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  1. #1

    Vhf aerial question

    Hello all,

    I am interested in performance differences of vhf aerial lengths if mounted in ideal boat location.

    Should you expect a large difference in transmission range with an aerial of same make and type but of a shorter length, or is it just the higher the aerial then the greater range to the horizon point?

    Previously I have had the standard 1.8 metre VHF whip but on my next boat they fit a 1.2 metre whip as a standard install due to it giving a better stored position when down.

    The 1.8 metre and the 1.2 metre are both stated to be 3dBi.

    Any ideas from the knowledgeable?

    Darren

  2. #2

    Re: Vhf aerial question

    The external length is for looks......... What matters is the the internal antenna length. I could be mistaken but I think that you would get the same performance from both....
    Remember to always log on before heading offshore.

  3. #3

    Re: Vhf aerial question

    VHF is 'line of sight', so not really Ozy, longer (higher) is better. However, mount the 1.2m aerial base 0.6m higher and they will be the same .

    However, there is probably a practical limit (???) where more height means you need more transmit power to make use of the extra height - and transmit power is limited by law. Over to Scotty or moose, this is more their area .
    Cheers
    Brendon

  4. #4

    Re: Vhf aerial question

    G'day
    I did find a chart some time back showing the different ranges with different heights. The difference was quite small between say a 1,8 and 2,4m antenna but there was a difference.
    Cheers
    Rod

  5. #5

    Re: Vhf aerial question

    Thanks guys,

    From what everyone is saying if the aerials have the same gain then it is just the difference in the line of sight.

    Assuming my aerial base is 1.8 metres above the waterline, then a 1.2 metre aerial will be 3.0 metres above water and a 1.8 metre aerial will be 3.6 metres above the water at top.

    When I run these figures through a line of sight aerial calculator I get the following range for each.

    1.2 metre aerial = 6.2 kilometre to the horizon (12.4 km to aerial of same height)
    1.8 metre aerial = 6.8 kilometre to the horizon (13.6 km to aerial of same height)

    Not much in it.

    http://www.hamuniverse.com/lineofsightcalculator.html

    Darren

  6. #6

    Re: Vhf aerial question

    There's a little more to it than just the length.
    All antennas have to be resonant on the frequency they are used on.
    To do so they must either have the a physical length equal to the wave length or in the case of short antennas the must helically wind wire around to achieve the correct length.
    (In some cases it will be a multiple of, or equal fraction)
    At VHF marine frequencies it's around 1.8m
    In general terms a full length antenna will outperform a helically wound one even if the claim the same gain. And even if mounted the same height (Assuming both are equal in build quality, impedance etc)
    The reason is more about how the signal is propagated (the shape as it were)
    They will also likely have a lower signal to noise ratio which is more important than power. Being understood is somewhat the point.

    It all gets very technical and it was so many years ago, so don't take the above as gospel, but as food for thought.


    If you can fit a quality full length unit do it. End of story.

    There's some good info here on the specifics of marine applications.
    http://www.icomuk.co.uk/Choosing-the...F-Marine-Radio
    Cheers,
    Owen


    The whole world's mad save thee & me (but I'm not too sure about thee)

  7. #7

    Re: Vhf aerial question

    Quote Originally Posted by Owen View Post
    There's a little more to it than just the length.
    All antennas have to be resonant on the frequency they are used on.
    To do so they must either have the a physical length equal to the wave length or in the case of short antennas the must helically wind wire around to achieve the correct length.
    (In some cases it will be a multiple of, or equal fraction)
    At VHF marine frequencies it's around 1.8m
    In general terms a full length antenna will outperform a helically wound one even if the claim the same gain. And even if mounted the same height (Assuming both are equal in build quality, impedance etc)
    The reason is more about how the signal is propagated (the shape as it were)
    They will also likely have a lower signal to noise ratio which is more important than power. Being understood is somewhat the point.

    It all gets very technical and it was so many years ago, so don't take the above as gospel, but as food for thought.


    If you can fit a quality full length unit do it. End of story.

    There's some good info here on the specifics of marine applications.
    http://www.icomuk.co.uk/Choosing-the...F-Marine-Radio
    That may be the case if the longer antenna was actually a different construction. In practice however most of them have a half wave length antenna mounted at the top of the whip (all the ones I have destroyed anyway). There are manufacturers that offer "high performance" antenna options that may indeed have different internal structure but it will come at a cost and the practicalities of small boats can mean that in some conditions they will create the effect of broken transmission as the boat pitches and rolls. Ultimately, height is your friend. The vessels that typically get the most range out of their VHF's (apart from ships) are generally yachts using a half wavelength (roughly a metre) antenna mounted at the top of the mast. with this sort of set up in a working condition, I could generally do a radio check with Bribie Island VMR on a simplex frequency from Manly boat harbour. On a powerboat with a full length antenna it was generally impossible or the transmission would be very weak.

    Ultimately, if you are looking for the best possible range, investigate some of the high gain antennas from companies like ZCG Scalar or Glomex. I wouldn't go any higher than about 6dB gain though or radiation pattern compression will create issues as I described. These antennas will be full height. Apart from that, in real practical terms, there would be very little in performance difference in the OP's description of his mounting situation given the same antenna internal construction and gains.
    "I soak the worms in rum. The fish love em and the worms die happy"
    "Alcohol is not the solution to your problems...................but then again, neither is milk"

  8. #8

    Re: Vhf aerial question

    Not so much more power but a different cable Brendon. In practical terms on a trailer boat there would be bugger all difference but at the top of a yacht mast, you can lose a very high portion of signal to cable losses if you stick with RG58 coax. On the meter it will show a good swr - all the power is going up and not coming back, but the range will be poor.
    "I soak the worms in rum. The fish love em and the worms die happy"
    "Alcohol is not the solution to your problems...................but then again, neither is milk"

  9. #9

    Re: Vhf aerial question

    I will only need about a metre and a half of coax to get from radio to aerial.

    Am I better off shortening the standard five metre cable and soldering on a PL259 connector?

    Thanks again for all the assistance.

    Darren

  10. #10

    Re: Vhf aerial question

    I snapped a std aerial and was surpised to find the wire inside only extended about 2/3 up the fibreglass, I guess the top 1/3 was for show.

  11. #11

    Re: Vhf aerial question

    Quote Originally Posted by Crunchy View Post
    I snapped a std aerial and was surpised to find the wire inside only extended about 2/3 up the fibreglass, I guess the top 1/3 was for show.
    This was the point I was trying to make.
    Remember to always log on before heading offshore.

  12. #12

    Re: Vhf aerial question

    Quote Originally Posted by Mr__Bean View Post
    I will only need about a metre and a half of coax to get from radio to aerial.

    Am I better off shortening the standard five metre cable and soldering on a PL259 connector?

    Thanks again for all the assistance.

    Darren
    This depends on the antenna. Some manufacturers include the cable length in their tuning from a design perspective and it can not be shortened, some don't. Check any installation literature that came with it or contact your supplier.
    "I soak the worms in rum. The fish love em and the worms die happy"
    "Alcohol is not the solution to your problems...................but then again, neither is milk"

  13. #13

    Re: Vhf aerial question

    Quote Originally Posted by Crunchy View Post
    I snapped a std aerial and was surpised to find the wire inside only extended about 2/3 up the fibreglass, I guess the top 1/3 was for show.
    Seen a few like that too Crunchy. As a general rule though, most of the antennas we went to the trouble to tear apart (if the client hadn't already done destructive testing) had the coax running up the centre of the whip with the antenna in the top half. It can make a difference dependent on your chosen mounting location - if mounting directly beside a windscreen or targa frame, you will get better performance with the antenna in the section of whip that is clear of the metal than one with the antenna in the lower half
    "I soak the worms in rum. The fish love em and the worms die happy"
    "Alcohol is not the solution to your problems...................but then again, neither is milk"

  14. #14

    Re: Vhf aerial question

    Penny has just dropped as to the reason why only half the length is typical.
    1.8m if a full wave at VHF frequency. Therefore it has a null at the horizon if mounted vertically. The signal is like this 8
    It will perform great if laid flat and used as a directional antenna in yagi or quad config but useless in a boat.
    Therefore a 1/2 wave is used with the ground plane forming the other half.
    Which fits exactly with the observations above re build.

    So if we assume they are all half wave and if the two lengths have the same gain, measured in the same way, then height is key.


    My apologies for confusing the issue.
    Cheers,
    Owen


    The whole world's mad save thee & me (but I'm not too sure about thee)

  15. #15

    Re: Vhf aerial question

    No apologies necessary, all info is appreciated.

    Darren

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