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VHF Marine Radio Communication

Discussion in 'Boat Electronics' started by James Hebert, Jan 8, 2005.

  1. James Hebert

    James Hebert Guest

  2. Bob

    Bob Guest

    off the top of my head, looking at the article, i think there's an
    error. he says EACH antenna needs to be 12.4 feet above the surface,
    but i think the SUM of the antenna heights needs to be this...IOW each
    antenna needs to be 6.2 feet high.
     
  3. James Hebert

    James Hebert Guest

    If two vessels are ten miles apart, they will each
    need antennas 12.4 feet high in order for their
    radio horizons to be in view (line-of-sight) of
    each other.

    The case presented shows how much margin there
    is in a typical circuit. There are many poor
    radio installations aboard recreational vessels
    which can barely talk to the marina office
    from its dock own gas dock, but this does
    not constitute a negation of laws of physics.
     
  4. Bob

    Bob Guest

    why is that? the equation sez 12.4 feet for antenna height. does that
    mean 1 antenna could be on the ground and the other at 12.4 feet?
     
  5. Me

    Me Guest

    this is where the Practical and empirical evidence shows that
    the math isn't showing what really is hapopeneing.

    I have two 1 watt Vhf handhelds, with rubber antennas.
    I can talk 16 miles over water with these two radios.
    Both myself and my wife are less than 6.5 ft tall.
    We are both standing at sealevel. (water lapping at our feet)
    Now how does you MATH explain this empirical DATA?

    Do my handhelds have receive sensitivity lower than atmospheric
    noise? Maybe the Laws of Physics cease to apply north of 58 degrees?
    Since this is a perfectly viable Path 24/7 and we have used it
    daily for the last 15 years, what is your explanation?

    Me who actually does know the answer.........
     
  6. Eric

    Eric Guest

    Sealevel is not a word. If you mean sea level, that does not equate to
    "water lapping at our feet". If you are claiming a 16 mile range, direct,
    when you are both at 6.5 feet above sea level, I say your full of crap. If
    this is a riddle, then I say your both at a seperate body of water with a
    valley between, or your communicating via a repeater.

    Eric
     
  7. Gary Schafer

    Gary Schafer Guest

    It means that the horizon is halfway between. If one antenna was at
    sea level and the other at 12.4 feet you would only talk half the
    distance.

    Regards
    Gary
     
  8. Me

    Me Guest

    Yes, Meindert (the engineer) come up with the answer. Vhf does indeed
    extend beyond Line of Sight, in all cases, and your math does not take
    this into account. The straight that I live on, is 16 Statute Miles
    wide, and my wife stands on one side, at the end of a road, at a boat
    ramp, and I stand on the other side, in front of my cabin, on the beach.
    Both our feet, are being lapped at by the water, at exactly Sea Level,
    at that moment, and YES, we do communicate with one watt, Motorola
    handheld radios with rubber antennas, and have for the last 15 years.

    You made a BAD Assumption, when you stated that VHF (156Mhz) RF
    Propigation was only Line of Sight. This is the downfall of your whole
    construction. When you have as many years as I have in RF
    Communications, you should, by then, have a better understanding of the
    basic Laws of Physics, that underpin the technology that your espousing.

    Me who knows the difference between Theory and Practice....
     
  9. Gary Schafer

    Gary Schafer Guest


    The old saying by the Motorola salesman selling low band radios to the
    farmers was "100 watts goes 100 miles".
    It would do that on many days but you dare not design a system around
    those figures.

    Regards
    Gary
     
  10. Doug Dotson

    Doug Dotson Guest

    Salesman's name was P.T. Barnum as I recall :)

     
  11. Gary Schafer

    Gary Schafer Guest


    By the way, the difference between radio line of sight and optical
    line of sight at 5 miles is only about .7 miles at that distance.

    The factor to multiply the height of the antenna by is about 1.33 for
    radio over optical. The difference is not great.

    There are phenomenon's such as ducting that take place on a regular
    basis over some radio paths. Usually not over 24 hours though.

    I can assure you that I can show you many places where hand held to
    hand held will not produce anywhere near 16 miles of range even over
    water but on rare occasions.

    Regards
    Gary
     
  12. James Hebert

    James Hebert Guest

  13. Jack Painter

    Jack Painter Guest

    James, don't confuse 1w-rubber-duck man (anonymous "me" contributor) with
    logic. Heck, after reading his story, I may pull down my antenna and replace
    it with a rubber-duck. After all, I only get 20-25 miles reliable range to
    surface craft from a 60' amsl antenna w/25w! This might have something to do
    with small craft's antenna rocking through an arc of 60 degrees at the
    entrance to the Chesapeake Bay!

    Tropospheric Ducting is a real problem with VHF-Marine. While Bruce has some
    interesting stories to tell about making use of that up North, we normally
    find it a real hindrance to good communications in the mid-Atlantic. Having
    five or more CG Groups trying to answer the same mayday, and hearing traffic
    from a hundred or more miles up and down the coast is not a good thing for
    vhf-marine radio.

    see http://home.cogeco.ca/~dxinfo/tropo.html for ducting forecasts

    Jack Painter
    Virginia Beach, Virginia
     
  14. Bob

    Bob Guest

    yes, as a ham i once talked from allentown, PA to n. carolina on VHF
    FM in the ham bands.

    While Bruce has some
    yes, we CG radio operators try to avoid that situation but it's
    sometimes inevitable. what's even worse is that we sometimes don't
    respond to a mayday, assuming it's in some other CG AOR.
     
  15. engsol

    engsol Guest

    My turn for a radio sea story.
    I was on Barter Island...north slope of Alaska near the Canada
    border (Kaktovik), when we had a three day RF 'event'. Whereas
    we generally could communicate with over-the-pole aircraft at
    ranges of 200 miles, we suddnely couldn't talk to them until they
    were just about overhead. Our radar also suddenly started painting
    the fuel tanks of the next DEW site west...but we couldn't see aircraft
    50 miles away. Talk about refraction!
    Just for the record, I'm an RF guy too...I worked for a few years doing
    path studies, with all the measuring gear to confirm theory. The end results?
    On average, the math worked...but there were days it didn't. The more
    marginal the predictions (in terms of range), the more it varied from day-to-day.
    The folks paricipating in this thread suggesting that the physical/optical model
    of the earth is not the same as the RF model are right on. 4/3K ring a bell?
    Cranky tonight Norm B
     
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