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Author Topic: J pole radiation pattern  (Read 2707 times)
K5DVW
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Posts: 2193




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« on: July 12, 2002, 11:47:59 AM »

I'm curious about j-pole designs for VHF and UHF. I'm mostly interested in the gain pattern, most notably the elevation slice. Since the j-pole is basically an end fed 1/2 wave antenna, should I expect the pattern to be basically omni directional, with elevation axis at 0 degrees, and 2.1 dBi gain? What about the colinear design with 1/2 over 1/2 wave. I've modeled it on NEC, but depending on how I feed it, I see the pattern change significantly, so I don't know what to believe.

Any information will be appreciated.

Thanks
K5DVW
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WB2WIK
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Posts: 20612




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« Reply #1 on: July 12, 2002, 12:23:18 PM »

I've always found the models to contain some built-in errors, but they're surely better than guessing.

In actually _testing_ 1/2-wave, base-fed VHF antennas like J-poles in our semi-anechoic chamber here at the lab, radiation angle is usually 16 degrees (above horizon), gain ~2.1 dBi as you state.  A nearly perfect replication of a J-pole, but one that's easier to adjust and also easier to mount, is the now 30 year-old Cushcraft "Ringo" design, by W1BX.  It's a J-pole that has an aluminum sleeve to facilitate easier mounting, and the matching section is wrapped into a flat ring at the base of the radiator, rather than sticking up in parallel with the main element.  Other than that mechanical variance, the Ringo is a J-pole.

Likewise, the original "Ringo Ranger," which is two 1/2-waves in phase stacked vertically, with the phasing line made of aluminum rod and fully adjustable, all base fed, exactly the same as one would do to make a "colinear J-pole."  That's Cushcraft model ARX-2, and it's also been on the market for 30 years, weighs almost nothing, and works fine.  It's radiation angle is about 7 degrees, measured, not modeled.

WB2WIK/6
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K5DVW
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« Reply #2 on: July 12, 2002, 01:48:34 PM »

Great information! The 16 degrees take off angle appears to somewhat match my model when I feed it balanced from the bottom of the U section, so maybe it's not that far off.

What I really need is an antenna with about a 5-10 degree down tilt. The model suggests if I do a 2x3/8 over 1/2 combination j-pole, I'll get something close. Guess I'll build it and find out.

Gee, I wish I had an anechoic chamber!

Thanks

Darrin
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WB2WIK
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« Reply #3 on: July 12, 2002, 02:15:38 PM »

Yep, managing electrical downtilt is fun and can be done a variety of ways.

One of the easiest ways is to use a conventional vertical antenna, and install it upside down!  (Don't laugh, it works, and provides very predictable downtilt.)

73 de Steve, WB2WIK/6
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K5DVW
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« Reply #4 on: July 12, 2002, 02:35:28 PM »

Well I used your suggestion about the ringo and did some research. They use a 1/8 phasing line instead of the 1/4 I was using. When I model a colinear 5/8 wave with a 1/8 wave phase shifter, I dont get downtilt, but I get really strong gain lobes at the horizon and only 1.5 dB down at 10 degrees.

This antenna is going to be used for a repeater at 7500 ft HAAT so a fat lobe at the horizon is good to have.

I've heard about the trick to mount a vertical upside down and we thought about it, but at our location we have only about 5" of vertical pipe to mount to... oh well!

Thanks for the great info!

K5DVW
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