Håller på med ett projekt om EH-antenner som
kommer att bli klart till sommaren.
090409. Nu har det material kommit som utgör 80 % av
bygget för EH-antenner kommit.
Så nu kan planering börja för konstruktion
och dokumentering av antennerna.
Mina demo antenner kommer att bli för 14, 50, 144 mHz
i olika tappningar.
Beskrivningen kommer att presenteras på denna sida
då allt labbande har gett det önskade
resultatet.
Conny / SM5DCO
conny.winrot@comhem.se
Skype: sm5dco
Sidan EH-antenner är under
konstruktion. Kommer att
innehålla byggbeskrivning m.m.
Conny 2010-04-20
W5QJR, has released a backpacker antenna design. See his web site
http://www.eh-antenna.com for more EH-antenna information. This
is Jack Arnold W0KPH version of his design.
The antenna is 22 inches tall and built on one inch PVC pipe.
(actually 1.34 inches) The cylinders are 7.5 inches long, and
spaced 1.34 inches apart. The cylinders are made using a thin
copper foil purchased at my local hardware store. A 12 X 30 inch
piece costs about $10.00.The coil was wired with #14 bare copper
wire. After you have put the cylinders on the PVC pipe, measure
the capacitance between the two cylinders. You will need this
value to compute the phasing network...If you don't have the
equipment to measure the capacitance , 7pF will get you close
enough..
I spaced the wire on the coil, using some heavy cotton thread
found in my XYL's sewing basket.... When I finished winding,
I removed the thread, and coated the coil with hot glue... The
Trimmers caps are 350 pF compression caps purchased at a local
radio components store...
The only problem in building the antenna was the feeding of the
wires inside of the PVC pipe. I really don't have any
suggestions on an easy way to do this.
The Network Design
program was used to compute the values of the components. The two
step 'L' and 'T' filter was used. the L was set
to 50 -25 ohms and the T filter 20-30 ohms. Most of these
antennas will measure 30 ohms radiation resistance... The
measured capacitance between the two cylinders on my antenna was
7pF. Compute the values of the coils and wind them...
When you finish building the antenna, I tuned it this way.
Place the antenna in the open away from obstructions. Disconnect
the LL coil from the 'L' Network, connect the MFJ 259B to
the lower end of the coil(the big one), and adjust the number of
turns on the coil until you get X=0 at the desired center
frequency. The bandpass of the antenna at the 2:1 Points is about
250KC each side of center. 'R' will be in the range of 30
ohms....Once you have the coil ,adjusted Set the input cap,
'CS' to the design values ,225 pf for my antenna. Connect
the L section back to the T section. Now put the antenna on the
pole and adjust the second cap,'CL' for a 1:1 SWR at the
design frequency.
I have not yet put the antenna on the air, We are having a heavy
snow storm, and I don't want to go outside to adjust
antennas. I did tune it up in the shack, and it looks good here.
Once the snow quits, I will get it outside and see just what it
will do.....
A Note Of Caution
1.
Be careful around the upper coil and the upper cylinder. These
points are at very high voltage and you can get a nasty RF burn
there..
2. Because of the small size of the capacitors, I do not
recommend Transmitter powers of over 50 watts. The caps might go
up in smoke at higher powers..
A 20 METER DIPOLE:
Now we will design and construct a very real and very practical
20 meter antenna.
This antenna can be scaled to other frequencies. For this
antenna, purchase a piece of
plastic pipe that has an outside diameter of about 1 inch. The
pipe is for water, thus the
pipe will be specified as an inside dimension. ¾ inch pipe
will have an OD of about 1
inch.
Step 1.
Wrap the pipe with aluminum foil, copper, or other conductive
material to
make 2 elements spaced the diameter of the pipe. You can put glue
on the pipe
or wrap the foil or metal with either clear tape or scotch tape.
We had some
thin sheet copper, and that is what you see in the
photograph.
Step 2.
Measure the capacity between the elements. Ours has a value of
about 7
pFd.
Step 3.
– Since we do not currently have an equation to predict the
value of
radiation resistance, from experiments I can tell you it will be
about 30 ohms.
Step 4.
Now we have the necessary information to calculate the network
values.
They are as follows:
·
C1 = 225 pFd and must handle a current of 1.4 amps at 71 volts
RMS for
100 watt transmitters.
E·
C2 = 291 pFd and must handle a current of 3.4 amps at 133 volts
RMS for
100 watt transmitters
Buy, beg or steal the necessary capacitors. Beware of the current
rating for
the power you operate with. Any capacitor will work for QRP.
Mica
compression trimmers are good for any power thru 100
watts.
·
L1 = 0.92 uHy. This translates to 2.5 turns on #12 wire around
the plastic
pipe. See the photograph for detail.
·
L2 = 13.61 uHy. This translates to 21 turns of #12 wire around
the plastic
pipe plus 4 turns between the antenna elements. Space the lower
coil about
1 diameter below the lower cylinder.
Step 5.A
– Tuning the antenna requires adjusting the amount of total
inductance
to set the desired resonant frequency. Course adjustment is
determined by the
number of turns, final adjustment is done by spreading the turns.
Alternately, I
put a small piece of wire soldered to the lower cylinder and
placed across the
gap. Bending that wire allows frequency adjustment of several
hundred KHz.
Step 5.B
– To achieve minimum VSWR it is necessary to adjust the
value of the
T capacitor (C2) and where it is tapped on the coil. C1 can be a
fixed value
because it is not necessary to adjust it. I prefer to do my
initial tuning with a
signal generator and a simple diode field strength meter. The
signal generator
allows changing frequency while the field strength meter
indicates the
frequency of maximum signal, then the relative signal power while
adjusting
the T capacitor. Antenna experimenters will have their own
techniques and
test equipment. Final adjustment is then done by trimming the T
capacitor and
spreading L1 for perfect VSWR. Once the VSWR is set, the
frequency can be
changed over a wide range with almost no change in
VSWR.
Step 6.
Record the 2:1 VSWR bandwidth. This one measured 245
KHz.
Step 7.
Measure the +/- 3 dB bandwidth. For this antenna it is 390 KHz,
about the
same bandwidth as a full size dipole. This is a Q of 36.4. Now,
since Q =
XL/R, then R = XL/Q. Since XL =1296, then R = 35.6 ohms. Since
the RF
resistance of the coils is 2.18 ohms (from the program), the
radiation
resistance is the total minus 2.18, therefore the radiation
resistance = 33.43
ohms. For fun, compare that to the radiation resistance of the
same antenna
length if it were a Hertz antenna. Previously we calculated a
value of 0.124
ohms. Now, we can calculate the efficiency as RR/(RR+RL)
= 94%. This is
equal to -0.27 dB, not bad for a very small antenna. Now you see
the true
effect of the EH Antenna concept.
Step 8.
Calculate the current thru the antenna. For 100 watt transmitter
power it is
P = I^2R, therefore I = (P/R)^.5 = 2.8 amps. For 5 watts QRP it
is 0.14 amps.
Step 9.
Calculate the voltage across the antenna where V =IZ, where Z =
the sum
of the radiation resistance and the capacitive reactance. For
a100 watt
transmitter V = 2.8*(33.4+j1144) = 3204 volts. For QRP the
voltage is 160
volts RMS. Just be careful of RF burns near the center of the
antenna.
Instructions for building an L Tee Network Dipole
The basic dimensions for an EH Dipole for any Amateur Band
are:
·
Outer diameter of insulative tubing (non-RF conductive, if it
gets hot in a microwave, don’t use it) should
be approximately the ham band divided by 10 with the answer in
inches.
Example: 20 meter band 20/10 = 2-inches.
The above sizing will provide nearly all if not all the band to
2:1 VSWR or less.
The bandwidth coverage is directly proportional to the diameter
to the antenna.
·
The length of the elements for a standard dipole is equal to the
circumference of the antenna.
For DX, make the elements longer. (Narrower
beam-width).
For closer in coverage, make the elements shorter, (Wider
beam-width).
·
The spacing between the elements must be the same as the
diameter.
·
To start with:
Assume the antenna capacitance to be 10 pF.
Assume the radiation resistance to be 32 ohms.
·
Use the EH Phasing Network Design in the Links section to
calculate the network values.
The only change will be to split the LL coil and place at least
last 4 turns of the coil between the elements.
This action causes the lead going to the top element to not have
the correct phase shift so as to radiate
inside the lower element.
·
For the capacitors used in the network we have had success using
mica compression type. For sources of
this type, just do a search for capacitors on the Internet. If
you plan on using more than 100 Watts you
might need a higher current rating.
·
Now that you have the antenna built, it’s time to tune
it.
First adjust the CL Cap. For minimum VSWR.
Next tune the transmitter to the frequency with the lowest
VSWR.
Then with CS and transmitter, tune to the desired
frequency.
Now adjust CL for minimum VSWR. If the minimum VSWR can not be
reached within the limits of CL,
then the assumption of 32 ohms is not correct and you will have
to go up/down on the CL capacitance.
·
Antenna height should be at least 1/8 of a wavelength.
TUNING AN L+T EH ANTENNA
Ted W5QJR
This note is intended to assist in getting the most from an EH
Antenna. There are
four (4) components in the network, two capacitors and two coils.
A previous note from
Stefano tells about an L+L network, so this note applies to the
L+T network.
Unfortunately, all four components are interactive, and to make
it worse, the
output impedance of the transmitter can have a major effect. We
have provided a
program to assist in calculating the values of the components,
and that is an excellent
starting point - if you know the impedance of the transmitter and
the impedance of the
antenna. Unfortunately, the impedance of he antenna can not be
directly determined, so
you need to make an educated guess. Most transmitters have an
output impedance of
about 30+ ohms with some reactance. Therefore, there is some
phase shift before the
network on the EH Antenna. Since we do not know the exact
characteristics of the
antenna, there are now 6 variables. Just to make it interesting
there is one more that is
new to antennas. The 7th variable is the amount of feedback
between the radiating
elements and the network. Due to the location of the network
coils and the lower
radiating elements, there is both capacity coupling and magnetic
coupling. The spacing
can be varied to affect the bandwidth and radiation
resistance.
Now that you are enlightened, we can begin tuning the antenna.
The objective is
to obtain maximum radiation and minimum VSWR. When this is
accomplished, you will
have maximum bandwidth for the system - unless you change any of
the 7 variables. For
example, changing the power of the transmitter will change
it's output impedance.
Changing any of the physical characteristics of the antenna will
affect the necessary
tuning. Any conductive object (including your hand) near the
antenna will affect the
tuning, primarily due to capacity. The capacity between the
antenna elements is only a
few Micro-Micro farads, so any near object can change the tuning.
This tells us that the
antenna should be hung from a rope as a vertical antenna away
from all objects.
Fortunately, the E and H fields of the antenna are contained
within the sphere of the
antenna, so we do not have a problem with the impedance of the
antenna changing due to
distant objects, such as chain link fences, that do have an
effect on Hertz antennas.
Therefore, the antenna can be tuned when hung from the ceiling
inside the Ham shack.
Ham shacks come in two varieties – those with adequate test
equipment and those
with none. For those with adequate test equipment – read
on. We believe if you are a
dedicated experimenter you will have the test equipment and
antenna related experience
necessary to achieve maximum performance from an antenna. If you
are lacking both test
equipment and experience, we recommend you purchase either an
antenna or a kit. The
kit will have the necessary instructions. If you read on you will
understand why the test
equipment is required to get the most from your EH
Antenna.
You will also understand
that experience with RF and related adjustments is also
necessary.
My definition of adequate test equipment is:
*) A quality signal generator with 50 ohm output and up to +20
dBm power output.
*) A frequency meter if the signal generator does not have
digital diplay of frequency.
*) A VSWR bridge compatible with the signal generator (see the
diagram of one on this
web site).
*) A good diode field strength meter.
*) A good capacity meter.
We begin the tuning procedure with the assumption that you
constructed the
antenna with recommendations from this web site. We also assume
you measured the
capacity of the antenna and estimated the values of the network
based on either the Bingo
program or the one on this site. All that is left to do is to
adjust the 2 capacitors and the 2
coils – only a combination of 4 items.
Because the signal generator has an output impedance of 50 ohms,
the input
capacitor for the L network can be a fixed value, so now there
are only 3 variables.
Adjust the signal generator to the desired frequency and adjust
the capacitors for
best VSWR. Check the signal strength and ensure that maximum
radiation occurs at best
VSWR. If the two do not agree and VSWR is not perfect, the
radiation will be less than it
should be. If they agree and VSWR is perfect, you have arrived.
If the radiation is
maximum when VSWR is minimum but VSWR is not perfect, you may
need to adjust
the small coil to improve the VSWR. If that did not fix the
problem, there are too many
or two few turns on the large coil. In other words, the antenna
can not be matched at that
frequency. Now is the time to change the signal generator to
other frequencies and try
readjusting. This will give a good indication as to the best
frequency for your antenna.
Frequency increments of about 2% are recommended. After you have
determined the
optimum frequency for that antenna, bring your antenna to the
desired frequency by
adjusting the main coil (more or less turns) and resetting the
capacitors.
Now that you have the antenna properly tuned at the desired
frequency, you need
to carefully measure the antenna parameters. With this
information you can determine the
true characteristics of the antenna. The data you need
is:
*) Resonant frequency
*) 2:1 VSWR bandwidth
*) +/- 3 dB bandwidth
*) value of the T capacitor
*) value of the coils
To measure the +/- 3 dB bandwidth, note the value of maximum
field strength –
then increase the signal generator output 3 dB. Measure the lower
and upper frequencies
where the radiation is the same as before the 3 dB increase. The
difference between the
two frequencies is the 3 dB bandwidth.
The inductance of the coils can be determined from the number of
turns, spacing,
wire size, and coil diameter. Consult programs or handbooks to
determine the inductance.
With this value calculate the value of inductive reactance at the
resonant frequency. Use
this value to calculate the value of the tuning capacitor. It
should be about 1.4 times the
measured value. If there is a significant difference, the cause
needs to be determined.
Also, determine the value of the loss resistance of the
inductors.
From the value of the T capacitor, determine the value of
radiation resistance.
This can be done by playing games with the network
program.
Try various values of R
until the value of C is found to be the same as the measured
value. The program should
now also indicate the amount of inductance you have
determined.
From the value of radiation resistance and loss resistance in the
coils, calculate the
system efficiency. If it is worse than 90% there is a big
problem. Note – 90% is a loss of
0.5 dB – not bad when you consider there are 6 dB in an
S-unit. .
If you think your EH Antenna is now performing properly, it is
time to take it to
it’s final resting place and connect it to the radio. If
the resonant frequency is not what
you want, the capacitors can be adjusted to make the VSWR better
at the desired
frequency, or you can change the coil inductance. This can be
done in several ways – a
copper slug can be moved in the coil to change the inductance, or
a copper or aluminum
sleeve can be moved over the coil to change the inductance. The
copper slug can be
moved by the use of a electric screw driver and a long threaded
screw. The control can be
remote from the antenna. Fortunately, on the higher bands the
instantaneous bandwidth of
the antenna allows full band coverage without tuning the antenna.
On the low bands you
may want to place a series resonant circuit in the center lead of
the coax. When the circuit
is resonant there will be no phase shift. As you adjust the
circuit you are effectively
changing the phase between the current and voltage. This will
alter the operating
frequency of the antenna. I have just presented two methods of
remote control – your
choice.
We indicated previously that the output impedance of your
transmitter may not be
the same as the signal generator. Therefore, you may need to
adjust the L section
capacitor to achieve good VSWR.
If you have a problem with RF feedback, In another area on the
web site we talk
about RF grounds for the antenna.
This may seem to be a lengthy process to achieve the desired
results from an EH
Antenna. However, compare this to tuning and matching a 75 meter
mobile antenna, or
installing a dipole and changing the angle to match the impedance
and adjusting the
length to the desired frequency. Those things must be done
outside, while the EH
Antenna can be tuned in the comfort of the Ham Shack. Also, if
you do complete all of
the suggested steps, you will know the details of the performance
parameters of you new
antenna, and that is more than you ever knew about a Hertz
antenna.
There is one more important thing about building your own
antenna, and that is
the satisfaction derived from building and using a portion of
your Ham station.
Best 73’s
Ted W5QJR
And God Luck Conny SM5DCO
COMPUTING THE PHASING
NETWORK
Original Design by Jack (W0KPH)
EH-antennas will be used for education and testing on
HAM-meeting for year 2010.
ANTENNAS
Ted Hart W5QJR
CEO EH Antenna Systems, LLC
INTRODUCTION: Ham Radio has been a birth place for and nurtured many
important inventions and discoveries. I have participated in some of those and benefited
from others over the past 61 years I have held the call W5QJR. Although I have
previously presented new concepts to Hams (including the Antenna Noise Bridge
in 1967 and the small High Efficiency Loop Antenna in 1984), and written many
Ham articles, I now have the privilege and opportunity to present one more, the
most important, an antenna concept that will benefit every Ham.
I have invented
and patented a not just an antenna but a new antenna concept, the first
practical change in antennas since Hertz developed the resonant wire antenna in
the 1880’s. What will it do for Hams? It will allow reduction in size and
reduce received noise. It retains high efficiency, wide bandwidth and virtually
eliminates EMI. Maybe size and improved noise are not important to you, but in
this modern world many Hams are off the air because of restrictions on
antennas. Those restrictions no longer apply.
Sound to good to
be true? That is what many have said until they experience the EH Antenna. You
can buy one from a company in Japan, but you can also readily build one with
inexpensive materials. We will include details of one in this article and
provide information that will allow you to build an EH for any Ham Band. You
can even build and test one for 80 meters inside the Ham shack.
THE CONCEPT: All antennas to date have been based on the Hertz
concept of resonant wires. Unfortunately those antennas have very large
Electric (E) and Magnetic (H) fields near the
antenna and do not create Poynting vector radiation until the fields have
traveled about 1/3 wavelength from the antenna. This is called the far field
boundary of the antenna. This is due to the fact that current is applied to the
antenna. A changing current creates a magnetic (H) field. A changing magnetic field creates an electric
(E) field. Because
one field creates the other these fields are 90 degrees apart in time, thus the
need to travel in time until they are in time phase.
Now, we will
consider the EH Antenna concept. With reference to the drawing below, note that
there is an Electric (E) field and a Magnetic (H) field. A high voltage is presented to the antenna
which develops the E field. Since the voltage is applied to the center of
the antenna and there is no voltage at the ends, the E field causes a
differential voltage to be impressed along the length of the cylinders, which
causes current to flow on the cylinders. That current is in time phase with the
E field (it is
developed simultaneously) and develops the H field which is also in time phase with the E field. Because
the two fields are in time phase at the antenna,radiation occurs at the
antenna. Because the two fields have the proper orientation relative to
each other and are in time phase they satisfy the Poynting Theory at the
antenna. This allows the fields to be efficiently integrated which allow the
antenna to have high radiation resistance. Because the EH antenna has large cylinders (compared to wires), the
capacity of the antenna is high. The combination of high capacity and high radiation
resistance provides large instantaneous bandwidth and high efficiency. Further,
because the radiating elements are large in diameter they can be very short
relative to a wavelength, typically only a few percent. The diameter of the
SFRT antenna is chosen to give the desired bandwidth at the lowest operating
frequency.
In the drawing
above note that the tuning coil inductance can be adjusted so that, in
conjunction with the capacity between cylinders, the system can be resonant at
the desired operational frequency. By selecting the proper tap on the tuning
coil and the source capacitor a very low VSWR match to the antenna can be
achieved. Typically the capacitor should have a nominal value of 200 ohms at
the operating frequency. However, it is not critical and that allows a variable
to be used. Varying the capacitor is much easier than changing the tap on the
coil.
PERFORMANCE: The antenna provides exceptional performance and
exhibits the following performance parameters:
HIGH EFFICIENCY
BROAD TUNING RANGE
BROAD INSTANTANEOUS
BANDWIDTH
EXCEPTIONAL LOW NOISE
RECEPTION
DESIGNED FOR NVIS
OPERATION
VERY SMALL SIZE
VERY HIGH RELIABILITY
ANTENNA SIZE: A small antenna will radiate and receive as well as
a large antenna if it is properly designed. Small wire antennas do not meet
these criteria.
INSTANTANEOUS
BANDWIDTH: If you examine the EH Antenna it is a simple series
resonant circuit (except for stray capacity). Therefore we can use the
following simple equations to define the antenna:
Q = XL/R=XC/R=F/BW
Q is a measure of
the antenna instantaneous bandwidth. At resonance XL and XC
are equal. These are the reactance of the inductance and capacitance
respectively at the operating frequency which is F. BW is the +? _ 3 dB bandwidth,
nominally 3 times the 2: VSWR bandwidth.
C can be
approximated by the following equation: =0.546*L*D+2.05* D where L is the
length of one cylinder and D is the diameter of the cylinder. Then XC
= 1/ (2pFC)
If we build the
antenna and measure the 2:1 VSWR bandwidth, we can then gain a good estimate of
the resistance of the antenna. By rearranging the equation above we have: R= XC
BW/F. R is the total R in the circuit comprised of radiation resistance (RR)
and loss resistance (RL)in the tuning coil. We can calculate XC,
we know the operational frequency, and we can measure the bandwidth therefore
we can calculate the resistance. Next, we need to know the loss resistance of
the tuning coil. If we assume it is made of good wire and the length to diameter
ratio is reasonable, the Q should be about 200. Since Q= XL/RL and
we know XL = XC, then RL= XC/200.
Typically, the loss resistance will be lessthan 2 or 3 ohms.
EFFICIENCY: The efficiency of the antenna can now be calculated as
a ratio of the output power to the input power. Since RL is defined as the
output and the sum of RR and RL is equal to the input power, the efficiency = RR/(RR+RL).
The calculated value of R based on the measured value of bandwidth should be
between 30 and 180 ohms depending on how the antenna is constructed. (more on
that later). If we assume the loss resistance is about 3 ohms and the measured
value of R is 30 ohms, the efficiency will be =(R-RL)/R = (30-3)/30 = 90% or
-1/2 dB.
IMPLEMENTATION:
Antennas come in
many physical configurations. We want to first detail a 20 meter EH antenna
with the associated feed network that any Ham can easily duplicate. All you
need is a piece of pipe about 1.25 inches in diameter that is made of
insulating material such as wood or, preferably, plastic pipe. Each of the
dipole elements for 20 meters will be about 7 inches long – yes, 7 inches on 20 meters – and can be made of aluminum foil from the kitchen wrapped around the
pipe. You will need some wire – preferably #14 magnet wire as used to wind
motors. The source capacitor needs to be about 47 to 56 pF. You can readily
build the antenna and tune it in a single evening. Then you can test it to
prove to yourself that it performs as well as that commercial 20 meter vertical
(16 feet tall) with a large ground radial system.
After you have proven the concept to your satisfaction you may want to
experiment with various sizes. Typically we prefer that each cylinder has a
length about 6 times the diameter and the spacing is equal to the diameter. If
you space the cylinders very close the capacity will increase, thus the
bandwidth will increase and the loss in the tuning coil will decrease because
there will be less wire in the coil. To do this, use a piece of glass for an insulator.
Cut the pipe and use hot glue to secure the glass between the cylinders. As
shown in the drawing below:
For lower frequency antennas use about 4 inch pipe on 40 meters for large
bandwidth or 2 inch for small bandwidth. And double that for 80 meters.
MOUNTING: To gain the most from the antenna the location of
the antenna can make the difference between performing well or poorly. We have
not fooled Mother Nature when we built a small antenna. However, she will
dictate the location of the antenna. If the antenna is mounted vertically for
chasing long haul communications, then the height of the antenna should be at
least 0.1 wavelengths above ground. On 20 meters that are only 2 meters. On 80
meters that are 8 meters. Since these are very low heights the antenna should
be mounted high enough so that the communication path is not blocked by trees
or buildings.
On the low bands,
the antenna is excellent for rag chewing if it is mounted horizontal and above
a reflecting screen. Aluminum screen wire works very well. The height need only
be about 3 feet on 40 meters and about 5 or 6 feet on 80 meters. Tests show
this configuration can provide more radiation than a ½ wave dipole.
FEED LINE
RADIATION: Many Hams have complained about the coax radiating
because there is RF current on the outside of the coax. When the antenna is
high in the air it is not in free space. Consider that the top cylinder is
operating at a high voltage relative to ground. This in effect says that there
is a capacity between the antenna and ground. The RF current on the coax is the
return current from that capacity. Unfortunately, it may be enough to alter the
tuning. That means that you must do the final tuning of the antenna after it is
placed it its operating position. Otherwise, it might as well be a wet noodle.
Now you understand why a large number of Hams have to say that the EH Antenna
does not work.
ALUMIMIUM REFLECTOR
Let me put this in perspective – I built a 75 meter
mobile antenna and placed it in a wooden rack on the back of my pickup truck.
Below that is a sheet of aluminum to act as a reflector. Keep in mind this was
an experiment. By comparative tests against a station with dipole 30 feet in
the air this antenna gave equal performance. That may be big news to some of you,
but the point here is that there was less than 5 feet of coax outside the cab
of the truck. 5 feet of coax on 75 meters is not much, and certainly not
enough, even if it were a very good radiator, to have a significant effect on
the performance of the antenna. So much for those that say it is the coax
radiating, not the EH Antenna.
A 20 METER DIPOLE
– SOME TIMES CALLED A BACKPACKER: Now we will
design and build a very real and very practical 20 meter antenna. For this
antenna a piece of plastic pipe with an outside diameter of about 1 inch was
used. The pipe is for water, thus the inside diameter is 1 inch (2.54
cm)...
Step 1: Wrap the pipe with aluminum foil, copper or other conductive
material to make 2 elements spaced about the same as the diameter of the pipe.
You can put glue on the pipe or wrap the foil or metal with clear tape or even
scotch tape. We used thin copper sheet, so that is what you see in the
photograph. Do not use any type of ferrous (iron) material.
Step 2: Measure
the capacity between the elements – if you have that capability. Ours has a
value of about 10pF. The reactance at 14 MHz is ohms.
Step 3: Wind a
coil to tune the antenna. It needs to have a reactance of ohms which is uH. The
estimated number of turns on the coil will be turns and cover about inches. The
coil should be spaced about 2 diameters or more below the lower cylinder.
Step 5: Select a
source capacitor of about 200 ohms reactance, which are about 56 pF. As
previously stated, a small trimmer capacitor would work well and reduce the
trial and error method of tapping the coil in the optimum position. Now, tap
the coil at about 1/10 the way up to the coil from the bottom.
Step 6: If the
feed wires are in place we are ready to begin the tuning process. To set the frequency
best to use a Grid Dip Meter to find the resonant frequency, then adjust the
turns on the coil to move it to the desired frequency. Best to remove turns
from the top of the coil. When you get close then spread the turns for fine
tuning.
Step 7: Adjust the
source capacitor and tap to give a low VSWR match.
Step 8: measure
the 2:1 VSWR bandwidth and multiply that value by 3 to give a good
approximation of the +/- 3 dB bandwidth. We measured xxx KHz which results in a
Q=
Step 9: Move the antenna
to the final operating location, connect the coax and note the operating
frequency and VSWR. If they have changed significantly it will be necessary to
change the tuning and matching. This is best done by having one person at the
radio and the second person at the antenna to make changes.
