IW5EDI Simone - Ham-Radio

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160 Meter linear loaded Tee vertical doublet antenna

Posted: 03 May 2016 10:31 PM PDT
http://www.iw5edi.com/ham-radio/2134...oublet-antenna


I decided to publish this website in order to pass on some insights about
this antenna that Ive garnered through extensive experimentation. A warning
though, some of the combined design aspects of the antenna may be unique
and unorthodox, a think out of the box antenna design. Note! I do not have
a B.S. or M.S. in EE, which makes me a true amateur amateur radio operator
not a professional amateur radio operator, so some of my antenna theory
explanations may be incorrect.

I have gained DXCC on 160 meters with 145 confirmed as of October 1, 2005,
using this antenna design, in approximately 4 years. 25% of the DXCC
contacts were via CW and 75% via phone. 103 DXCC contacts were with 140
watts PEP, 17 with 800 watts PEP.

What I have done is to simply identify the basic inherent weaknesses of the
average 1/4 acre city lot 1/4 wave inverted L with a 30-50 foot vertical
section and a few 1/4-1/8 wavelength radials and have devised methods to
overcome these weaknesses. This antenna design is not meant to be a rival
to a 4 square vertical array but can compete with a full 1/4 wave vertical
with 60 1/4 wave ground mounted or buried radial wires, if designed
correctly.

First of all let me say that Im not a professional broadcast radio
engineer. My background is in the sciences, i.e., climatology, meteorology,
oceanography and space plasma physics. Im just a true amateur experimenter,
antenna modeler and voracious reader of every book on antenna theory and
design that I have been able to get my hands on, some 50 years old. As an
avid antenna experimenter, I have spent approximately 10 years in the field
experimenting with this antenna design and its variants (1/4, 3/8, 1/2 wave
L/Tee Vertical), between 1993 and 2002 and have also done extensive
modeling using EZNEC 5.0. My good friend K4TR Joe Dube of Brooksville, FL
has also been experimenting with this design between 1997-2002.

Along the way I have come to the conclusion that some of present day
antenna theory is just that theory, in general concepts not totally proven
by controlled scientific experiment and/or overemphasized and therefore to
be taken with a grain of salt in some instances! I have also concluded that
allot 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 now
grossly miss informed about the basics.

A Broadcast Radio Engineer may come along and poke holes in some of the
following antenna theory and concepts, as Ive explained them. I have been
told repeatedly that I know nothing about antennas. Even if the theory of
operation of the linear loaded voltage fed Tee Vertical as I explain it is
flawed in any way, one thing that cant 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 to include 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 the average backyard poorly conducting soil conditions, to
include an inferior ground radial system and low radiation resistance, a
measure of antenna efficiency, due to the typically short (30-50 feet)
vertical radiating element section of a 1/4 wave inverted L.

The proper definition of radiation resistance is; the total power radiated
as an electromagnetic radiation, divided by the square of the current at
some defined point in the system. To put it in simplest terms, a measure of
antenna transmitted signal efficiency.

A 1/4 wave radiator will focus its current field in the ground immediately
around its feed point and as you extend the vertical section past 1/4 wave,
the highest current point moves up the vertical section and outward on and
in the ground surface. With much effort the near field transmitted signal
losses can be reduced to a point that you improve antenna efficiency to
maybe around 50-75%. However the average backyard 1/4 acre location makes
it difficult to overcome signal losses in the mid field (200-500 feet) on
160 meters and signal losses in the far field (between 500 and 1000 feet
for a 1/4 wave vertical and around 52,000 feet for a true 1/2 wave L/Tee
Vertical) (Fresnel Zone) is out of reach for all of us.

The linear loaded voltage fed Tee Vertical antenna places the highest
current point at or near the top of the support structure gaining the
following advantages. The elevated highest current point of the antenna is
above allot 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 linear loaded
voltage fed Tee Vertical, the noise level has been reduced to S1-2. Of
course the actual amount of noise reduction will vary from QTH to QTH.
Another advantage of elevating the highest current point is, reduced
radiated signal absorption and pattern distortion, away from
omni-directional. In a sense you can say that the highest current point is
getting a better omni-directional look at the radio horizon. Actually
though its best to have the highest current point say approximately 25-50%
below the flat top to assure vertical polarization. Remember the linear
loaded voltage fed Tee Vertical is a DX antenna with a null overhead and
therefore little high angle radiation close in for rag chewing.

Another advantage of elevating the highest current point, per the ARRL
Antenna Handbook edition #15, is the reduction of capacitively coupled
transmitted signal loss between antenna and lossy ground. Logic dictates
that placing distance between the highest current point of the antenna and
lossy ground possibly reduces capacitive coupling losses in the near field.
Of course though due to the wavelength involved, the reduction in loss will
be not the same on 160 meters versus say 40-10 meters.

The agreed upon standard for the number of ground radials for least near
field loss for a 1/4 vertical antenna is 120 1/2 waves but you see a
diminishing point of return after approximately 24 1/8 wave or 16 1/4 wave
radials and there is virtually no difference (approximately 0.07 db)
between 50-60 1/4 waves and 110-120 1/2 waves. Also basically your ground
radials need not be any longer then the length of the vertical section of
your antenna. An alternative to ground radials is an elevated counterpoise,
which will be covered further into the text.

Radiation resistance, which as stated previously is a measure of
transmitting antenna efficiency, is obviously a very important but
difficult to accurately measure variable, basically the higher the value
the better. Once again the proper definition of radiation resistance is;
the total power radiated as an electromagnetic radiation, divided by the
square of the current at some defined point in the system.

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 15 ohms of radiation resistance and couple it with a poor ground
radial system say 50% efficiency at best and you still have a very
inefficient signal radiator. By the way, if you feed a 1/4 wave vertical at
one end then the feed point impedance becomes the same as radiation
resistance. However bend the radiator like an inverted L and the two are no
longer the same.

Another method used to improve radiation resistance is to employ a capacity
hat top loading system. A traditional capacity hat in the form of at least
three flat top or sloping wires spaced approximately 120 deg apart and tied
together at their ends in a ring shape, is employed to make up for the
missing part of a short vertical antenna. Basically each top hat wire
length should be at least the same length as the missing part of the
vertical. On 160 meters an 1/8 wave vertical with an approximate length of
64 feet should have a three top hat wire lengths of 64 feet. This method of
top hat loading increases the radiation resistance of the short vertical,
(much like a linear load which is normally placed at the bottom of the
vertical) only even better and moves the highest current point up the
vertical portion of the antenna. The highest current point on my voltage
fed Tee Vertical is elevated approximately 60 feet above ground using this
method. If at all possible mount the top loading wires as high on the ends
as in the center because dropping the wire ends effectively shortens the
vertical section of the antenna. At my QTH the best I can do is to get the
ends of the top loading wires 70 feet above ground versus 80 feet at center.

There are several methods that can be employed to reduce near field ground
losses and in some cases increase radiation resistance and henceforth
transmitting antenna efficiency, excluding the laying out of dozens of
ground radials. One is to place 3-4 ground radial wires into an above
ground counterpoise system (for a typical backyard 1/4 wave inverted L
antenna). Four 1/4 wave wires approximately 15-30 feet off the ground, can
rival 120 1/2 wave radials on the ground, as far as connection losses
(which can 10-40 db) and lowest takeoff angle but not necessarily
concerning near field ground losses (which has been measured at
approximately up to approximately 5 db by W8JI). Unfortunately though
raising radial wires into an elevated counterpoise also effectively
shortens the vertical section of the antenna, similar to top loading wires.

It would seem logical that the linear loaded voltage fed Tee Vertical
antenna would require a less extensive ground radial or counterpoise system
in the near field at the antenna feed point, as the antenna is much longer
than a 1/4 wave and has the highest current point elevated well above the
ground surface and also well away from the feed point on the ground
surface. However there will still be some losses in this nearer field but
just further out from the antenna feed point. The problem though is that
its difficult to get enough wire in the ground to overcome the ground
losses at the further distance, on a typical 1/4 acre suburban lot.

Another method is to lengthen the transmitting antenna. As mentioned
earlier, in theory the radiation resistance measured at the end feed point
of a 50 foot vertical section inverted L is around 15 ohms, a linear loaded
1/4 wave L is near 16 ohms, a full 1/4 wave vertical is 36 ohms, a full 3/8
wave vertical is 300 ohms and a full 1/2 wave vertical is 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. The added length of the
antenna can be placed in a linear load configuration.

As mentioned earlier, the average backyard 1/4 acre location makes it
difficult to overcome signal losses further out in the near field (maximum
concentrated ground current is approximately 3/8 wave length out from the
feed point with a 1/2 wave vertical) on 160 meters. Reducing signal losses
in the far field at the first reflection point (Fresnel Zone), which is
around 52,000 feet for a true 1/2 wave vertical, is completely out of reach
for all of us.

To recap the various methods of improving antenna efficiency and
performance; lengthen the antenna past 1/4 wave using a linear load, add a
capacity hat in the form of a three wire flat top, elevate the highest
current point, use a radial counterpoise system.

So thats it in a nutshell, the linear loaded voltage fed Tee Vertical can
overcome most all the inherent weaknesses of the average 1/4 acre city lot
backyard 1/4 wave inverted L.

Now lets discuss the benefits of using the linear loaded voltage fed Tee
Vertical on 80 through 10 meters, as a multi-band antenna. As the length of
a transmitting antenna exceeds a full wave 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 half wave dipole at 1/2 wave height.
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.

Using the Tee Vertical antenna as a multi-band antenna on 80-10 Ive had
very good results. On 17 meters I have worked 100 DXCC countries with
minimal time and effort.

It is strongly recommended that a high voltage handling parallel network
matching device be used to load up the linear loaded voltage fed Tee
Vertical antenna. Also as a tuner will see at least 1,000 ohms of feed
point impedance on 160 meters with a linear loaded voltage fed Tee
Vertical, your average store bought Tee network tuner cant deal with such a
high impedance and voltage. My matcher is a parallel network consisting of
high power components, one 700 pf split stator 5 kw variable capacitor and
a 28uh 5 kw roller inductor.

It is also recommended that the parallel network tuner at the antenna end
feed point be fed 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 ladder line from the antenna end
feed point, to a balanced network tuner just inside of the shack.

Attaching one 1/4 wave radial for 80 through 10 meters, to the ground side
of the tuner and tuning the radials for maximum current with say 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 40 through 10 meters
using the linear loaded voltage fed Tee Vertical but have gotten rid of it
easily using the above mentioned method. Also making up some stub lengths
of wire that make the total length of the antenna on each band of operation
an odd quarter wave multiple, moves the first highest current point at the
matching network and removes all shack RF.

Im constantly experimenting with different radiator lengths and layouts. As
of 10/01/02 my configuration of the linear loaded voltage fed Tee
Vertical/Doublet antenna is:

A linear loaded voltage fed Tee Vertical antenna with the entire vertical
section and linear load section made out of 450 ohm ladder line. The
vertical section is 80 feet high, with a 47 foot linear load horizontal
section one foot above ground that terminates in the tuning doghouse, to a
legal limit plus rated home brew parallel matching network and driven
against one 1/4 wave radial on the ground, four 10 foot long ground rods
and a 150 foot deep well casing. The capacity hat is comprised of three 144
foot wires using #12 stranded wire, spaced one foot apart and sloping down
to 70 feet.

Of course the ground rods and well casing dont do much if anything as far
as reducing near field ground losses and are actually part of my DC
lightning ground. My ground system is sitting over very wet and highly
conductive muck soil with swamp and ponds in the near field and Fresnel
zone of the antenna. I also have a near zero local QRN level even on the
transmit antenna, lucky me!

Ive also had similar good performance with a voltage fed Tee Vertical using
three 200 foot capacity hat wires, a 52 foot vertical section, a 75 foot
horizontal linear load one foot above ground, with nine 1/8 wave
counterpoise wires 5 feet above ground.

Per the EZNEC 5.0 modeling program, my 80 foot Tee Vertical has a near
perfect textbook circle radiation pattern, with 1.95 dbi gain at a takeoff
angle of 20 degrees, a 3 db beam width of 51.2 degrees, F/B of 0.30 db,
feed point impedance of 628.6-j19350, a 1 mile mV/m of 134.22 using 1000
watts, with the highest current point elevated at approximately 60 feet
above ground. However for all intents and purposes the highest current is
nearly equally distributed along the 80 foot vertical section thanks to the
capacity top hat and 47 foot linear load horizontal section one foot above
ground. See links below for model diagrams of the Tee Vertical antenna.

If you zig zag sections of wire, that cant be placed in a vertical
position, versus using a coil, its much more efficient then a coil and
radiates to a certain extent. Actually, if the linear loaded sections are
designed right, they can add to the current on the vertical section, of a
1/4 wave L. Its an idea I borrowed from VE3DO and discussed in ON4UNs book
Low Band DXing.

Remember once again, the linear loaded voltage fed Tee Vertical is a DX
antenna with a null overhead and therefore virtually no high angle
radiation close in for rag chewing. Put your linear loaded voltage fed T
antenna on a pulley and you can lower it at will, roll up one leg (100
feet) of the 200 foot flat top into a ball or place an isolation relay to
electrically remove one leg, the antenna then becomes and inverted L
electrically and performance wise.

However thanks to the creative ingenuity of Joe Dube, K4TR of Brooksville,
FL., who owns D & G Antennas there is another option. Joe came up with the
idea of turning our linear loaded voltage fed Tee Vertical into a ladder
line fed all band doublet/dipole. By flipping a switch which actuates a
SPDT 12 volt relay at the antenna feed point in the dog house, the Tee
Vertical becomes a 160-10 meter horizontal doublet with lots of gain.

K4TR D&G ANTENNA MFG & SALES

At times due to the nature of propagation on 160 meters during propagation
disturbances, a low dipole can outperform a Tee Vertical on DX and is also
quieter. I also have the added benefit of switching to the regional big
signal cloud warmer low noise dipole to overcome high summertime lightning
induced QRN for rag chewing. I use the dipole antenna set up in conjunction
with a good performing Time wave DSP-9+ for summer operation. Click on the
link below to see a diagram of the remote relay switching arrangement.

Having field tested the K4TRs doublet aspect of the antenna design on 80-10
meters during the summer of 2002 I can verify that it works very well as
and all band rag chew and DX antenna. I use a homebrew Tee network matching
box to tune out inductive reactance.

Also last but not least, a personal observation concerning short monopoles.
When I model a 52 foot vertical with one 200 foot top hat wire using EZNEC
3.0 on 1830 kc, then add two more 200 foot top hat wires, the near electric
field in V/m RMS increases, the total electric field at 1 mile increases
and the feed point impedance increases a little. When I conduct the same
modeling exercise on 180 kc I see the same results as at 1830 kc but cannot
verify it in the field. With no top hat wires attached a 52 foot vertical
antenna obviously has capacitive reactance and therefore inductive top
loading wires are needed or a linear load or at a last resort a lossy coil.
With the 52 foot vertical and three 200 foot top hat wires the antenna feed
point becomes inductive and feed point impedance high enough for the
necessity of a parallel matching network. When you feed a 90 degree
monopole at its ground end the feed point impedance and radiation
resistance are basically synonymous, lengthen the monopole to 135 or 180
deg and of course feed point impedance and radiation resistance become
different but the added electrical length does seem to increase radiation
resistance.

Ive done extensive experimentation with radials on vertical antennas on 160
meters during the past 18 years.

Back in 2001 a MF broadcast engineer friend of mine using professional
broadcast measuring equipment, took near field measurements of the electric
field in V/m RMS. The antenna was a 1/4 wave inverted L with a 64 foot
vertical section and (1/8 wave) 64 foot long radials laying on the ground
surface.

I found the following:

There was little measurable difference between 0 and 4 radials, a small
measurable difference between 4 and 8 radials, a medium measurable
difference between 8-16 radials, a large measurable difference between 16
and 32 radials, a small measurable difference between 32 and 64 and no
discernable measurable difference between 64 and 120 radials.

We then conducted another experiment using conventional (1/4 wave) 128 foot
radials and found the data to be exactly the same as the 1/8 wave radials.
To me this proved the theory that the radials need not be any longer than
the vertical section is tall.

I have never had the opportunity to do the experiment with a full 1/4 wave
vertical.

This statement will be controversial. Using a voltage fed electrical 1/2
wave tee antenna with a 64 foot vertical section and three 200 foot long
top hat wires, in the near field we measured only a very small difference
between 1 radial and 64 1/8 wave radials. We measured no difference between
1 radial and 64 1/4 wave radials.

The ground conductivity was pretty good at the location of the experiment.
It was a typical Florida hammock swamp that had been filled in but always
had black mucky soil and a high water table. The conductivity was
approximately .03 S/M with a dielectric constant of approximately 20. Ive
always presumed that the results might be different over ground with poor
conductivity.

Here are some modeling results for the linear loaded voltage fed Tee
Vertical antenna using EZNEC 5.0. Click on the links below to see the
results. Link #1 shows current distribution which is very similar along the
length of the 80 foot vertical section but peaks at approximately 60 feet
up, link #2 shows takeoff angle and total pattern.



article by W4HM originally available at http://www.wcflunatall.com/w4hm9.htm

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