> Contrary to the belief of many, a beam located at a lower level on the
> tower does not "shield" any top loading which is above that level.
> The lower beam merely has less top loading effect than if it were
> mounted higher. I can see no difference in this if the beam is
> side-mounted.
While "shield" may be a poor choice of words, a large yagi or yagis
mounted low on a shunt-fed tower can have a detrimental effect on
system efficiency if they are large enough or enough of them. I
prefer to think of it as "robbing" the upper part of the antenna of
current.
Antenna radiation is directly related to the physical linear distance
between two points we move current over. What we want is the
percentage of current to be as uniform as possible and as large as
possible over the entire length of the structure. Adding a low
antenna increases the current over a short distance, and that reduces
the ampere-feet of the radiating system while increasing current in
what could be a lossy ground system.
It is much like mounting a capacitance hat low on a mobile antenna
system (like many people do) instead of at the very top. Loop
radiation resistance is decreased (the antenna appears to occupy
somewhat less height) and efficiency can suffer.
Doesn't mean it won't work, just that it can often make things worse
than they could be.73, Tom W8JI
W8JI@contesting.com
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