Comments on: Jpole vs. Coaxial Dipole EZNEC Shootout http://www.hamradio.me/antennas/jpole-vs-coaxial-dipole-eznec-shootout.html Hams helping hams make the most of the hobby of amateur radio. (This site is moving from www.hamhelpdesk.com to www.hamradio.me) Fri, 27 Aug 2010 00:29:03 +0000 http://wordpress.org/?v=2.8.4 hourly 1 By: edwardsharriman http://www.hamradio.me/antennas/jpole-vs-coaxial-dipole-eznec-shootout.html/comment-page-1#comment-137 edwardsharriman Fri, 26 Jun 2009 03:02:38 +0000 http://www.hamhelpdesk.com/?p=492#comment-137 Gentlemen: I've done a fair amount of modeling and of building bazooka (similar to coaxial) and end fed Zepp (similar to jpoles) antennas over the last few years. I'd like to point out that of the three antennas modeled above, the jpole is the easiest to construct and has the fewest problems with feed line current. To be complete, the feed line should be included in the models. One quickly sees that, even with a perfect choke at the base of the coaxial antenna, significant feed line currents are generated. The monopole has less of a problem and the jpole has nearly none. Great article! ward Gentlemen:
I’ve done a fair amount of modeling and of building bazooka (similar to coaxial) and end fed Zepp (similar to jpoles) antennas over the last few years. I’d like to point out that of the three antennas modeled above, the jpole is the easiest to construct and has the fewest problems with feed line current.
To be complete, the feed line should be included in the models. One quickly sees that, even with a perfect choke at the base of the coaxial antenna, significant feed line currents are generated. The monopole has less of a problem and the jpole has nearly none.
Great article!
ward

]]> By: admin http://www.hamradio.me/antennas/jpole-vs-coaxial-dipole-eznec-shootout.html/comment-page-1#comment-112 admin Fri, 17 Apr 2009 02:54:19 +0000 http://www.hamhelpdesk.com/?p=492#comment-112 Hello James, In the coaxial dipole EZNEC model I have one source at the bottom end of the upper element. Actually this source position is automatically centered in the bottom most segment as I recall. For simulation purposes only, the four wires for the external legs are all attached to the bottom of the top wire. One thing you should add to make the simulation more complete is the addition of one more wire also attached to the bottom of the top wire. This simulates the outside of the coax braid to some extent and reveals why these antennas can be difficult. I design 900 MHz antennas for our UAV airplanes and I had to use a couple of ferrite cylindrical beads around the coax just beneath the lower extent of the four wires to "choke" the current from flowing further down the coax. If I don't do this, the whole coax becomes part of the antenna. Your description is apt... it gets ugly. Folding back the braid keeps things simple, but you still need to choke that current on the rest of the coax to get what you want. Sounds like you are on the right track. Good luck. John Hello James,

In the coaxial dipole EZNEC model I have one source at the bottom end of the upper element. Actually this source position is automatically centered in the bottom most segment as I recall. For simulation purposes only, the four wires for the external legs are all attached to the bottom of the top wire.

One thing you should add to make the simulation more complete is the addition of one more wire also attached to the bottom of the top wire. This simulates the outside of the coax braid to some extent and reveals why these antennas can be difficult.

I design 900 MHz antennas for our UAV airplanes and I had to use a couple of ferrite cylindrical beads around the coax just beneath the lower extent of the four wires to “choke” the current from flowing further down the coax. If I don’t do this, the whole coax becomes part of the antenna. Your description is apt… it gets ugly.

Folding back the braid keeps things simple, but you still need to choke that current on the rest of the coax to get what you want.

Sounds like you are on the right track. Good luck.

John

]]> By: jlerch http://www.hamradio.me/antennas/jpole-vs-coaxial-dipole-eznec-shootout.html/comment-page-1#comment-111 jlerch Fri, 17 Apr 2009 01:20:30 +0000 http://www.hamhelpdesk.com/?p=492#comment-111 With regards to the coax dipole, how many sources are you using? Are the 4 external legs connected to the center conductor or the shield? I ask as I'm attempting to model a popular 910mhz ant used in RC airplanes to transmit video. The ant is nothing but a piece of turtle necked coax, with 1/4 of center conductor exposed and a 1/4 wave of the coax insulation turned back on itself. When I modeled this, it gets ugly.. I was refered to this web page, which shows a coaxial dipole, but I wonder if we are working on the same idea or not.. Thanks for your time James Lerch Tampa, FL With regards to the coax dipole, how many sources are you using?

Are the 4 external legs connected to the center conductor or the shield?

I ask as I’m attempting to model a popular 910mhz ant used in RC airplanes to transmit video. The ant is nothing but a piece of turtle necked coax, with 1/4 of center conductor exposed and a 1/4 wave of the coax insulation turned back on itself.

When I modeled this, it gets ugly.. I was refered to this web page, which shows a coaxial dipole, but I wonder if we are working on the same idea or not..

Thanks for your time
James Lerch
Tampa, FL

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