loss between the antenna
and your average poorly conducting soil conditions and low radiation
resistance, a measure of antenna efficiency, due to the typically short
(25-50 ft) vertical radiating element section of a 1/4 wave inverted L.
With much effort the near field transmitted signal losses can be reduced
to a point that you improve antenna efficiency to around 50% but the
average backyard location makes it impossible to overcome signal losses
in the mid field (1000-2000 feet) on 160 meters and signal losses in the
far field (around 52,000 feet)(fresnel zone) is out of reach for all
Amateur Radio Operators.
The 160 meter linear loaded 1/2 wave L antenna places the highest
current point at the top of the support structure gaining the following
advantages. The elevated highest current point of the antenna is above
the majority of the local vertically polarized noise field. At my QTH my
1/4 wave inverted L noise level was always S9 to +5 over. With my 256
foot 160 meter linear loaded halfwave L, the noise level has been
reduced to S2-3. Of course the actual amount of noise reduction will
vary from QTH to QTH. Another advantage of elevating the highest current
point is, reduced to nearly eliminated radiated signal absorption and
pattern distortion, away from omnidirectional. In a sense you can say
that the highest current point is getting a better omidirectional look
at the radio horizon.
Another advantage of elevating the highest current point, is the
reduction of capacitive coupling signal loss between antenna and ground
and gaining the advantage of laying down less ground radials. Logic
dictates that placing distance between the highest current point of the
antenna and ground, reduces the coupling losses. The agreed upon
standard for number of ground radials for a vertical antenna is 120 1/4
waves but you see a rapidly diminishing point of return after 16-20 1/8
to 1/4 wave radials. An alternative to ground radials is an elevated
counterpoise, which will be covered further into the text.
Radiation resistance, which as stated earlier is a measure of
transmitting antenna efficiency is obviously a very important variable,
basically the higher value the better. A 1/4 wave inverted L with a
vertical section of 50 feet, will have a very low radiation resistance,
around 15 ohms (very inefficient), increasing to near a theoretical 36
ohms as you approach a vertical length of 1/4 wave. Take this 36 ohms of
radiation resistance and couple it with a poor ground radial system and
you still have a very inefficient signal radiator.
There are several methods that can be employed to increase radiation
resistance and henceforth transmitting antenna efficiency, excluding the
laying out of dozens of ground radials. One is to raise 1-4 ground
radials into an above ground counterpoise system. Four 1/4 wave wires
approximately 15 feet off the ground, can rival 120 1/4 wave radials on
the ground, as far as transmitted antenna efficiency goes but not
necessarily concerning absolute lowest radiation angle. Another is to
lengthen the transmitting antenna. As mentioned earlier, in theory the
radiation resistance measured at the end feedpoint of a 50 foot vertical
section of an inverted L is around 15 ohms, a 1/4 wave linear loaded L
is near 30 ohms, a 1/4 wave 36 ohms, a 3/8 wave 300 ohms and a 1/2 wave
1000 ohms, a very efficient figure indeed! Basically as you lengthen the
radiating element the radiation resistance increases and it decreases as
you shorten it, it also varies with the diameter of the radiator.
Antenna input impedance varies according to where you feed it.
So that's it in a nutshell, the 160 meter linear loaded 1/2 wave L
overcomes all the inherent weaknesses of the average city lot backyard
1/4 wave inverted L.
Now let's discuss the benefits of using the 160 meter linear load 1/2
wave L on 80 through 10 meters, as a multiband antenna. As the length of
a transmitting antenna exeeds a fullwave on the operating frequency
interesting things begin to happen. Gain starts to increase and the
radiation moves inward towards the axis of the transmitting wire, versus
the 90 degree broadside you see on a halfwave dipole. As the
transmitting antenna continues to become even longer in comparison to
the operating frequency, multiple lobes of radiation form on the wire in
response to the numerous highest current points that exist. The
following table lists by band the number of highest current points on
the wire (1/2's), the increase in gain and the radiation angle with
respect to the antenna wire axis, with 90 degrees being broadside to the
wire and 0 degrees being off the ends.
256 Feet
FREQ KC #1/2 Waves Gain(dbd) Rad. Angl
1845 1.01 0.0@ 90
3888 2.14 0.5/1.9* 52
7225 3.99 1.3/3.0& 35
10115 5.58 2.2 29
14263 7.86 3.0 24
18139 10.00 4.0 21
21338 11.83 4.8 20
24960 13.87 5.6 19
28400 15.73 6.3 18
@- some slight increase in gain over a 1/4 wave
*- collinear antenna, 2 1/2 waves in phase &- 4 1/2 wave In phase
Using this antenna on 17 meters I have worked 107 countries with minimal
time and effort.
It is strongly recommended that a parallel network tuner be used to
load up the L antenna, as in a sense the tuner is part of the antenna.
Also as a tuner will see approximately 10,000 ohms of feedpoint
impedance on 160 meters, your average store bought T network tuner can't
deal with such a high impedance. My tuner consists of one 250pf variable
capacitor and a 28uh tapped inductor.
It is also recommended that the parallel network tuner be placed at
the antenna end feedpoint, with a high quality run of Belden 9913/RG-8U
or Belden 9258/RG-8X coax back to the radio shack. For 80 Through 10
Meter operation, it is recommended that you use 450/600 ohm ladderline
from the antenna end feedpoint, to the parallel network tuner in the
shack or as KK4TR and I do, forget a feedline altogether and bring the
end of the antenna into the shack to the parallel network tuner. We
realize that the no feedline concept is taboo in Amateur Radio but it's
a zero loss, highly efficient method that works very well. Attaching one
1/4 wave radial for 80 through 10 meters, to the ground side of the
parallel network tuner and tuning the radials for maximum current with
the MFJ-931 Artificial Ground removes 100% of any stray RFI in the shack
to zero. I have found a minor amount of shack RFI on 30,12,10 meters
using the 256 foot L but have gotten rid of it easily using the above
mentioned method.
My current 1/2 wave L antenna configuration is as follows: one 64 foot
leg 9 feet off the ground that I consider a single wire feedline, one 64
foot leg in the vertical plane and a 128 foot leg sloping from 64 feet
up down to 7 feet up towards the NNE. I also have 4 1/4 wave
counterpoise wires up at 15 feet.
73 and GUD DX from KN4LF and KK4TR.
http://members.tripod.com/~kn4lf/index.hml
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