[TowerTalk] 15m monster finally up

[john at kk9a.com]

That is a good point Jim.  I removed the "source" from the four yagis on one 
side of the array.  I believe that EZNEC sees this as a short.  I could 
place a small gap in the middle of the DE and see what this does.

John KK9A


To:towertalk at contesting.com
Subject:Re: [TowerTalk] 15m monster finally up
From:Jim Lux <jimlux at earthlink.net>
Date:Wed, 26 Sep 2012 16:14:12 -0700
On 9/26/12 10:16 AM, john at kk9a.com wrote:

After studying this a little more, I do not think that the beamwidth can
be easily changed with switching.  In a model I tried driving only one
side of the array and the pattern looked similar to the fully driven
stack, just slightly skewed. One thing for sure, this will be very very
strong in the direction it is pointing.


Very interesting


I just ran a quick model using a 3 element Yagi (from Cebik.. it's the 
sample with 4nec2 called 3lyagi20) I duplicated it 50 feet over (the driven 
element is about 34 feet long)
With both driven, gain is 16dBi, HPBW in the horizontal plane is 40 degrees


With one driven, the other shorted (E=(1E-5,0)) the gain is 13.11 max at 4 
degrees towards the shorted element, HPBW is 70 degrees

With one driven, the other open, the gain is 9.34 dBi, pointed 8 degrees 
away from the undriven antenna), HPBW is 70 degrees

With the undriven antenna shorted, the gain is almost exactly 3 dB less, 
pretty much what I expected.

All I can figure is that with the driven element open, the reflector and 
director are still coupled to the original antenna, and sucking some power 
away, and reradiating it with a bad relative phase.