On Fri, 02 Jan 2004 18:06:06 GMT, Gary Schafer
wrote:

On Fri, 02 Jan 2004 13:00:34 GMT, Shortwave Sportfishing
"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


The method of feeding the antenna has nothing to do with the take off
angle of an antenna.

The stuff you are reading in the handbook about ground effect on
vertical angle are discussing HF antennas where due to the low height
compared to wavelength and the propagation method at hf the ground has
a great effect.

An antenna at VHF is usually several wavelengths above ground and
ground has little effect on take off angle. Given a 1/4 wave ground
plane antenna with radials at 90 degrees, the ground plane of that
antenna will be the controlling factor. It does tilt the angle up a
little from what a 1/2 wave dipole would be.

If you take the ground radials and bend them down and folded over the
coax feed line so that they are vertical you then have a half wave
center fed vertical dipole antenna.
This particular version would have the coax going up through the
center of the folded down ground radials. A tube or pipe could be
substituted for the folded over ground radials. In most cases that is
exactly what is done. A metal tube is used in place of the radials.
This is commonly referred to as a "coaxial sleeve antenna". Just about
all of the fiberglass type VHF antennas have some form of this type of
ground plane in them.

Another type of 1/2 wave vertical antenna that does not need a ground
plane is one that is fed at the end of the 1/2 wave length rather than
at the center as above. The METZ type half wave is one such type. It
uses a coil at the feed end to transform the low impedance of the coax
to the high impedance feed end of the antenna.
At a high impedance feed point the current is much less than it is at
a low impedance feed point (center of 1/2 wave) so not much of a
ground plane is needed. In this case the coax shield acts as the
ground plane for the 1/2 wave antenna. It is not the best ground plane
but again not much is needed in this case. The high current point is
in the center (quarter wave point) of the antenna.

Once you get a few wavelengths above ground additional height does not
much effect the radiation lobes (pattern) of the vhf antenna. But
height does effect the line of sight or in this case radio line of
sight, (which is slightly greater than visual).

What Larry is talking about with the donut shaped vertical pattern is
a result of gain in the antenna. The higher the gain the flatter
(sharper) the vertical pattern of the antenna. Any tilt of the high
gain antenna will raise the pattern above the horizon or tilt it into
the sea. It will not be at the horizon where it does most good.

A lower gain antenna, like the 1/2 wave, has a much fatter pattern.
Like a fat doughnut or a ball. Tilting it one way or the other still
maintains about the same amount of radiation at the horizon. This is
because it normally radiates in a wide vertical pattern. With a wide
vertical pattern a great portion of the signal is wasted as it gets
radiated at high and low angles that are not useful. But the advantage
is that it can be moved (tilted) a long way and still maintain about
the same amount of useful signal.

To get gain, a gain antenna narrows the wide vertical pattern. It robs
some of the power normally radiated at high and low angles and places
more of it at the horizon. Thus the narrower vertical pattern.
The advantage is a stronger signal. The disadvantage is that if it is
tilted very far the signal drops off sharply because the vertical
angle is very narrow.

Excellant Gary - that's what I was trying to get at by taking some
leading questions and seeing where the discussion went. There were
some details that I think were confusing and I just wanted to clear
some of it up without it sounding like I was a newbie big shot trying
to take over the subject.

Nice job.

Regards,

Tom, NM1Q
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