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Author Topic: Questions: Homebrew 2 meter/70cm collinear antenna  (Read 4456 times)
KD5GR
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Posts: 101




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« on: November 12, 2012, 01:54:21 PM »

I had given up on building a collinear antenna based on a couple of articles I’ve read, but the ARRL Antenna Book has renewed my interest.  The one shown on page 15-10 of the Book is the same as the one shown here http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12968 and it raises a few questions.  [NOTE:  For some reason, the hyperlink isn't going to the correct page, but if you copy the URL and past it in your browser, it will take you to the correct page.]

First, is there any reason why I can’t place the ¼ wave aluminum/copper sleeve balun inside the PVC or fiberglass rod with the ferrite beads below, and a female N-type connector below that?  I plan to mount the connector to an aluminum plate so I can attach the ¼ radials.  (I'm only going to use 4 half-wave dipoles to keep the antenna length shorter.)

Second question:  According to the ARRL Antenna Book, this is the method used in the Comet GP series of antennas and some of those like the GP 3 is a 2 meter/70 cm dual band.  Is a choke used in the Comet antennas?

Finally, I have the EZNEC software that came with the Antenna Book, and I could like to model this antenna if possible.  Has anyone modeled a collinear antenna with EZNEC?

Thanks,
Chas.
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WB6BYU
Member

Posts: 13337




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« Reply #1 on: November 12, 2012, 03:33:50 PM »

Quote from: KD5GR
...The one shown on page 15-10 of the Book is the same as the one shown here http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12968 and it raises a few questions.



A lot of commercial antennas are built like that, but performance suffers as the velocity
factor of the half wave sections decreases below 1.0 because of the difference between
the current distribution inside the coax (where the velocity factor applies) and the current
flowing on the outside of the shields that is responsible for radiation (where the velocity
factor would be different.)

Commercial designs use copper pipe and wire with various sorts of spacers to make coaxial
lines with a velocity factor very close to 1.0 instead of coax cable sections.


Quote

First, is there any reason why I can’t place the ¼ wave aluminum/copper sleeve balun inside the PVC or fiberglass rod with the ferrite beads below, and a female N-type connector below that? 



That should work.



Quote

I plan to mount the connector to an aluminum plate so I can attach the ¼ radials.  (I'm only going to use 4 half-wave dipoles to keep the antenna length shorter.)



Why are you adding radials?



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KD5GR
Member

Posts: 101




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« Reply #2 on: November 12, 2012, 05:58:10 PM »

Quote from: KD5GR
...The one shown on page 15-10 of the Book is the same as the one shown here http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12968 and it raises a few questions.



A lot of commercial antennas are built like that, but performance suffers as the velocity
factor of the half wave sections decreases below 1.0 because of the difference between
the current distribution inside the coax (where the velocity factor applies) and the current
flowing on the outside of the shields that is responsible for radiation (where the velocity
factor would be different.)

Commercial designs use copper pipe and wire with various sorts of spacers to make coaxial
lines with a velocity factor very close to 1.0 instead of coax cable sections.


Quote

First, is there any reason why I can’t place the ¼ wave aluminum/copper sleeve balun inside the PVC or fiberglass rod with the ferrite beads below, and a female N-type connector below that? 



That should work.



Quote

I plan to mount the connector to an aluminum plate so I can attach the ¼ radials.  (I'm only going to use 4 half-wave dipoles to keep the antenna length shorter.)



Why are you adding radials?

The information you provided about the velocity factor is what I read before and it's why I previously decided against trying to build a collinear antenna.  I'm disappointed the ARRL Antenna Book didn't cover this issue.

I read that using less than 8 1/2 wave segments results in a higher SWR and that using radials helps to lower it.  There was a reference to the use of radials in the Comet GP series of antennas.  (This was in other articles, not the ARRL Antenna book.)

You're always a great resource and I appreciate the information.  I'm going to stick to building the yagi and just buy the Comet GP 3 for the omnidirectional antenna.

Thanks again,
Chas.
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N4CR
Member

Posts: 1672




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« Reply #3 on: November 15, 2012, 06:16:22 PM »

Here's another article that has more details that might be useful.

http://www.repeater-builder.com/antenna/wa6svt.html

Originally from '73' magazine.
« Last Edit: November 15, 2012, 06:19:17 PM by N4CR » Logged

73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
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