At 12:45 AM 9/21/99 EDT, KI7WX@aol.com wrote:
>
>Anyone know if the study attributed to Mr. Gordon was limited to
observations
>with yagis mounted at tower tops only. e.g. above the guys? Seems this
>would be fine, but is a different situation than stacked yagis where the
>antennas are within the guy wires and sometimes quite close to them.
Hi Mark -- The K4VX study (8/93 QST) evaluated one stacked yagi case and
one sloper case. The stack was for 20 meters, with antennas at 109 and 54
feet, the lower one inside continuous guys tied off to the tower at 53 and
103 feet. He does not say, that I could find, whether the guys were
modeled as floating or grounded. A good model would also assume some value
of resistance between the guys and the tower and the ground plane.
To be fair, the article says only that you should *consider* continuous
guys. In the sloper case, where the interaction produced some gain
increase, he cautioned that this was a single specific case.
To my mind, that caveat is the most important sentence in the whole
article. The article does seem to show that a given antenna/tower/guy
arrangement can operate with only minimal effect from continuous,
conductive guys, but it could be a serious mistake to extrapolate from that
and assert that *any* installation will see no destructive effects from
such guys.
I have my entire station in a model, including guys. Acknowledging that
the geometry of the model is not a perfect representation of the physical
reality, it's a useful tool. Recently, I modeled my 97-foot Rohn 25 tower
with Force 12 C-3s at 97 and 69 feet. The guys (at 31, 62 and 95 feet) are
insulated at the tower and grounded at the outer ends. The upper guys have
21-foot fibreglass insulators at the inner ends.
With this configuration, the modeled impact of the guys on the pattern of
the stack was significant, and for the lower antenna used alone it was
profound. The main culprit appears to be the top guy set, and it may be
that there is a critical length issue there. I haven't tried breaking it
up in various standard ways yet, to see if that helps.
Whatever the case, it does illustrate the importance of doing the modeling
for your situation, not taking the K4VX article as a general rule.
73, Pete Smith N4ZR
n4zr@contesting.com
Sometimes a tower is just a tower
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