To: <topband@contesting.com>
> Date: Tue, 31 Mar 1998 16:43:07 -0500 (EST)
> Eric, N7CL commented that Ewes perform better over better ground.
I agree with him, it worked that way for me...and it worked that way
for W8LRL..... and about a dozen others who answered a poll I took a
year or so ago.
> This might be true only because the ground conductivity greatly affects
> the TOA (in this case, the angle of max reception), but this is true for
> all antennas, no?.
Not necessarily. "TOA" is affected by ground a long distance out from
a vertically polarized radiator. That has little to do with ground
systems near and under the antenna.
A EWE is really two vertical radiators, with a single wire phasing
line that feeds them. The current is 180 degrees out of phase in the
verticals, plus delayed by "S" (the space of the elements or length
of the wire).
In order to have good directivity, the current in the system has to
be uniform. That requires the top wire to NOT radiate or NOT be over
(and coupled to) lossy soil. It also requires a good non-radiating
termination at each end.
"K9AY" loops are the same way.
Stick them on a crummy ground, and they very likely won't work as
well as they can.
Since the 70's, I've used two and more element phased arrays for
receiving. I tried an EWE, mainly to eliminate the feedlines and
broadband matching devices my systems normally require. I had very
poor results until I installed a ground screen under the EWE. The
ground screen was so much work, I abandoned the idea (the goal was to
save effort, not make life more difficult).
> Eric also said that Ewes are size-critical, but he said this is because
> if the Ewe is small, it doesn't produce enough signal. Any Ewe, large or
> small, can be made resonant on the desired frequency by inductively
> loading it at the feedpoint. Doing so does not alter its directional
> pattern and, no doubt, will produce a much larger signal.
I would certainly agree that is true IF the EWE is a real mess to
start with. But a properly terminated EWE has travelling waves, NO
standing waves. Adding a reactance in series with the system will
produce a degradation is signal, unless you simply haven't
conjugately matched the feedpoint to the receive system and the added
reactance is forming part of an impedance transformation system.
The last thing in the world you want is an inductively loaded EWE,
loaded at only one end. Any non-symmetry at one end of the
system would destroy the symmetry of the two vertical sections. Those
vertical sections MUST have equal currents with a phase shift
of 180-S for a unidirectional pattern, and EQUAL currents in the case
of equal size vertical sections for a complete backfire null.
> I previously posted the parameters for optimum-size Ewes modeled over
> "very poor", "poor", "good" and "very good" soil conductivity. Anyone
> who didn't read that post and would like to have the info, e-mail me
> direct and I'll send it to you.
Even the author of EZNEC cautions against putting too much faith in
accuracy of models with low horizontal wires near ground, or antennas
connected to ground. The error between a low dipole and the real
world thing using full blown NEC-2 is about 5 dB, according to
published data. If it can't accurately model a dipole close to earth,
it probably can't accurately model an EWE.
73, Tom W8JI
w8ji.tom@MCIONE.com
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