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Author Topic: antenna help  (Read 1028 times)
VK4TJF
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Posts: 94




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« on: January 25, 2013, 06:01:54 AM »

hi there
I would like to make an end fed half wave vertical antenna for 40 meters that is only 10 meters in length and is made with wire.
I would like to have a loading coil on the bottom to make up the loss in inductance. what is the value of inductance that I need to make my loading coil?
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W5DXP
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« Reply #1 on: January 25, 2013, 06:24:39 AM »

I would like to make an end fed half wave vertical antenna for 40 meters that is only 10 meters in length and is made with wire.

What are you hoping to accomplish by turning a 1/4WL monopole into an electrical 1/2WL monopole. It's radiation pattern will be essentially the same as a 1/4WL monopole and it will have lots of loss in the loading coil. Are you trying to avoid a radial system?
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
G8HQP
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Posts: 124




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« Reply #2 on: January 25, 2013, 07:17:26 AM »

What is the value of capacitance which your antenna will have? Tell us that, and most of us can do the simple calculation to find the inductance which will have exactly the opposite reactance at your operating frequency.

If you can't tell us that, any answer we give will necessarily be wild guessing.
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WB6BYU
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« Reply #3 on: January 25, 2013, 09:01:18 AM »

You can't get there from here.

Let's back up and start from the beginning...

First, a half wave antenna is 20m long, not 10m long.  A 10m wire is 1/4 wavelength.
So the first step is to clarify why you want a half wave antenna that is really a
quarter wave antenna.

My guess is that you've heard that a half wave antenna doesn't need radials (which
isn't quite true) but don't have space for one.  There certainly are commercial "half
wave antennas" that aren't really that long.  We'll assume, then, that what you
want is a quarter wave wire antenna with a high feedpoint impedance.

Actually a quarter wave wire can be a good antenna in its own right.  It does
want radials for good efficiency, of course, but often can be used without a tuner.

But let's look at how to shorten a half wave antenna so that there is a high impedance
at the feedpoint (which will require some sort of matching network, of course.)  We
can put a coil in the middle of a wire of almost any length less than 1/2 wavelength
and adjust it until it resonates with the wire on the desired frequency.  Maximum
radiation from an antenna comes from the portions with the highest current, which
is in the middle of a half wave antenna.  If we have a wire that is shorter than 1/2
wavelength we can add a coil in the center and adjust it until the antenna is
resonant. Unfortunately the coil is in the highest current portion of the antenna, and
while this reduces the amount of coil required, it also means it replaces the portion
of the antenna that radiates most efficiently.  It's not a bad approach in general as
long a the wire isn't too short.  In your case, that would put the loading coil in the
middle of your vertical wire, which might or might not meet your needs.

Another approach is to split the coil into two sections placed symmetrically either
side of the center of the wire.  This allows the middle portion with the high current
to radiate, while effectively shortening the ends that don't radiate as much.  This,
however, requires two loading coils, and each of them might be larger than the one
required in the center.  The values of the coils depend on the exact wire lengths.
A reasonable approach would be to put the coils 1/4 to 1/3 of the way in from each
end - now you have two coils in the middle of the wire, which again, might or might
not be convenient in your application.  The closer to the ends we put the coils, the
larger they have to be, and the more touchy the tuning. You can't really put them all
the way at the ends, because they need some wire or capacitance hat beyond them
to provide capacitance to resonate with the coil inductance, otherwise you need
a theoretically infinite coil.

It gets more complicated if you only want the inductor on one side of the antenna.
Imagine the traditional current distribution on a dipole:  high in the middle, dropping
to zero a the ends.  The voltage is high at the ends and low in the middle.  As a result,
tends are high impedance and the center is relatively low impedance, with the
impedance being a smooth function as you go along the wire.  When we add a coil
in the antenna, the resistive component of impedance is the same on either side
of the coil (only the reactance changes.)

Which is why you can't add a loading coil to a quarter wave wire to make it a half
wave antenna:  the resistance is still that of the quarter wave wire, but with added
reactance.  That is NOT the same as adding a quarter wave of wire, which also allows
the resistance to change.  There is no value of inductance that you can add to
a quarter wave wire to make it electrically a half wave antenna with a high feedpoint
impedance
.  Well, you might be able to wind a self-resonant coil, but that is not
going to be very efficient due to circulating currents, and tuning will be difficult.


So I'd suggest that we start at the very beginning (a very good place to start) and
understand what your requirements are, and we may be able to suggest antennas
that will meet your needs, because the direction you appear to be heading is a dead
end.
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W5DXP
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« Reply #4 on: January 25, 2013, 01:09:26 PM »

There is no value of inductance that you can add to a quarter wave wire to make it electrically a half wave antenna with a high feedpoint impedance.

Actually, it would be pretty easy to add a large inductance that will result in transmission line effects and make the antenna 1/2WL long electrically. But it won't make the antenna 1/2WL long physically and the physical length is what determines the radiation pattern. The electrical length only affects the feedpoint impedance and I don't see how going from 35 ohms to 3500 ohms could be an improvement.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
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Posts: 13341




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« Reply #5 on: January 25, 2013, 01:37:58 PM »

If the input impedance of the antenna is 35 ohms, adding any pure inductor
in series with that don't increase the resistive component, but will simply
add a reactive term.

Once you start considering the other real-world properties of the coil such
as self-resonance, self-capacitance, transmission line effects, etc., then
I agree things can get a bit more complicated.

That's why I added the disclaimer sentence about the self-resonant coil.


The usual reason given for wanting a high feedpoint impedance is because it
reduces the need for an extensive radial system.
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W5DXP
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« Reply #6 on: January 25, 2013, 02:58:34 PM »

If the input impedance of the antenna is 35 ohms, adding any pure inductor
in series with that don't increase the resistive component, but will simply
add a reactive term.

Sorry, that is based on the lumped circuit model and is only true for small inductors that are less than maybe 10 degrees long. For large inductors, e.g. 90 degrees, the lumped circuit model falls apart and the distributed network model must instead be used. If you need proof, at 4 MHz, model a 75 uH helix coil in EZNEC vs a lumped 75 uH coil. The results are absolutely nothing alike. If you need the EZNEC files, I'll be happy to provide them.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
VA7CPC
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Posts: 2393




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« Reply #7 on: January 26, 2013, 09:14:34 AM »

If you want to build an (electrical) 1/4-wave antenna with a loading coil (at the center, or at the base), find a downloadable copy of this program:

      "Mobile.exe"

It's very old (DOS-compatible!), but easy to use.  It'll give you the coil-winding instructions.

.      Charles
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N5KNG
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Posts: 103




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« Reply #8 on: January 29, 2013, 05:41:12 PM »

Hello James,

I looked at your question here and your other question, and perhaps this document will provide the solution for you: http://www.earchi.org/92011endfedfiles/Endfed6_40.pdf

I also thought about a vertical dipole, but I am imagining your particular situation requires the feedline to come in at the base, and you have very little ground (dirt) area under the end of the antenna.  Also, the 10m height is a little short for an effective dipole at 40m frequencies.

So, hopefully this is close to what you need.
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