[Antennas] High Take-off Angle of 1/4 wave Vertical??

Thu Mar 24 16:08:16 EST 2005

de WB2CPN 2005年03月24日
Here's my 2 cents worth before this dies:
The pattern, or the "take-off angle" as you
call it, of a vertical antenna, or any other
antenna, is determined by the point-by-point
summation of the field(s) produced by the
movement of electrons in the radiating element(s).
Every inch of the wire radiates to some degree,
and that depends on the value of antenna current
at that particular inch. These little mini-fields
all interfere with each other by adding or
subtracting their values. Now here's where phase
comes in, and the summation mentioned above has to
include the phases of the fields being summed. The
famous doughnut dipole pattern is a summation of
fields from every inch of the radiator, and from
the point of summation it's a different distance
to every inch of the radiator, so that means every
field has a different phase at summation time.
And, except for polarity, there's not an inch of
the radiator that generates a field equal in value
to any other inch. Gads, what a mess.
With vertical antennas there's always a ground or
something under it that can reflect or redirect the
fields which are radiated. Look at the field
produced by the inch which is half way up the
radiator. That field travels out in all directions,
interacting to some extent with all the other fields
along the way, and the part of the field that goes
down at a 30 degree angle will hit the ground out
there some place. Then get reflected back upward
as it continues its travel. But it is in a mess
of fields with which it must meld. All the summation
taking place out in front of the antenna produces
the various nulls and lobes and whatever.
Ground losses will reduce the amplitude of the
reflected wave, but not the phase or direction. But
amplitude is a factor during the summation process,
so the ground reflectivity will change the position
of the nulls and lobes. That's why ice and snow or
rain under your antenna will make a noticeable
difference in stations heard.
-30-
73 Clete