How High to mount radar dome on CD25?

[Ron on Meander]

Pete In NY,
I see from the pictures of Seasprite in your album that you chose the Garmin radar. How do you like it?
I have the Garmin 3006 chartplotter and I am thinking of adding the radar to it, so I would be very interested in any feedback you or anyone else may have on the Garmin Radar.
Thanks
Ron

[Sea Angel]

I did not notice any mention height above the water, though assumed. The heigher the better to increase your 'radar horizon'. This has a practical limit such as if you are in dry storage and have to pay by the height restrictions.

Greater peak effective radiated power can be obtained with a larger antenna to decrease the beam width; i.e., better focus of the RF envelope.

Everything is a compromise. I went with the 4KW Raymarine Ant., but the 2KW would have done me just fine. I wanted the narrower beam width for target discernment and rain penetration... all in my ego for my application.

The guys here have done an excellent job in presentating the hows... and it is still magic to some of us. The duty cycle of On then OFF is the secret and is right there. Remember that you and the radar can't listen or 'see' if we are talking. The longer we listen the further away the sounds can be heard without being over run.

Sea Angel

[Pete in NY]

Ron,

I chose the Garmin radar since I already had the 3010C and the sounder module. I got the 4kw dome since it was the same dimensions as the 2kw and I wanted to see through rain and snow longer. For the extra few dollars it seemed to make sense at the time. I installed it spring 2005 and had limited use of it so far due to healthy issues I needed to take care of that hit me in August. I did use it, however, in returning from the July 4th Macy's fireworks display in the dark and found it quite useful in the mass of boats that left the East River and heading up Long Island Sound after the show. The radar overlay is great since the return gives you an orange blip on the chart that is visible before navigation lights on another vessel in the dark and alerts you to other traffic. Here in our crowded waters not everyone follows nav rules and some do not even have nav lights on. I have had no problem seeing kayaks, buoys, mooring floats, rubber dinghys during practice in the daytime.

I wired the radome power to a separate accessory switch on the helm panel and can turn it off and on easier and faster than wading through the chartplotter's menus. This is most useful to shut down the radome in a flash when someone decides or needs to go forward in a hurry.

Regards'
Peter

[tpbrady]

This is the first time I have looked at this thread, but I noticed a lot of discussion on power output. I think the rating also includes gain of the antenna. For example, if I inject one watt of energy into an antenna with a 3 dB gain, the effective radiated power would be 2 watts, meaning power doubles for every 3 dB of gain. In the case of a radar, a 10 watt input into an antenna with 23 dB of gain would just about generate 2KW of effective output power. The gain works both ways in that the antenna array also amplfies the much weaker return signal.

If you go back and look at a microwave oven, the ratings on those are in the range of 600 to 1000 watts. That is raw power, because the antenna involved doesn't have appreciable gain. The last time I looked inside one, it was a piece of waveguide directing the energy into a rotating metal fan blade that dispersed it throughout the cooking compartment.

[Sneaks]

[tpbrady]

Thanks for the correction. I am still baffled how 28 watts in can generate 2.2kw out. Next time I find a radar engineer I am going to ask. The answer is probably over my head since I was a forestry major in college.

[journey on]

Well, radar works by sending out a pulse of radio waves called a chirp, starting to count time, and then waiting for the reflection and stopping the time count. Speed of light is 186, 000 mph, and distance is speed X time.

The energy in the pulse is 2000 watts, but average power into the radar is ~25 watts.

If you transmit continously, how do you know when to stop counting time of flight? The reflection would be continuous, and mixed up with the transmitted power. Deep space probes transmit continuously, and measure time with a long PRN code which is turned around at the spacecraft. Sending out chirps is easier.

So the difference is peak vs. average.

Sorry for the lecture, but there you go.

Boris

PS, my dad was a forestry major in college, and sometimes I wish I was.

[Helen O]

[journey on]

Now you know why I wish I was a forestry major.
All I remember is 1 foot per nanosecond. Am I right?

Boris

[Ron on Meander]

Pete,
Thanks for the info on the Garmin radar. It sounds like it works pretty well. I appreciate the info.
Ron

[Sea Angel]

Old Man thoughts..

12.36 uSec is the RadarRange Mile [2 way travel] and a 1uSec Pulse should travel 0.186 miles, or 328 yards; as I best remember..

Art

[Helen O]

[Minnow]

I had a chemistry professor in college, that told me that he was stationed at an early warning radar installation in Alaska during WW2.
He said that when they got cold, that they would walk out in front of the dish to warm up. Was he just pulling my leg?

[tpbrady]

I once met a Navy Admiral named Grace Hopper. Retired at the age of 80 something on the deck of the USS Constitution. She used to go to lectures and hand out nanoseconds. Her nanosecond was a piece of wire just shy of 12 inches long and it represented the distance light traveled in a nanosecond. She was better known for writing the computer language Cobol.

[CAVU]

Minnow,
I don't think those early warning sites were built until the 1950's. I think he was pulling your leg.