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Author Topic: 40 meter wire vertical  (Read 5073 times)
W5WSS
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« Reply #15 on: August 25, 2012, 09:22:23 AM »

Hello Cecil yeah it is interesting to find differing results with eznec perhaps one should compare the simulations to more powerful software.
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W5WSS
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« Reply #16 on: August 25, 2012, 10:04:54 AM »

Two tuned ,elevated radials positioned opposite each other is sparse but should serve to cancel the horizontal radiation some but depends.
Toms point is well taken though with regards to problematic radial counterpoise inadequacies forcing common mode driving the feedline via the earth surface relationship relative to the radials i.e. when they are not say covering an estimated every 5 degrees or so of spacial area about the vertical element. So if one can not remedy the inadequacy at the source then a 1:1 choke balun can allow the system to work as intended.
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KJ4I
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« Reply #17 on: August 25, 2012, 11:05:11 AM »

Common mode has definitely been something that crossed my mind on this project. I have dealt with it on several occasions over the years and it's not a lot of fun. Good thing is though that I have some baluns and some 1:1 choke baluns laying around if needed. It will be a pretty simple setup so it will be easy to adapt for experimentation. I'm not yet sure what kind of issues with interaction I'm going to have regarding the proximity to the tower but that's all about the experiment I guess. I might end up considering the fiberglass pole option a little later on.
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WB6BYU
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« Reply #18 on: August 25, 2012, 11:24:19 AM »

A delta loop might not be a bad choice in that situation, as the points of maximum
radiation are spaced futher out away from the tower.

It's just wire, rope, and coax:  experiment and see what works.
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W8JI
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« Reply #19 on: August 25, 2012, 11:52:13 AM »

I'm not sure I understand the EZNEC results. I modeled the antenna over mininec ground. The result was -0.04dBi at 23 deg TOA. The current in each of the radials was 0.5 amps, half of the supplied current. I then added a wire from the junction of the two radials to mininec ground to simulate a common-mode path. The result was 0.0 dBi at 22 deg TOA. There was 0.15 amp in the simulated path to ground and 0.57 amp in each of the radials. The simulated common-mode path to ground increased the current in each of the radials by 14%.


It's well known, and has been publicized for a long time, that a few elevated radials are problematic for common mode. Also, any ground path to earth increases loss.

Even a groundplane with four 1/4 wave radials requires a common mode isolation method or the antenna will be sensitive to feedline and mast lengths, and ground path changes.

There is some optimal ground path impedance that presents the most loss. A mininec ground may or may not allow that impedance.
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N8CMQ
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« Reply #20 on: August 25, 2012, 03:17:38 PM »

Hi!
  Can you put a radial field under your tower?
If you can, then try gama loading the tower, you may be surprised!
Have fun!
   CMQ
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W5DXP
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« Reply #21 on: August 25, 2012, 07:44:51 PM »

Hello Cecil yeah it is interesting to find differing results with eznec perhaps one should compare the simulations to more powerful software.

As in any other system, the magnitude of the common-mode problem probably depends on the length of the common-mode path to ground, i.e. the length of the coax.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
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