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how does marine vhf antenna work?

Discussion in 'Boat Electronics' started by RB, Feb 2, 2006.

  1. RB

    RB Guest

    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.
     
  2. Larry

    Larry Guest

    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.
     
  3. 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
     
  4. Gary Schafer

    Gary Schafer Guest

    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
     
  5. Larry

    Larry Guest

    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.
     
  6. James Hebert

    James Hebert Guest

    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.
     
  7. 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
     
  8. James Hebert

    James Hebert Guest

    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
     
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