On Wed, 22 Dec 2004 09:22:09 +0100, Enzo wrote:
>Now I wonder why was I able to match the antenna on 160m only
>with the two elevated radials?
Probably because resistance to ground of such a limited ground
system is fairly high, which adds to the low radiation resistance
of your antenna to get you closer to 50 ohms. In other words, you
may have gotten closer to a "match" by introducing more loss, but
the loss sucks up power.
>From an equivalent circuit point of view, there are three
resistances in series -- the Radiation Resistance of the antenna,
which accounts for the power that is actually radiated by the
antenna, the resistance of the wire, and the resistance of the
ground system. Thus, you can think of it as simply a voltage (and
power) division between the radiation resistance of the antenna
(good), the wire resistance of the antenna (bad), and the loss
resistance of the ground system (bad).
In general, you want the radiation resistance to be large compared
to the other two (or the other two to be small in comparison to
the radiation resistance). With a short antenna, you make the
radiation resistance larger by making the antenna longer. You make
the ground resistance smaller by using more radials and making
them longer. You make the wire resistance smaller by using fatter
wire.
Hope this helps. To understand this better, study one or more of
the antenna books.
Jim Brown K9YC
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