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Oops, please disregard previous email of this antenna as I accidentally
sent the rough draft version!
=A0=A0=A0=A0Let 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 backyard 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 city lo tbackyard
1/4 wave inverted L or low dipole by leaps and bounds.
=A0=A0=A0=A0KK4TR 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.
=A0=A0=A0=A0To be certain, an Electrical Engineer may come along and
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.
=A0=A0=A0=A0The 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.
=A0=A0=A0=A0Another 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.
=A0=A0=A0=A0Radiation 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.
=A0=A0=A0=A0There 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 or current
pattern. 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.
=A0=A0So 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.
=A0=A0=A0=A0Now 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 =A0 =A0 =A0 1.01 =A0 =A0 =A0 =A0 0.0@ =A0 =A0 =A0 =A0 90
3888 =A0 =A0 =A0 2.14 =A0 =A0 =A0 =A0 0.5/1.9* =A0 =A0 52
7225 =A0 =A0 =A0 3.99 =A0 =A0 =A0 =A0 1.3/3.0& =A0 =A0 35
10115 =A0 =A0 5.58 =A0 =A0 =A0 =A0 2.2 =A0 =A0 =A0 =A0 29
14263 =A0 =A0 7.86 =A0 =A0 =A0 =A0 3.0 =A0 =A0 =A0 =A0 24
18139 =A0 =A0 10.00 =A0 =A0 =A0 =A0 4.0 =A0 =A0 =A0 =A0 21
21338 =A0 =A0 11.83 =A0 =A0 =A0 =A0 4.8 =A0 =A0 =A0 =A0 20
24960 =A0 =A0 13.87 =A0 =A0 =A0 =A0 5.6 =A0 =A0 =A0 =A0 19
28400 =A0 =A0 15.73 =A0 =A0 =A0 =A0 6.3 =A0 =A0 =A0 =A0 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.
=A0=A0=A0=A0It 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 5,000 to 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.
=A0=A0=A0=A0It 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.
=A0=A0My 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.html
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