Don wrote:
The "cone of silence" issue brings up another topic. I first heard of the
term in W1WCR's Beverage antenna book. I recall that ON4UN makes a few
references to W1WCR's publication in his own book. However at least three
things in W1WCR's book cast doubt on the entire work.
(1) He claims that for 1.8 mHz and above, the ground systems at each end of
the antenna should be bonded together with an insulated wire lying on the
ground directly under the antenna wire for its entire length. This seems
contrary to anything else I have ever read about Beverage theory.
**** From the point of view of induced signal level, this will (depending on
local soil) either do nothing or cause reduction of induced wavefront tilt
and therefore the signal level for low arrival angle signals. From the point
of view of terminating the antenna to make it unidirectional, it wouldn't be
necessary. I can't imagine why this is recommended so if you run across any
further explanation, I'd love to hear it.
(2) He recommends sloping each end of the antenna to reduce pickup from the
vertical drop. Don't you have exactly the same vertical drop, regardless of
whether it occurs at one point, or over a length of 50 or 60 feet? To me,
this is somewhat like saying it takes less energy to move an object up a
gradual incline than to use a vertical lift up the same elevation
(neglecting friction, the energy expenditure is exactly the same).
**** I don't see the benefit. For my Beverages, I usually run the ends
straight down. Mine were long (600 to 1900 feet), so the pickup from the
vertical section was many dB down from the main antenna's signal. With a
shorter, higher Beverage I do think this is an issue. At MW frequencies
(where I listen most), the relative efficiency of the Beverage was about a
twentieth that of an equal length vertical. Your mileage will vary depending
on your soil conductivity and dielectric constant. Were I working with high
(10 feet or more) , short (several hundred feet) Beverages over good ground
I would try coax as the downlead with the shield grounded. But in any case,
I see no need for sloped termination
(3) All the rf transformers he shows are autotransformers, grounded at one
point, with a tap for the coax feedline. With any antenna of this kind,
including the K6STI loop, the flag, ewe, etc., the recommendation is always
to use a transformer with separate primary and secondary and
electrostatically shielded if possible, with the bottom end of transformer
secondary strapped to the coax shield but NOT grounded to the same point as
the primary. The purpose of this configuration is to avoid common mode
noise pickup. These three questionable items make everything else in the
book suspect, even though at first glance most of it appears to be based on
sound principles.
**** The only advantage that an autotransformer has is that it is - to
some - easier to construct. In the field, I'd say the advantage of isolated
grounds will either be none in an electrically quiet location to significant
in terribly noisy locations. As you can't lose with isolated grounds, I use
them these days.
**** I'm also not a big fan of the cone of silence concept. The wave
velocity can easily vary 10 or 15% depending on local soil type, so it's
hard to cut the antenna to the COS length. The idea also largely ignores two
big advantages of longer Beverages: narrower beamwidth and a sort of spatial
diversity. The narrower beamwidth of the longer antenna may or may not be
the most important parameter, but if so the COS concept goes out the window.
Also, my experience is that there is a "smoothing" factor with reception
with Beverages that I have always attributed to a space diversity. I haven't
heard of any proof and don't have any myself so feel free to ignore that
part. And there's a bigger factor I don't pay attention to the COS thing: I
always had phased Beverages and therefore had the capability to put a null
where I wanted it. I preferred a long, long antenna and then created a null
where needed on a case by case basis. Lastly, the COS idea is by definition
narrowband, and I used my Beverages from 120 meters down to 600 meters.
Chuck
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