how does marine vhf antenna work?

[RB]

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.

So, how do they manage to radiate? Maybe the coax outer shield forms a
counterpoise. Don't know. Just curious.

 

[Larry]

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

The big antennas are phased arrays of cheap little wires encased in
fiberglass to protect the wires and make them look "bigger". There are a
series of dipole antennas inside the long ones with phasing networks to
make them work together to produce a flattened pattern from the halfwave
dipole's radiation donut pattern. If you step on a donut, it gets wider
and that's what the phased array does to the radiation donut of the
halfwave.

Unfortunately, the squished donut is always perpendicular to the plane of
the whip so when the boat is heeled over or rocking around in the waves,
too much gain from too many phased dipoles is a bad thing. The flat donut
on one side of the boat is pointing into Davy Jones' Locker and the other
side of the boat it's pointing to space, not the target. Halfwave
antennas, like the Metz, with fat donuts and wider radiation patterns are
better on sailboats and small boats for that reason.

 

[Bruce in Alaska]

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. 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.
The really High Gain ones, are Colinear Arrays of 1/4 Wave Segments
over a 1/4 wave Ground Sleeve, again shoved up a hollow fishing pole
and expoxed in place. Cheap to build and no tuning elements to mess
with like your endfeed 1/2 wave whips.
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.

Bruce in alaska

 

[Gary Schafer]

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


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

Meindert

Yes that's true the sleeve is the other end of the dipole. The
difference is with the sleeve the antenna is essentially center fed
just like a dipole or a ground plane. In the case of the sleeve if you
picture a ground plane antenna with radials but instead of the radials
sticking out you fold them down over the coax in the form of a sleeve.

With an end fed ½ wave like some antennas are (metz is one) the coil
is at the bottom but it still requires some counterpoise. In that case
the outer shield of the coax serves as such.
Being a high impedance feed the ground currents are very low so it
doesn't take much of a ground for them to work rather well.

The collinear gain type antennas, as Bruce says, have stacked elements
starting at the bottom with the sleeve antenna. Often the stacked
elements are nothing more than quarter wave lengths of coax with the
center conductor and shield swapped on each section. Very cheap to
make.

Regards
Gary

 

[Larry]

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

Yes. It performs the same function and gives the coax cable that must feed
the center of the dipole a shielded way to keep from being part of the
antenna and becoming a radiator itself, by putting it inside the sleeve.
It's still a dipole.

Impedance of the sleeve dipole is around 65-75 ohms, but that's "close
enough for government work", as we used to say in the Naval Shipyard. To
get 50 ohms of match, the sleeve needs to be a skirt out at around 45
degrees from the horizontal, like those ground plane base antennas with the
4 or so radials at 45 degrees, halfway between horizontal and vertical.
Flat out, like mounting a 1/4 wave whip against the metal top of a cabin or
car roof the impedance is near 30 ohms.

 

[James Hebert]

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.

 

[Bruce in Alaska]

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

 

[James Hebert]

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


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