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Author Topic: Bottom fed dipole, not Center?  (Read 5579 times)
W4JST
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« on: December 13, 2012, 06:23:48 PM »

I attached a picture that may help.

I made a 1/2 wave dipole to use vertically for 2 meters.

It consists of two 19" pieces of 1/2" copper pipe joined with a 1/2" PVC coupler.

At the very bottom, my 1/2" hardline terminates to RG-8X coaxial cable running INSIDE the bottom 19" section of the dipole.
THIS is wear I made the ground connection, at the bottom, not at the center of the antenna.
I then ran RG-8X INSIDE the bottom 1/2" pipe 19" section to the center of the dipole and soldered the RG-8X center conductor to the bottom of the top element.

Is this equivalent to a 1/2 wave dipole or is there any type of cancellation or anything going on?

I have never seen another dipole where the ground connection was made on the far side from the center and wasn't sure if it made any difference or not.


http://i50.tinypic.com/2rmvztj.jpg
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WB6BYU
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« Reply #1 on: December 13, 2012, 06:52:36 PM »

No, it's not the same.  A proper dipole would run the coax up to the top of the
lower half and connect the coax shield to the top of the bottom half of the antenna.

The way you have it connected, there is a 1/4 wave portion connected to one
side of the coax and a 1/2 wave section connected to the other side.  That isn't
conducive to having a good match.
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KD2CJJ
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« Reply #2 on: December 13, 2012, 07:27:19 PM »

Why wouldn't you just run the feedline in the antnenna and just create a vertical dipole?
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73

Mike
KD2CJJ
KC9Q
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Posts: 49




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« Reply #3 on: December 13, 2012, 07:53:01 PM »

It looks to be a variant of a Coaxial Dipole Antenna.  Normally the radiator is a whip antenna, but this one uses the same tubing that the groundplane is made from for the radiating element.  Should be an unbalanced antenna with a 50 to 75 ohm impedance.  Lookup coaxial dipole antenna and compare.

73,
Mike 
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W4JST
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« Reply #4 on: December 14, 2012, 07:27:26 PM »

Thanks for the responses everyone.

The reason I did this is because at the very bottom of the antenna I soldered a PL-259 connector to the bottom piece of 1/2" copper pipe. That made a ground connection right there so I did not join it at the top.

Is this a performance issue?

The SWR is fine but I do want maximum performance for 2 meters.
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WB6BYU
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« Reply #5 on: December 14, 2012, 09:21:09 PM »

So we have a 50 ohm coax up to the connector, then an approximate quarter
wave of 50 ohm coax (electrically slightly longer than 1/4 wave due to the
velocity factor) running up to the feedpoint.  There the upper half of the dipole
is connected to the center conductor - so far so good.  But there is no
connection to the braid for the other half of the dipole.  That means that the
RF currents at the feedpoint flow down the outside of the RG-8.  You must
have equal currents flowing from both sides of the feedline, so if there is any
RF in the top portion of the antenna there must be some on the outside of
the coax.

But the first thing that the RF on the outside of the coax sees is a shorted
quarter wave coaxial stub formed by the shield of the RG-8X down to the
connector and up the inside of the lower half of the antenna.  In some
cases this will act as a high impedance, but in this case we'll assume some
RF is actually being radiated, in which the transit time through the 1/2 wave
of conductor down through the stub means that there is a 180 degree
phase shift by the time the RF starts flowing down the outside of the
lower half of the element.  That makes the current out of phase with any
radiation from the upper half.  But there is also nothing to keep the RF
from flowing all the way back down the outside of the coax into the
shack.  In each wavelength along the way there will be in phase and
out-of-phase currents radiating, so the net effect is to raise the
angle of radiation and waste your power in unwanted directions.  (Unless
you are trying to talk to a satellite overhead.)


So the radiation from the nominal antenna portion is out of phase and
likely will cancel itself at the horizon, and the current continuing to flow
along the entire length of feedline will cause a multi-lobed pattern with
most of the radiation at high angles and little towards the horizon (though
there may be a minor lobe there, depending on the feedline length.)
Neither of those is beneficial for your attempts to work distant terrestrial
stations.
 
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W4JST
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« Reply #6 on: December 15, 2012, 09:40:03 AM »

WB6BYU thank you. Your responses are always very helpful and educational.

I will look into doing something different.
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KC4MOP
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« Reply #7 on: December 17, 2012, 03:28:17 AM »

After I looked at your picture I agree with B6BYU's response, below....

So the radiation from the nominal antenna portion is out of phase and
likely will cancel itself at the horizon, and the current continuing to flow
along the entire length of feedline will cause a multi-lobed pattern with
most of the radiation at high angles and little towards the horizon (though
there may be a minor lobe there, depending on the feedline length.)
Neither of those is beneficial for your attempts to work distant terrestrial
stations.So the radiation from the nominal antenna portion is out of phase and
likely will cancel itself at the horizon, and the current continuing to flow
along the entire length of feedline will cause a multi-lobed pattern with
most of the radiation at high angles and little towards the horizon (though
there may be a minor lobe there, depending on the feedline length.)
Neither of those is beneficial for your attempts to work distant terrestrial
stations.
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W9GB
Member

Posts: 2656




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« Reply #8 on: December 18, 2012, 10:24:33 AM »

Quote from: W4JST
I made a 1/2 wave dipole to use vertically for 2 meters.
It consists of two 19" pieces of 1/2" copper pipe joined with a 1/2" PVC coupler.
At the very bottom, my 1/2" hardline terminates to RG-8X coaxial cable running INSIDE the bottom 19" section of the dipole.
--
THIS is where I made the ground connection, at the bottom, not at the center of the antenna.
I then ran RG-8X INSIDE the bottom 1/2" pipe 19" section to the center of the dipole and soldered the RG-8X center conductor to the bottom of the top element.
--
I have never seen another dipole where the ground connection was made on the far side from the center and wasn't sure if it made any difference or not.
http://i50.tinypic.com/2rmvztj.jpg
Aaron -

I remember coaxial style antennas (like this) sold in 1970s for VHF Aviation Band usage.
http://www.zcg.com.au/coaxial-dipole-base-station-antennas.htm
The coaxial cable running inside the lower element creates a stub and can be used as a coaxial capacitor.

Homebrew Coaxial Dipole for VHF or UHF
June 2009 QST article by John Portune, W6NBC
http://www.repeater-builder.com/antenna/pdf/qst-2009-07-coaxial-dipole.pdf

The Stick VHF Vertical Bazooka Antenna
D.E. Sanders, W4BWS/HC4
http://www.hamuniverse.com/w4bwsverticalbazooka.html

Practical Antenna Design: 140-150 MHz VHF Transceivers
By Elpidio Latorilla
Look at CD-2 antenna on page 52
http://books.google.com/books?id=_Pd9z-mYd6IC&pg=PA52&lpg=PA52&dq=coaxial+dipole+vhf&source=bl&ots=t6NqpZkTYX&sig=PVQkCOeo4Fxb0iX3j77A2dthlOQ&hl=en&sa=X&ei=-bjQUKzfB6bUyQHRyoCQCw&ved=0CEYQ6AEwBA#v=onepage&q=coaxial%20dipole%20vhf&f=false
« Last Edit: December 18, 2012, 10:49:23 AM by W9GB » Logged
W4JST
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« Reply #9 on: December 19, 2012, 08:09:04 PM »

Thank you for the links.
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WY9C
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« Reply #10 on: December 25, 2012, 01:06:27 PM »

I hope that I am in the right area as I am new to this forum. I have a few questions to that relate to the feeding of a vertical dipole. I am considering putting up a 160 foot tall steel pipe on which I will be hanging two 125 foot sections of 6 gauge insulated stranded wires about four feet apart. I will be feeding these wires at the bottom with ladder line. The bottom of this dipole would be about 35 feet from the ground. I am getting back into the hobby after about a 20 year hiatus so my knowledge is very limited. I would appreciate any advice on how you think this antenna would work. Thanks. Robert WY9C
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N4CR
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Posts: 1694




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« Reply #11 on: December 25, 2012, 07:18:57 PM »

For WY9C... I'm just guessing, but for most HF, 6 feet separation will act like a continuation of the feed line.

Essentially each half will cancel the radiation of the other half. And save your money, 6 gauge won't make a hill of beans different from 12 gauge other than making some copper mine owner wealthy.
« Last Edit: December 25, 2012, 07:23:32 PM by N4CR » Logged

73 de N4CR, Phil

Never believe an atom. They make up everything.
WY9C
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Posts: 6




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« Reply #12 on: December 26, 2012, 03:07:54 AM »

Thank you Phil for your advice. Robert WY9C
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M6GOM
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« Reply #13 on: December 26, 2012, 04:16:21 AM »

Also by using a steel pipe, the pipe would become part of the antenna. I would look at reworking it to deliberately include the pipe, maybe by using a ceramic insulator at the pipes's base.
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W5WSS
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Posts: 1775




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« Reply #14 on: December 26, 2012, 06:13:47 AM »

M6gom very clever! Cool
 wonder if he could outer sleeve or somehow improve the conductivity leaving the structural integrity intact?
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