"Meindert Sprang" wrote in message
...
"RB" wrote in message
.. .
Most of our antennas have some kind of counterpoise---a ground plane,
radials, etc. This is for the rf in the radiating part to "push" off of.

A question is how the common fiberglass marine VHF antennas work. In
many
installations, there is no visible counterpoise.

These antennas are half-wave antennas and they don't need a counterpoise.
Only a quarter-wave antenna needs one, to account for the other missing
quarterwave part, so to speak.

Meindert


That's correct.

Another way of saying it is that the "counterpoise" is built in to the
marine antenna.

The basic "half wave" antenna is called a dipole and its shown at this web
link:

http://www.flashwebhost.com/circuit/...ansmitters.php

This link calls it a "horizontal dipole" but all you need is to rotate it to
vertical to make it a perfectly good vertical dipole. Some variation of
this antenna is what is used for marine VHF. Notice that one "leg" of the
dipole is connected to the center wire of the coax and the other leg is
connected to the shield.

Notice farther down the page that they show a picture of a "vertical
antenna". Its more accurately called a "vertical ground plane" antenna.
They have simply replaced half of the "dipole" that was originally connected
to the shield of the coax with four horizontal stubs connected to the shield
of the coax.. The stubs work electrically almost exactly like the missing
half of the dipole. If you have a metal vehicle like a car then you can let
the conductive body of the car replace the four horizontal stubs (also
called the ground plane). This is why you see so many car antennas with
only the single quarter wave stub sticking up. They are using the metal
body of the car as the lower half of the antenna.

http://www.packetradio.com/ant.htm#2mdipole

Here is a link to pictures of a more exotic antenna called the J-pole which
uses a matching stub to help transfer the radio energy from the coax into
the resonant length of copper pipe. Farther down the page is a "J-pole"
cleverly cut from 5 feet of cheap TV twin lead.