TopBand: Re: Fact and Fantasy about Beverage Antennas
Earl W Cunningham
k6se@juno.com
Wed, 30 Jul 1997 16:23:35 EDT
On Wed, 30 Jul 1997 12:09:20 -0400 (EDT) K3BU@aol.com writes:
> (snip)
>My operation in 1986 from portable site in VE3 during 160m contest:
>I strang 1600 ft Beverages on gently sloping hill. First night my inv
Vee and >Slopers were outhearing the Beverages. Second night I cut the
suckers in half >and voila! 160m sounded like 20 with Eus booming in,
killing the other >antennas.
> (snip)
============
Fantasy: "The longer the Beverage wire, the better".
Fact: There is an optimum length for the Beverage antenna. This is
because the wire (along with the ground) is in effect a
transmission line and exhibits that phenomenon called
"velocity factor". Consider this: In a unidirectional Beverage,
the sky wave first strikes the antenna at its far end and is
propagated down the wire toward the receiver. Additional sky wave
stikes the wire at increments further down the wire and is also
propagated though the wire in its way to the receiver. However,
the sky wave travels at the speed of light (velocity factor =
1) and the signal propagated down the wire travels at somewhat
less than the speed of light (velocity factor = approximately .95
to .97). This difference in the propagation velocity of the two
signals causes an ever-increasing difference in the phase of the two
signals. When the phase difference is less than 90 degrees, the sky
wave signal cooperates in increasing the amplitude of the signal
in the wire, however, when the phase difference is 90 to 180
degrees, the signals subtract from each other resulting in less
amplitude than when the wire is at that optimum length where the
phase difference at the receive end of the wire is 90 degrees. It
has been calculated (by others, not by me), that this "optimium"
Beverage length is in the vicinity of 2 wavelengths on 1.8 mHz, or
about 1050 to 1100 feet.
Apparently this phase phenomenon (sky wave vs. signal in the wire) in
Beverages is not taken into consideration in today's modeling software,
but it helps explain Yuri's experience with his 1600-foot vs. his
800-foot Beverage.
Fantasy: "The Beverage antenna is horizontally polarized - that's why
it's a quiet receiving antenna."
Fact: The Beverage wire responds mostly to the vertically polarized
component of the received signal. I submit that the reason for
its quietness is the fact that it's such a highly directional
receiving antenna - you receive noise only from the desired
direction, and not the total from all directions.
Fantasy?: "The termination resistor for a Beverage antenna should be
about 600 to 800 ohms"
Fact: The first Beverage I constructed was an 1100-footer, made of #17
gauge galvanized steel electric fence wire. This wire was 7'
high, with the last 30' at the ends sloping gradually down to
ground level (to avoid any omni- directional pick-up).
Someone (I've long-since forgotten who) once told me that to find
the correct termination value, I should apply a small amount of
RF to the antenna and walk down the wire with a field strength meter.
The best value of termination resistance would be when the
sine-wave fluctuations observed while I "walked" the
wire were least, i.e., "flat".
I first "walked" the wire with the far end open. This resulted
in the expected maximum voltage observed at the far
end, decreasing to zero as I reached a point 1/2-wave from the
end, then increasing for the next half wave, etc. Next I
grounded the far end (4' ground rod and one 1/4-wave
radial). "Walking the wire revealed the same sine-wave type voltages
along the wire, but 180 degrees opposed to the first results (as
expected). Then I terminated the wire with 680-ohms. Results of
"walking" the wire showed the same sine wave characteristic, but
with much smaller swings in amplitude. There was a
maximum at the far end of the wire, which told me the resistance
value was too high. After a few tries with various values of
resistance, I concluded the best value was about 300 ohms. This value of
termination resulted in a "flat" standing wave along the full
length of the wire. (It actually gradually decreased going
from the feedline end to the terminated end, which was
obviously due to loss in the wire).
My subsequent four Beverages, ranging from 550' to 1100' long, were all
terminated with 300-ohm non-inductive resistors. All were outstanding
performers and very definitely better than my transmitting antenna (2-el
phased shunt-fed towers) for S/N ratio on receive. 60 dB of difference
was noted on signals when switching opposite-direction Beverages.
Front-to-side appeared to be just as good.
One day I obtained a spool of #16 enameled copper wire and decided to
replace my 1100-foot Beverage with copper to reduce wire losses. The
results were very disappointing. The copper wire, installed in the exact
same place as the old galvanized steel wire, was noisier and had poorer
F/B ratio. I quickly took it down a reinstalled the steel wire. After
giving some thought as to why the steel wire performed better as a
Beverage than the copper wire, I concluded that greater wire loss in this
case was an asset. Here's why: A Beverage wire is actually a
bi-directional antenna. Most of the signal propagated down the wire from
the unwanted direction is absorbed in the termination resistor, but not
all of it. Some of the opposite-direction signal is reflected from the
far end back down the wire to the receiver. I concluded that the lossier
steel wire was an asset in this case because the amplitude of signals and
noise from the unwanted direction was more greatly attenuated before it
reached the far end of the wire, therefore less unwanted noise and signal
was reflected down the wire toward the receiver.
Sadly, I no longer have any of those Beverages due to population growth
in this area. My 330' X 330' lot doesn't quite permit long Beverages. I
now rely on my 2-el phase verticals for receive, which is still better
than 1-el, hi.
Incidentally, I used that same #17 galvanized electric fence wire for my
radial system, about 5 miles of the stuff in a total of 192 radials (96
under each tower). After all, it was cheap ($20/mile), but now, after 21
years, much of it has rusted, even here in the desert. Does anyone know
a source of cheap copper wire??
Anyway, I thought my experiences with Beverages might be of use to
others.
73, good hunting,
de Earl, K6SE
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