On Wed, 27 Sep 2006 18:13:57 -0500, Kelly Taylor wrote:
>Sevick showed in the 1970s that if proper attention is paid to radials,
>loading and feedpoint impedance, shortened verticals can work very closely
>in performance to their full-size counterparts.
Yes, that's been reprinted a number of times. BUT: "proper attention to
radials" takes on a somewhat different meaning on 160 as compared to 40
meters. :)It's all in the ratio of the radiation resistance to the ground
loss resistance, and how many radials it takes to achieve a ratio that isn't
burning much of the power in the ground system. The shorter the antenna, the
lower the radiation resistance (the radiated power), so the shorter the
antenna, the more (and longer) radials are needed to achieve the same
efficiency. There's an excellent piece on this in the current (20th) ARRL
Antenna Book (I think by Rudy Severns).
As an example, to achieve 2 dB of loss in average ground with a quarter wave
vertical (on the order of 30 ohms Rr), he shows that you need 24 radials 1/8
wave long. If the antenna is half that length, Rr is likely more like 15
ohms, so you need a lot more (and longer) radials to achieve that 2 dB
number. Or, with the same number of radials, you'd have something like 3 dB
more loss with the shorter antenna.
For me, it all comes back to the same point -- do the best you can, and put
it on the air.
73,
Jim K9YC
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