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  #1  
Old 04-02-2007, 02:57 PM
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Default J-Pole antenna

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Originally Posted by mplafleur View Post
I'm building my own antennas. A J-Pole for the winglets. I've got a prototype built but haven't done any testing with it.
So - wat b a j-pole? a one pole, ungrounded, j shape?
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  #2  
Old 04-02-2007, 03:35 PM
ddillon ddillon is offline
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Default J-pole

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Originally Posted by Dust View Post
So - wat b a j-pole? a one pole, ungrounded, j shape?
The J-Pole is an easy to construct antenna with two elements arranged in a sort of J-shaped arrangement... a long element and a short elemenet, both parallell. It is a popular base-station antenna for home-constructed base-station antenna setups because it can be made of copper water pipe.

It also makes for a convenient portable antenna for backpackers, hikers, and users of portable handheld radios because a roll-up j-pole can be constructed from a piece of coax cable and some ladder line or twin-lead (that stuff made of two parallell wires and an insulator).

I used to always keep two roll-up j-poles in my flight bag. One for for my handheld radio as a backup and another one for my handheld amateur radio.

Back at that time, I had access to a telephone-link via my handheld radio. It was convenient to call a friend and let him know that I was 20 minutes from landing and we could meet at the restaurant in 30 minutes.

I have had bad luck with handheld radios in the cockpit and came to the conclusion that they are almost worthless when used from within metal airplanes using the little rubber antenna. I've got one of those early Yaesu handhelds with the nav function. If I ever have to use that to do an IFR or night approach during a radio or electrical emergency, I had better have made my peace with the higher powers.

Both worked WAY better than that rubber antenna supplied by the radio manufacturer, and could easy clip onto the sun-visor of the cockpit.

The J-pole is a sensitive creature, likes to be vertical, and does not like to be near other metal. One of the benefits, however, is that the coax cable feedline is attached to the end of the antenna rather than the center.
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  #3  
Old 04-02-2007, 03:53 PM
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would be interesting to see a pic of these antennas

mp has one of these analyzers - tested mine ages ago - i think i remember that they tested well - i think i remember that
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dust

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  #4  
Old 04-02-2007, 04:30 PM
ddillon ddillon is offline
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Default J-pole antennas

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Originally Posted by Dust View Post
would be interesting to see a pic of these antennas

mp has one of these analyzers - tested mine ages ago - i think i remember that they tested well - i think i remember that
Hey Dust,

Are we going to see copper water pipe sticking out of the roof of your new hangar so that you can sand and listen to the approach frequency?

I can't find any photos anywhere for roll up j-pole antennas for aviation use, but a quick search shows that MFJ sells them pre-made for 2-meters (about 144MHz). Link here with a clickable thumbnail to see a photo.

Diagrams and instructions for the roll-up-j-pole can be found here. The top section is for the copper pipe j-pole and the bottom section of the web page has a diagram and instructions for the roll-up j-pole. It has a calculator on the page that doesn't seem to work.

Another site that shows some photos are here.

And, yet another info page.

My aviation j-pole was pretty big when unrolled... and if I recall correctly, it went from the sun visor either to the floor or over the backrest of the passenger seat.
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*Chapter 6 - in progres, fuselage assembled without a bottom, step 1 completed.

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  #5  
Old 04-02-2007, 07:13 PM
Nathan Gifford Nathan Gifford is offline
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I cannot remember any of the high altitude balloon people I know using j-poles. Of course, they are operating above 50kft!!

I will have to go back and check on the radiation pattern for these antennas. True enough, they would be cheap and easy to install on the vertical stabilizers, but I am not sure how well they work at altitude.

They are great ground level antennas.
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  #6  
Old 04-02-2007, 11:28 PM
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I went through many calculations and a number of tries at making this antenna before I found something that worked with my analyzer at the center of our comm band. The final result was an antenna that was about the same length as the distance from the bottom of our winglet to the tip. I think I'll need to make the winglet slightly taller to make it fit. It uses a combination of copper tape and 300 ohm twin lead. I have yet to check out its' characteristics.
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Old 04-02-2007, 11:52 PM
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SO - is this a 5/8 ths wave? seems the benifit is in making a lonnger wave antenna in the same space as a dipole.

Also - think i like the wire for the com antenna instead of the foil cause it may have better forward performance

is that the case?
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dust

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  #8  
Old 04-03-2007, 12:23 AM
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Quote:
Originally Posted by Dust View Post
SO - is this a 5/8 ths wave? seems the benifit is in making a lonnger wave antenna in the same space as a dipole.

Also - think i like the wire for the com antenna instead of the foil cause it may have better forward performance

is that the case?
It is a 5/8 wave which will have better gain than the dipole.

The tape gives broader bandwidth than wire.
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  #9  
Old 04-03-2007, 11:03 AM
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Quote:
Originally Posted by mplafleur View Post
It is a 5/8 wave which will have better gain than the dipole.

The tape gives broader bandwidth than wire.
But - does the tape do that on edge?
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dust

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