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Antenna Ratings
On Fri, 02 Jan 2004 05:12:36 GMT, (Larry W4CSC) wrote:
On Thu, 01 Jan 2004 18:06:55 GMT, Shortwave Sportfishing
wrote:
How can an antenna work without a ground plane? At the frequencies
we're discussing, the ground effect in FM is about the same as it is
in AM if I understood your discussion points (deleted from this post)
correctly.
Modulation has no effect on antenna physics. The object is to fit at
least 1/2 wavelength onto a conductor. Where it is fed is of no
consequence to radiation, but does effect the feedpoint impedance.
As to the "ground plane".....
~~ snippage ~~
Well, you kind of danced around the answer, but I'd still like to know
how the ground plane effects the radiation angle which logically would
also have an effect on reception of a signal. On page 3-9 of the ARRL
Antenna Handbook (16th addition - sorry, it's the latest I have at
hand at the moment) states:
"The total current in the antenna consists of two components. The
amplitude of the first is determined by the power supplied by the
transmitter and the free-space radiation resistance of the antenna.
The second component is induced in the antenna by the wave reflected
by the ground. This second component, while considerably smaller than
the first at most usefull antenna heights, is by no means
insignificant."
So it would seem that the "ground plane/wave" is not an umimportant
consideration when considering antennas.
Further on that same page, is the following:
"Changing the height of the antenna above the ground will change the
current flow assuming that the power to the antenna is constant."
Again, it would appear that the "ground plane/wave" is not
insignificant.
Now, as I understand it, at VHF frequencies, the methodology of
providing energy to the antenna (loading/feed) is not as important to
the generation/reception of the signal as is height. In fact, if I
read the pattern charts correctly, the height of the antenna has more
to do with the lobe pattern (the donut you were discussing) than the
method of feeding the antenna.
Yes/No?
Later,
Tom
S. Woodstock, CT
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"I object to fishing tournaments less for
what they do to fish than what they do to
fishermen." Ted Williams - 1964
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