On Fri, 21 Mar 1997, Doug Brandon wrote:
>
> I've been using a roof mounted HF6V vertical (at about 16') for the
> past 5 years with reasonable results. I'm currently looking into putting
> up a tower, but in the meantime I'm curious if a roof mounted beam
> (say a Force C3/C4 or Cushcraft A4) at around 20' - 25' would do any
> better than the vertical.
Doug,
As a rule of thumb, you can use the following numbers: a horizontal
antenna at 1/2 wl height has an elevation angle of maximum radiation
(take-off angle) of 25-26 degrees with about a +/-8 - 10 degrees
vertical beamwidth (angular distance to half power points)
At 1 wl up, the takeoff angle is about 13-14 degrees with a narrower
vertical beamwidth.
The height in fractions of a wavelength will vary from band to band for
the beam, but the 25' you propose is in all cases under 1 wl, even at 10
meters. On 20, it is below 1/2 wl. However, you will have some
directivity in both reception and transmission, and--depending on what you
are presently experiencing--that may be worth something.
Hence, you may (or may not) be disappointed in the results you get,
relative to the investment involved--unless the elevated vertical is
troubled by unspecified factors (tree clutter and the like). However, if
you view the 25' installation as a first step toward a taller installation
using the same antenna, then it may make economic sense to get started
this way.
Notice that the analysis very quickly gets out of pure performance data
into other factors that only you can know.
-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
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