To: <topband@contesting.com>
>Date: Tue, 31 Mar 1998 21:13:37 -0800
>From: Bill Hohnstein <k0ha@navix.net>
>
>> N7CL wrote:
>> >
>> > Modeled Beverages simply go dead when you make the ground under
>> > them perfect. This also squares with my field experience...
>
> I might have agreed with that last year. The ground conductivity
>here is very high. Last year my Beverages generally were lucky to be
>as good as my parasitic vertical array for reception to Europe. Since
>then I improved my transmit antenna's European director and I have
>received better reports from Europe on its performance. I also have
>changed my Beverages--lowered their height to about one meter above
>ground, changed matching on coax to some, changed some lengths, and
>changed the termination resistance a bit. This year I rarely saw
>improved reception while using my transmit antenna...
>
The operative word in my statement was "perfect". Even sea water
isn't actually perfect. Also, even though I didn't make it very
clear, I was describing what happens to a working _model_ of a
beverage which has been optimized for operation at some height
over some less than perfect ground constants suddenly has the
ground constants changed to "perfect". If you do the same thing
to a working EWE model, things change but it doesn't go
completely dead. And it is relatively easily reterminated into
proper operation.
I have observed instances of both situations in the field on real
antennas. We weren't able to get the beverage working correctly
in the time alloted. It _may_ have been possible to resurrect it
with matching changes and retermination (both attempted but not
extensively or systematically). We were able to hear stuff on it
but we could not get much directivity out of it. Almost any
other wire we had in the area was a better RX antenna.
This particular beverage was 1.5 wavelengths of field wire 5 ft
above a steel (forget the name of the stuff) temporary runway
which had been abandoned after WWII use for bomber pilot training
at Eglin AFB in northwest Florida. Move the antenna off into the
field over plain dirt (sand really) and it plays beautifully.
>> > The beverage is a traveling wave antenna. The EWE is
>> > not. It is more properly thought of as a terminated loop.
>
This is probably a misstatement on my part. I'm used to seeing
the current phase vector rotate along the conductor(s) of
traveling wave antennas. I did not see this on the EWE models.
But it was probably due to the relatively small size of the EWE
not being large enough to easily observe the shift. On the
classical traveling wave antennas, the conductor lengths are long
enough that the phase rotation is obvious.
Actually, the more I think about it, the more I like to think
about the EWE as a directional coupler loop in a waveguide one
surface of which is the earth.
>What software was used?
I use several different software packages. Each package and even
different versions of the same package has its own set of
strengths and weaknesses. I don't have Brian's stuff (yet) but I
probably should get a copy to see what its strengths are.
The EWE modeling was done on several different versions of Roy
Lewallen's software up to and including EZNEC V2.0. I have
access to unix based NEC4 stuff at work. But it is too much
trouble to feed. The relatively minor difference in the
parameters I am interested in aren't usually important enough for
me to fool with that for playing around with amateur antennas.
>To quote from a response that I received to my past topband
>posting on modeling Beverages from K6STI:
>
>> Bill, NEC-2 cannot accurately model all aspects of antennas with
>> grounded conductors.
Actually, I am less concerned about grounded conductors than
about horizontal ones in close proximity to real ground. But of
course, the above is a true statement. The question is which
aspects are compromised and by how much. Also, over perfect
ground, there is no problem with grounded vertical conductors or
low horizontal ones.
>> This particularly applies to the pattern of traveling-wave
>> antennas. See the manual where I'm sure this issue is
>> discussed.
>>
>> Brian Beezley, K6STI
>
>
>73, Bill K0HA
>
>
I know there is some problem with horizontal wires over real
ground. And the large traveling wave antennas like the beverage
certainly fit this description. But I have seen pretty good
general agreement in at least the gross characteristics of the
pattern between model and real antenna. We just have to accept
the limitations of the various packages, learn how to avoid them
where possible, and understand that modeling results aren't
perfect.
For example, it isn't necessary to connect any wires to ground to
model a beverage. The results won't be perfect, but they will
have avoided the ground connection limitation and will be a
pretty good representation of what can be expected. When it
comes time to field the modeled antenna, it is probably going to
be necessary to tweak some parameters to get it working as
expected. My experience has been that the tweaking process is
grossly shortened when I use a model as the starting point. It
helps me tremendously to have already had some modeling
experience with how the thing responds to changing various
parameters before I'm out in the weather making changes.
73, Eric N7CL
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