Topband: Re: Topband : Beverage Termination

[Sam Dellit]

g'day

if i can be permitted to add a possible disclaimer to my earlier comments
about there being no need to taper beverage feeds and terminations.

scanning Colin & Zucker "Antenna Theory - Part 2", i quote the following
without pretending to understand the maths or theory
from chapter 21 "surface-wave antennas":

"the actual current distribution along a surface-wave excited at z=0
has a large hump at the feed end, an unavoidable characteristic of
surface wave excitation, and is mirrored on a greatly reduced scale
near the termination, where the discontinuity at z=l, in addition to
diffracting the incoming surface wave, excites a reflected surface wave
travelling back towards  the feed.  the amplitude of the reflected surface
wave can be made very small by tapering the terminal discontinuity (ie
allowing x to decrease to zero over , say half a wavelength distance centred
on z=l and the resulting standing wave along the antenna is then small"

i am not at all sure whether this is directly relevant to beverages, seems
to be more directed at a dielectric slab type antenna, where perhaps exactly
matching terminating resistances (reactances?) are harder to achieve. at
least if tapering is a furphy, then this may be the source of it. certainly
the taper will not cure the vertical response problem, but perhaps it
ensures that the antenna does see the termination as we intend it. over to
the experts
.........

73s
sam dellit vk4zss




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