If anyone is still interested in antennas, the following excerpt from
another list may be interesting;-
Just a short note on antennae. The co-ax must indeed be of the same
impedance as the output of the radio (in this case 50 ohm). Now not
only
must the impedance of the cable match that of the radio so too must that
of
the antenna match that of the cable (each time you have an impedance
mismatch you get a reflection of the signal which results in a standing
wave
in the cable and that much less of you signal going out into the air).
These simple antennas we are using are quarter wave antennae which if
cut to
that length (1/4 wave length) result in a 50 ohm impedance. As far as
the
ground plane goes it is there to provide the quarter wave antenna a
reflecting surface so it can have an imaginary second element going the
other direction (like a di-pole). The ground plane should be at least
1/4
wave length in radius from the mounting point of the antenna (that is a
1/2
wave lengths diameter).
The bit about not being allowed to cut the co-ax is only true if you
have a
mismatched impedance on the antenna. Along the length of the co-ax the
resistive and reactive components of the impedance change as the wave
travels down the cable (that is the standing wave which gives you the
SWR
you may have heard about). If you have a mismatch then you have this
situation and can reduce the SWR by cutting the co-ax to the exact
length to
give you an impedance transformer between the 50 ohm of the radio and
the ??
ohm of the antenna. But if you stick to the 1/4 wave length and the
ground
plane then your antenna impedance is only resistive and your cable can
be
any length you wish.
For a fast calculation of a quarter wave length use L=234/F where L is
the
length of a 1/4 wave in feet and F is the frequency in MHz. So for
121.5
MHz the 1/4 wave antenna should be 1.9259 feet long.
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Hope this adds light to the subjest !
Martin
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