g'day tom & the topband gang
roll on 1 Jan 04 (aussie morse requirement voided)!
re tom's post :
my em theory is not as strong as i would like & i read far
more than i absorb, but nevertheless some miscellaneous
comments which may be relevant
(1) radiation from vertical / sloped / snaked ends of
beverage antennas is NOT trivial and its minimisation
has always been a significant aspect of beverage design
ever since the beverage was invented (tangentially : it is
not clear to me from my reading yet that Bev invented the
single wire "beverage" antenna, Bev seems to have claimed
primarily the twin wire wave antenna that Misek promotes
in his book)
(2) Bev's anecdotal response to end radiation, (i think they
called it "vertical effect" in the golden age) was that vertical
effect is not important if the antenna is 100 times longer than
it is high. This is a good anecdote in my practical experience
& theoretical understanding. For an am bc dxer where we will
be looking for longer antennas due to lower freqs, most bev ants
will satisfy the anecdote : typically 300m+ long and 3m high, but
shorter topband bev ants are getting dodgy in my opinion at 3m
high. i take Bev's assertion of "not important" to mean about 20dB
down on the maximum. of course, most of us would regard any
response above 40dB down as worthy of interest
(3) tom has tackled the problem from the radiation perspective, but i think
there is a simple estimate available from a reception perspective
(of course a full MoM solution would be nice but NEC2 is garbage
on bev ants & it is not clear to me yet that NEC4 addresses all the issues
either). to a first approximation : the wave tilt effect will generate a
travelling
wave of about 10% of the magnitude of the incident vertically polarized
wave for moderate ground conductivity (a little more for poor conductivity,
a little less for good conductivity). the travelling wave will build up
along the
antenna in proportion to its length. the direct incident field will produce
a
voltage at the end of the vertical section being 100% of the incident field
strength
times the length. thus a 3m vertical produces about the same voltage as 30m
of horizontal wire, so to get the vertical effect 10 times (20dB) less than
the
travelling wave effect a 300m long bev ant is needed. first order
approximation
is simple; second order actually calculate the wave tilt (quite easy, jordan
& balmain
have an example calculation), third order : see the lawrence livermore
national lab
site and read lotsa stuff by burke & miller which hopefully has all been
incorporated
into NEC4
(4) side issue is that there is a phase relationship between the induced
vertical
& travelling wave components dependent upon the conductivity itself. another
side
issue is that we have both termination end vertical effect and feed end
vertical effect,
which also will have a phase relationship depending upon the bev ant length
(the basis
of the EWE ant)
(5) on a vaguely related note, i grow tired of seeing all these lovely NEC2
bev ant
model results which are clearly not even worthy of first order approximation
status.
we all know from experience that the bev ant is a fabulous low TOA ant,
especially
when located at the coast, yet all these models show deep minimas / nulls at
low
TOAs. i am at the point of assuming that the error lies in the absence of
full (any)
consideration of the sommerfeld surface wave, but still not certain.
(6) is there a web site which has published results of NEC4 bev ant models
complete
with vertical leads & ground stakes?
(7) can someone with a better understanding of em theory explain received
surface waves
on this reflector. i have a reasonable understanding of how radiation from
an antenna can
be modelled in terms of a space wave and a surface wave (propagated along
the surface
of the earth), but what is the reciprocal situation for an incident received
plane space wave
at the surface of the earth. clearly the space wave is received both
directly and by reflection.
do the incident space waves excite a surface wave which propagates along the
surface. if so
does this surface wave decay rapidly (which i assume generally happens) or
can it actually
build up to a high level (which is what i assume happens for the special
case of a coastal
bev ant, with the surface wave reaching high levels along the salt water
interface, then cascading
on to the land based bev ant)
(8) on a more positive note, xmas came early at my shack with the
neighbouring sand mine
operator agreeing to let me use 100+ acres of their land at the rear of my
property
for antennas. took me all of 3 hours to set up 500m bev at 6m on 100degT
right thru
central america. another 500m on 280degT to follow shortly. may the solar
flux keep
dropping
73s gd dx de sam dellit vk4zss brisbane / tamborine
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