I have received a few questions on the elevation patterns of the
self-contained vertical wire antenna family. I add some notes on the
subject to my web page, along with some elevation plots to clarify the
situation.
For anyone interested, here is the text--without pictures, of course.
How High Should I Place an SCV?
This question is reasonable, because the behavior of the elevation pattern
of SCVs is far less familiar than patterns for dipoles and Yagis. In fact,
it may come as a surprise.
The elevation angle of an SCV is largely controlled by the peak height of
the antenna. For the lowest lobe, this ranges from about 20 degrees for a
maximum height of 50' down to about 13 degrees for a maximum height of 90'
up. Of course, there is serious energy below the elevation angle of
maximum radiation.
However, some folks are surprised to learn that as they elevate one of the
SCVs, a second lobe appears and comes to dominate the pattern. Consider a
40-meter right-angle delta with vertical dimensions of 20 to 50', 40 to
70', and 60 to 80' upward. 50' is under 1/2 wl (closer to 3/8 wl). Its
pattern is the purest, with only a single lobe and a field maximum at 20
degrees. The model with a maximum height of 70' (just over 1/2 wl) shows
the appearance of a secondary lobe at a much higher elevation angle. The
lower lobe, although at 15 degrees, is not stronger than that of the model
20' lower, and radiation at 10 degrees and below is insignificantly
different. The model at the highest level (90' or about 5/8 wl) has a
lower lobe at about 13 degrees, but it is now lesser in strength than the
main lobes of the lower models. The maximum radiation appears at nearly 50
degrees, and the two lobes blend together for single wide elevation pattern
between about 10 degrees and about 60 degrees.
This same pattern development is inherent to all versions of the SCV
antenna family--a genetic trait. One can see this in a half-square modeled
at vertical dimensions of 11 to 50', 31 to 70', and 41 to 90' up. A
secondary lobe at a high angle appears at the middle height, while that
secondary lobe becomes the primary lobe at the upper height.
What does all this mean? The answer depends on two key factors. First, as
the upper-angle lobe grows, so too does higher angle QRM/QRN to reduce the
signal-to-noise ratio of low-angle signals. since gain at the lowest
levels does not change significantly, the added noise will make DX harder
to hear.
Second is the type of operating an individual wishes to do. For DXers, the
heights above the lowest level may do more harm than good. However, for
general operation in both domestic and international competition, the
highest model may provide the broadest signal coverage, although the
middle-level models may equalize the stronger closer signals with the more
distant weaker signals. Remember that these patterns affect the strength
of both transmitted and received signals, and both should enter into one's
deliberations.
These patterns hold good for the lower HF bands when model heights are
scaled for the frequency of operation.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://funnelweb.utcc.utk.edu/~cebik/radio.html
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Sponsored by: Akorn Access, Inc. & N4VJ / K4AAA
|