>By making the tower be "behind" the reflector it (I think) will have a
>negligible effect on the antenna's pattern....whenever I do the side mount on
>the leg/face of the tower it ends up near an element and although it is
not re
>sonant I think the tower MUST do some degrading of the beam's pattern.
There are two main scenarios that degrade the pattern of Yagis.
First is any conductor near resonance anywhere in the area
of the antenna and in a plane to interact. The second is from
any conductor not near resonance that is very close to one of the
Yagi elements. This situation will cause changes due to
capacitive loading, and therefore detuning, of the nearby element.
This second scenario is the one that typically applies with a tower
up through the Yagi. But with separations of a few feet or more
between the tower and the nearest element then this problem is
minimal.
>How about it, you yagi sages....at 10 meters these antennas will weigh in at
>about 20 lbs. (including the coaxial feed line)....I am not worried about
>upsetting the "balance" of the physical load on the Rohn 55 they are mounting
>to...if they were bigger beams at a higher frequency I could see being
>worried but these are overgrown TV antennas.
A stack of end mounted Yagis may not make for much mechanical
load but can add considerable torque to the tower during high
winds. You can probably get away with end mounting on a fixed
tower--I would never try it with a rotating tower. Just too much
wind torque.
>DOES THE FACT THAT THE OTHER ANTENNAS BELOW IT ARE NOT "directly" BELOW CAUSE
>ANY POTENTIAL QFU? (Seem that when you use swinging gate side mounts and
>ringrotors you are never directly underneath the top beam, anyhow.
>
Yagis in a stack must be "lined up" in the direction they are pointing.
One that is "closer" or "further" from the intended direction than the
others is no different than having more or less coax in that feedline.
It is possible for the antennas to be out of phase if one 10M beam
on a 34 ft boom is rear mounted and another 10M beam is stacked
with it that is center mounted. The driven elements are
approximately 17 ft different in their distance to the intended "target".
When a ring-rotator-mounted Yagi is "stacked" with a Yagi that
us mast-mounted, the driven elements may be offset a bit side
to side but if mounted correctly they are equally distant from the
Target and are still in phase. If the two Yagis are pointed
different directions so that their patterns don't overlap then it
doesn't matter if they are then "non-coincident" as far as their
driven elements are concerned since the two patterns are not overlapped
and any signal differences in the sidelobes of one is many dB down
from the main lobe of the other. Adding or subtracting a very
small number from a big number makes little change in the big
number--so the phase of the small number (sidelobe) is unimportant.
I think the interaction of the tower with the Yagis is minimal based
on my experience with 8/8/8/8 on 10M. Of course the tower
goes through the elements in an area where the directors are starting
to get spaced pretty far apart. This array was mounted on a
rotating tower and there were never any indications of lobe distortion
and the magnitude of my EME echoes on 10M seemed to confirm
the theoretical numbers and I was quite satisifed with the S/N
ratio on EME--it being as much a 15 dB on one occasion.
Again the secret to mounting a beam with a tower running up
through the Yagi is to keep the elements as far from the Yagi as
possible--at least two feet or more on 10M. And keep those
driven elements directly over each other or be prepared to add
a delay line into any that are "closer" to the intended target.
73 John W0UN
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