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Re: [TowerTalk] Modelling Dilemma

To: N4CW@aol.com
Subject: Re: [TowerTalk] Modelling Dilemma
From: Terry Conboy <n6ry@arrl.net>
Date: Thu, 09 Sep 2004 19:26:05 -0700
List-post: <mailto:towertalk@contesting.com>
At 05:59 PM 2004-09-09, you wrote:
Using EZNEC 2.0, I model a one-wavelength wire at 50 feet above real ground
("normal"), and feed it at 50% from one end. I get a pattern that has nulls off
the ends of the wire. If I change the feed point to 25% from that same end,
the pattern has nulls off the sides ("normal" to the axis) of the wire.
I didn't realize changing where an element is fed would alter its radiation
pattern.
I am plotting the "total" field in an azimuthal plot.
I'd appreciate someone explaining why this happens or pointing me to some
readily-available reference material.

Bert,


In the first case (50% point), what you have created is often called "Two Half Waves in Phase". It has a high impedance feed and if you look at the currents, you'll see that they are indeed in phase, which is why you get a nice broadside pattern.

In the second case (25% point), you have made a long wire antenna. In a long wire, currents alternate in phase by 180 degrees every half wavelength. In your case, you have two half-waves out of phase, which produces a null normal to the wire. The feed impedance is relatively low (100 ohms or so) since you are feeding at a current loop. If you make the wire 2 wavelength long, and feed at 12.5% from one end, you will see that the main lobes get closer to the ends of the wire and you pick up 2 side lobes, too.

When I first got EZNEC, I stayed up all night playing with Roy's darn program!

73 Terry N6RY

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