In a message dated 7/22/01 7:19:33 PM Pacific Daylight Time, KD4OL@aol.com
Request info on obtaining Antenna Modeling software regarding stacked yagis
(2 possibly 3) for 10 meters. Looking for software that is very user
friendly. Your assistance would be appreciated.
Eznec works just fine. However I did discover someting about stacked
vertically polarized beams on a horizontal boom also applicable to horizontal
stacking. A general roll of thumb is this. The optimum spacing is 1/2 wave
for 2 element beams, 5/8 wave for 3 element bams and wider as the gain of
each beam increases. If less than optimum spacing is used there is less gain
and and more upset of F/B and SWR between the beams. If wider than optimum
the 3 dB gain increase stays the same with the lobe sharpening a bit and 2
ears form. 10M and higher beams can be rotated on a horizontal boom to a
vertical pick up antenna for a pattern check, the spacing modified as needed
for "No Ears" and/or then changed to horizontal stacking or use Eznec.
The problem I detected is with 2 element beams using 1/2 spacing and Plumbers
Delight construction. The boom acts like a resonant element for both DE's.
It's hot with RFand then it couples to the mast and tower. I did a series of
tests in Eznec and it showed what the different spacings do for RF Spill Over
on to the booms, mast and tower. You can see the current loops and relative
amplitude on the screen with the antenna. A current chart will give you the
relative current level all over the array also. The higher gain beams tend
to have spacings where the connecting booms are less resonant and "reduce the
problem". I willl run tests on 2M on this concept with different gain beams
with the elements grounded to the boom and then insulated masts etc. Perhaps
the hot or at least warm booms, mast and tower explains one reason why the
F/B is frequently ruined with stacked beams.
I have a similar installation on 6M with 2 element vertically polarized beams
spaced 1/2 wave and I didn't quite get what I thought I should compared to
Eznec in where I didn't use connecting metal booms between the elements and
boom between the beams and connecting mast and tower to ground. I added
metal booms (betwen the gamma matched elements) and mast and they were hot.
Remember any resonant element connected to the center of the DE will absorb
RF also. It travels all directions. I just added an insulated boom between
the beams and will retest. I will add toroids to the coax if the shield is
connected to the center of the DE like on a gamma match. Balanced feed is a
better way to go. If balanced coax is used, do not connect the shield to the
boom. You might try it and see if the SWR changes.
In Eznec booms are seldom added and all that connects to it. There is no
feedline as the "RF Source" is connected to the DE and there is no RF Spill
Over on to a coax feedline as--there isn't any. So always add the boom and
all it's actual connections after the ideal free space tests. The mast or
tower section supporting the stacked beams have always been assumed to be
"Immune to RF coupling" just because it's the opposite polarization--NOT SO.
Insulating elements from the boom stacked or not has shown to be in Eznec a
good idea. More on this later. K7GCO.
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