Re: [TowerTalk] Modelling Dilemma

[Terry Conboy <n6ry@arrl.net>]

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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