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Antenna Ratings

Discussion in 'Boat Electronics' started by sk, Dec 29, 2003.

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  1. sk

    sk Guest

    Know anywhere I can find ratings to help decide which antenna?? I have a 23
    foot center console. Generally range 10-15 miles offshore and plan to mount
    the antenna on my
     
  2. Harry Krause

    Harry Krause Guest

    I have an 8' "Digital" brand antenna hooked up to my VHF on my 25'
    Parker pilothouse. Works fine in the rock and roll of the Atlantic Ocean.

    Visit the boat at:

    http://photos.yahoo.com/hakrause
     
  3. Jack Burton

    Jack Burton Guest

    So, how does the horizon work into the gain equation?
    Interesting - all the USCG small boats around these parts use the
    Shakespeare 396-1 which is a center-fed 1/2 wave.

    I use one and it's been great antenna.

    Later,

    Tom
    S. Woodstock, CT
     
  4. ~~ major snippage ~~
    Yes, that is true enough on a sail boat because you are talking about
    height in that case which can conquer a multitude of conditions nine
    foot antenna or three foot base loaded (end fed) stainless steel.

    I'm speaking about a small boat as in, oh say, a 23 foot CC Ranger
    with a T-top or a 32 foot Contender with a T-top. I would think that
    even in your scenario, the angle of the signal to the horizon when it
    leaves the antenna would work the same base load or center fed and
    cause signal capture problems at the receiving end regardless.

    By the way, neither of those boats would ever be in a situation with
    30 foot rollers - if they show up, I'm long gone. :>)
    How can an antenna work without a ground plane? At the frequencies
    we're discussing, the ground effect in FM is about the same as it is
    in AM if I understood your discussion points (deleted from this post)
    correctly.
    Wellm to each their own I guess. :>)

    Later,

    Tom
    S. Woodstock, CT
     
  5. ~~ snippage ~~

    Well, you kind of danced around the answer, but I'd still like to know
    how the ground plane effects the radiation angle which logically would
    also have an effect on reception of a signal. On page 3-9 of the ARRL
    Antenna Handbook (16th addition - sorry, it's the latest I have at
    hand at the moment) states:

    "The total current in the antenna consists of two components. The
    amplitude of the first is determined by the power supplied by the
    transmitter and the free-space radiation resistance of the antenna.
    The second component is induced in the antenna by the wave reflected
    by the ground. This second component, while considerably smaller than
    the first at most usefull antenna heights, is by no means
    insignificant."

    So it would seem that the "ground plane/wave" is not an umimportant
    consideration when considering antennas.

    Further on that same page, is the following:

    "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
     
  6. Gary Schafer

    Gary Schafer Guest


    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.

    Regards
    Gary
     
  7. Larry, nobody's suggesting 9 db antennas. They are for shore stations
    and real ships. 6 db, on the other hand, seems to work fine on my 17'
    boat. I talk to dive buddies who are theoretically out of range, and
    they come in fine, with no fade in/out as the boat rocks.

    --
    Chuck Tribolet

    http://www.almaden.ibm.com/cs/people/triblet

    Silicon Valley: STILL the best day job in the world.
     
  8. snipped

    Regards
    Gary[/QUOTE]

    As always a very excelent explanation, Gary.

    Bruce in alaska
     
  9. 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
     
  10. Gary Schafer

    Gary Schafer Guest

    All antennas are two terminal devices.
    There is no such thing as a single point feed antenna.

    Changing a horizontal antenna to a vertical antenna at the same height
    does not improve the radiation angle.
    If it did everyone would have their horizontal antennas in the
    vertical position.

    AM radio stations depend on ground wave signals not sky wave.

    A horizontal antenna theoretically does not have a ground wave so
    verticals are used for AM stations as they produce a ground wave
    signal.

    For sky wave signals height is important in order to produce better
    lower angle of radiation. Lower angle radiation provides longer hops.

    Adjusting the height to adjust the impedance of the antenna is not to
    be worried about. That's what matching systems are for.
    With multielement antennas (beams) the feed point impedance can be
    very low even if the antenna is high in the air. Some other means of
    matching the antenna to the line is required.

    Regards
    Gary
     
  11. Gary Schafer

    Gary Schafer Guest


    TV and FM stations do not depend on ground wave propagation. It is
    line of sight. Nothing to do with polarization or ground wave.

    FM stations have had dual polarization (both horizontal and vertical)
    for many years. Only because cross polarization with the receiving
    antenna greatly attenuates the signal.

    Ground wave propagation as in AM stations is not a sky wave. It
    follows the curvature of the earth staying close to the ground (ground
    wave).

    A horizontal antenna does not produce any significant ground wave.
    (wave that stays close to the earth) The ground wave from a horizontal
    antenna will fall off in just a few miles.

    Yup that's what I said. "For sky wave signals height is important in
    order to produce better lower angle of radiation. Lower angle
    radiation provides longer hops."

    The propagation that you experience on 80 and 40 meters with the guys
    around the state is the result of high angle radiation and reflections
    as you state.

    If you want higher angle, shorter hops, keep the antenna low.


    Tuned antenna? I assume you are referring to a "resonant antenna"?

    A non resonant antenna radiates just as well as a resonant antenna.
    Only difference is the impedance presented to the feed line. But even
    a resonant antenna may not provide 50 ohms to your coax. Depends on
    how high it is. :>)

    Regards
    Gary
     
  12. Gary, maybe you can help me out with something here, because either
    I've forgotten or I was taught incorrectly.

    Over short distances (Up to five to eight miles), the ground wave
    component of any VHF signal is actually the primary method by which
    VHF signals are received and not by Line Of Sight.

    Not true?

    Later,

    Tom
    S. Woodstock, CT
     
  13. Gary Schafer

    Gary Schafer Guest


    I don't think so. There is some bending, refraction of the signal that
    makes the VHF radio horizon slightly longer than line of sight but it
    is due to the atmosphere and not ground wave propagation.

    If you have a VHF station on a high mountain peak or a very high tower
    there could be no ground effect at all.

    Regards
    Gary
     
  14. You are making me unpack my reference books now. :>)

    Later,

    Tom
    S. Woodstock, CT
     
  15. Gary Schafer

    Gary Schafer Guest

    Some clarification of "ground wave" is in order here. The term ground
    wave is sometimes lumped into several different modes that are not
    "sky wave" modes.

    SURFACE WAVES:

    The ground wave that I am referring to here is sometimes called
    "surface wave". It is in the ground wave family.
    A surface wave travels close to the ground actually touching the
    ground. AM radio stations depend on this method of propagation. At AM
    frequencies it is good out to about 100 miles. The signal strength is
    quite weak at that distance. But in the shorter distances it provides
    a very reliable and predictable constant coverage. Unlike sky wave
    signals that come and go between night and day.

    At higher frequencies that distance is much shorter as the signal gets
    highly attenuated. So "surface waves" are not very useful at
    frequencies much above the broadcast band for any significant
    distance.

    The old 2 MHZ marine band used to depend on "surface wave"
    communication. With good conditions range can be better than VHF but
    usually quite noisy on those frequencies. Also a more complicated
    antenna system was needed and higher power than what is used on VHF.

    DIRECT WAVES:

    Line of sight propagation is also a form of ground wave propagation in
    that it does not become a sky wave and reflect off the upper
    atmosphere. It may follow the earths curvature and may or may not
    touch the earth. This type of propagation is commonly called "direct
    wave".

    As you asked about "the ground wave component of VHF signals being the
    primary method by which VHF signals are received and not by line of
    sight".

    The primary method is by line of sight which is technically a form of
    ground wave as explained above. But usually when someone mentions
    "ground wave" they are usually referring to "surface wave".
    Line of sight is usually used when referring to "direct wave" VHF
    communications.


    In line of sight communications there are reflections from the earth
    or other objects that can add or subtract to the signal that is being
    received directly. The reflected signal gets shifted 180 degrees as it
    is reflected. At VHF the reflected wave (because it is rather short
    compared to the distances involved) can shift rapidly in phase. This
    is what causes flutter on the signal at times.

    With horizontal polarization at low HF the wave length is long
    compared to the distances involved in the direct and ground reflected
    waves. They arrive almost always 180 degrees out of phase. That highly
    attenuates the direct wave signal.

    NO SURFACE WAVE FROM HORIZONTAL ANTENNAS: from ARRL antenna handbook.

    With a vertical antenna the electric field is perpendicular to the
    ground. It is a law of electromagnetics that electric lines must touch
    a conductor (the earth in this case) perpendicular or else they would
    have to generate infinite currents in the conductor. The earth is a
    rather good conductor below about 10 mhz so waves at these frequencies
    are mainly vertical when traveling close to the ground. If they were
    horizontal they would try to induce large currents and be absorbed by
    the earth. They get shorted out.

    This is why a horizontal antenna has little ground wave or surface
    wave. It dies out very rapidly. In addition horizontally polarized
    direct waves at low HF get canceled by ground reflections. (see above)

    Thus the reason for vertical AM broadcast antennas.

    At VHF antenna heights are usually several wavelengths above ground
    and polarization does not matter. Plus the earth is not a good
    conductor as to short out the horizontal waves as it does at low HF.

    The reason for vertical polarization being mostly used at VHF is it is
    much easier to obtain omni coverage with simpler antennas. Especially
    portable and mobile antennas.

    Regards
    Gary
     
  16. ~~ snippage ~~

    Thank you. :>)

    Later,

    Tom
    S. Woodstock, CT
     
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