radar antenna height

[gator79]

what is the minimum height for a marine 24 mile radar? I have a 24 foot Grady White with a radar arch that is about 9 feet above water level. I would like it to be able to reach the 24 mile range. thanks

 

[18rabbit]

Re: radar antenna height

Radar is microwave radiation. It is recommended, as a rule of thumb, the antenna be placed a minimum of 5-ft above the heads of all passenger to prevent burns. It doesn’t really need that much clearance but you want it high enough so that if someone stood upon the transom or on top of the cabin they aren’t sticking their head into the beam.<br /><br />Also, the higher the antenna is mounted, the more ‘near’ targets are missed as the beam shoots over them. What you get to ‘see’ will be a factor of the antenna height, power, resolution (parabolic vs. array), RPM’s, and the motion of the ocean. I remember reading that at about 10-ft height, the radar can be looking 20-35 miles out, depending on the previously mention factors.

 

[mattttt25]

Re: radar antenna height

Range = 2.2(h1+h2)<br /><br />h1= antenna height<br />h2= target height<br /><br />this is in no way exact, but a rough estimate.<br /><br />edit: disregard this crap. thom is correct.

 

[ThomWV]

Re: radar antenna height

Unless you are almost unbelievably wealthy there isn't a chance on this earth that you will attain the full 24 mile range for spotting any surface craft. As an example, without doing the math, I would say that if you were able to mount your antenna at 100 feet above the water you would still be unable to see a typical container ship, fully loaded, at 24 miles. Not a chance on earth to tell the truth.<br /><br />While the formula is slightly different, because there is actually some bending of the micorwaves, you can use the line of sight formula that you use with our VHF radios. That is the square root of the height of each antenna (measured in feet) times 1.54 and then add the two numbers together to get the maximum range in statute miles. If your antenna ia 10 feet above the water it contributes 4.7 miles (square root of 10 is 3.16, multilpy that times 1.54 for 4.7 miles). That means that the target would have to be 152 feet tall to see it at 24 miles (the remaining 19 miles divided by 1.54 = 12.33 and square that for a total of 152 feet tall).<br /><br />Now what you can see at 24 miles is approaching weather, and sometimes planes way out there.<br /><br />Look, let me tell you something about Radar if you haven't had it before. When you need it you'll find that its a rare day when you have the range set to over 4 miles, and that most of the time you'll want it set to something more like 2 miles. Forget about all that crap about it taking just a couple of minutes for two approaching targets to hit each other. What matters is that you see it comming and make a turn.<br /><br />Thom

 

[gator79]

Re: radar antenna height

my main reason for the radar is to watch aproaching storms. I will ussually be 30 to 40 miles offshore and if there is a storm comming you need to get back closer to shore.

 

[ThomWV]

Re: radar antenna height

Good luck.

 

[Boatist]

Re: radar antenna height

With a 10 foot high antenna you could see a refelecting object 5 feet high at 9 miles.<br />10 foot high object at 10 miles.<br />25 foot high object at 11 miles.<br />100 foot high object at 18 miles.<br />250 foot high object at 25 miles.

 

[mattttt25]

Re: radar antenna height

 

[ThomWV]

Re: radar antenna height

Here is a link to a chart I made up some time ago that will give you maximum distances for different antenna heights. Notice that somehow I screwed up the first column but it will be clear to you how it works if you just move the word "Height" to the left. If you put your antenna into the first column, which is for 8 feet above the water line, then go down the column to you see 24 miles (24.7 actually) you'll see that the target (which I call the second antenna) would have to be 200 feet tall. Take a look:<br /><br /> Maximum Distance Chart

 

[18rabbit]

Re: radar antenna height

With the antenna mounted at a height of about 10-ft, microwave radiation from radar is detectable at 20-35 miles out, depending on the variables mentioned above. Whether or not a return from that radiation is usable will depend on the signal processing electronics.

 

[Boatist]

Re: radar antenna height

Gator<br />Here is a pretty good information page. Hope it helps.<br /><br /> http://www.boatus.com/husick/nd_radar.asp

 

[ThomWV]

Re: radar antenna height

Well, our radar is 9 feet above the water (23' walkaround with what you might call an arch, we don't have a hard top) and its a 2 kW unit (Raytheon SL-72). Its advertised as a 24 mile radar, but of course that's a bogus number. One thing about it though, we can watch storms build, move, and disipate out to our full range, which is 24 miles. Of course that's only because of the elevation of the storms themselves, you sure aren't going to get a useful picture from anything else, other than mountains behind a shoreline, at that sort of range.<br /><br />By the way, I'm curious, what radars are you other guys using on your boats? I was sort of torn between the one we have now and the Furuno 1712. I was also looking at Simrad and Si-Tex when I bought it. What have you got now and how do you like it?<br /><br />Thom

 

[BrianTindall]

Re: radar antenna height

Here is a simple rule with radar, if you can see it (clear day, with binoc's or scope if needed, but who uses radar on a clear day?) your radar will see it. Use the line of sight theory and you will be safe. Yes radio waves will bend with the earth to some extent, but at 9' above the water, it won't mean much. A VHF with the weather alert feature is very good way to go. When NOAA sets off the alert you get it right away. Now you can decide what you want to do, stay or go.