In article ,
James Hebert wrote:

In article ,
Bruce in Alaska wrote:

Morad 156HD's are actually an Endfeed 5/8 Wave with a matching element
built inside the aluminum 1/4 Wave ground sleeve. Mechanically, very
rugged, and electrically extreamly rugged. They are the most common
VHF Antenna used in the North Pacific, and Bearing Sea.

Thanks for mentioning the Morad antenna. I had never heard of it
before. It looks like an excellent antenna. Apparently it is
seldom sold in the recreational boat market.

It is found on non-commercial vessels more, on the Left Coast, and
in Alaska. They also make a really nice little Sailboat Antenna,
but the model number escapes me at the moment. Senior Moment...
Maybe Old Chief Lynn, or one of the other west coasty guys can chime
in...

Bruce in alaska
--
add a 2 before @

In article ,
"Meindert Sprang" wrote:

"Bruce in Alaska" wrote in message
...
In article ,
"Meindert Sprang" wrote:

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

Well, that is not actually true in most cases.

I see. My Shakespear is a 1/2 wave whip with a coil in the base.

If you actually take a
Shakespear Fiberglass VHF Antenna apart, you will find that they are
1/4 wave driven elements, with 1/4 wave of Ground Sleeve shoved up
the inside of a hollow fiberglass fishing pole and epoxed in place.

But doesn't this ground sleeve produce the "other end" of the dipole?

Meindert

Generally vertical antennas which are shorter than a half-wave are
usually worked against a ground plane, however that is not to say that
antennas which are longer than that could not also benefit from being
operated above a conducting ground plane. Any antenna, vertical or
horizontal, which is operated above a ground plane will produce more
gain due to the mechanism of an "image" antenna being developed in the
ground plane.

The quarter-wave series-fed vertical worked against a ground plane is a
common antenna in part because:

--it has a good radiation resistance (about 37 ohms) for matching to
50-ohm coaxial transmission lines and makes for a very simple, direct,
series fed antenna.

--it has favorable radiation characteristics

The typical half-wave marine antenna is a shunt fed antenna. Its
impedance at the base is quite high. There must be some matching network
to transform the antenna impedance down from a quite high value,
probably over 1,000 ohms, to match the 50-ohm transmission line. This is
often done with a tapped coil arrangement which is shunted across the
antenna to "ground" which in this case is the shield of the feed line.
The coil is not a "base loading coil" in the sense that one uses that
term with short vertical radiators (like a 27-MHz CB antenna which is
only a few feet long) where the coil is in series with the feed, but
rather it is an impedance matching coil which is shunted across the feed.

There are also arrangements where vertical antennas are not fed at their
base but rather at some elevated point. This technique is used in some
longer marine antennas where the transmission line enters the antenna
inside or coaxially with the bottom radiating element. The feed is often
made one quarter-wave from the base of the antenna, as this will provide
a good impedance point. Also, there may be another quarter wave of what
appears to be an antenna but is really a decoupling stub to suppress
flow of antenna currents on the transmission line.

I tend to favor a series-fed antenna as there can be little doubt about
where the transmitter power goes--it goes right into the antenna
radiating elemet. Shunt fed antennas have the possibility that some of
the power remains in the shunt element. If the Q of the shunt element is
not high, there can be losses. This accounts for the rather large size
of some of the base matching units on half-wave antennas, even though
they are used with modest power transmitters.

de K8SS