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Broadband Radar vertical beam width?

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I have bought a Simrad 3G radar and am trying to figure the best height to mount it on the mast. According to the 3G/4G manual it is best to mount it so that you get a clear view from the radar beam at a 25 degree angle down. Less acceptable is an angle from the radar of 12.5 degree according to the pictures. So to me the specs should be a vertical beam width of 50 degrees not the 25 degree that is listed?

For the Simrad 2K radar the specs say it is a vertical beam width of 30 degrees and their pictures show that the radar should have a clear view of 15 degrees. If this is correct it appears that the broadband has almost twice as much vertical beam width. Does this seem right?

Last point, if this is true then I would guess that broadband radar would be less susceptible to heeling affecting the quality of the radar signal? If so I would be more likely to go to a fix mount vice self leveling.....

thanks for any input,


2 Replies

  • Richard, I have a PDF of the 3G/4G installation manual open, and I think I understand the confusion.

    First let me explain that no radar's vertical beam measurement describes a hard edge. The beam's highest effective power is on center (i.e. focused toward the horizon) and the power tapers off in the vertical dimension. I think vertical beam width is usually defined by the points where the effective power is -3 dB the maximum power.

    Almost all radars are rated as a 25 degree vertical beam, with 12.5 above and below the horizon. That does not mean that you can tilt the radar 12.5 degrees and maintain peak performance.

    If you look at the specs in the back of the Broadband manual you'll see that Simrad also defines its vertical beam width as 25° or more specifically
    "25°+/-20% (-3 dB width)". Then on pages 8 and 9 you'll see that they're recommending keeping that whole 25° as free of obstructions as possible for better performance.

    What Simrad is doing different is recommending that you keep a full 50 degrees unobstructed for "Best Performance". In short, I don't think the Broadband 3G or 4G has a larger vertical beam width than other radars. But it does have less effective power, so I guess every little bit more may help. Broadband radars also seem more sensitive to onboard obstructions, so that's another reason to keep the beam as free as possible.

    Incidentally, I also think that due to the tapering nature of radar beam power, any boat motion off the horizontal affects performance. The question is how much, and, more important, if it's enough to justify the extra complication of self leveling.

  • Thanks Ben, that totally makes sense! As you say the weaker power and ability (sensitivity?)to pick up close targets makes it more important to get the radar head high enough and to use the 25 degrees as the proper angle above and below the horizontal.