How High to mount radar dome on CD25?

P

Pete in NY

New member
I have never used radar and I am trying to figure where to safely mount a new radome on my CD25 Sea Grace.

Since the radar beam radiates 12.5 degrees above and 12.5 degrees below the horizontal on the possible house top, or pedestal mount or radar arch locations, how far back in the cockpit can you go before a 6'1" person (me) is painted by the beam? Higher is better sincet he "umbrella" under the dome is higher, but what is the minimum height you would need to keep the radar beam clear of the cockpit and above your head ? Obviously you would be hit with the beam if you are standing on the foredeck. This is not about range, but about safety on board. My radome install instructions say to keep personnel away from a transmitting radar dome. I presume this is out of its beam.

My trig calculations say that if the radome is mounted 10 inches higher than the house and above the windshield that you get cooked if you step into the cockpit on my CD25 or if I put it on a radar arch like the new brochure shows, someone gets cooked if they are in the v-berth.

Do you have rules about where you go on the boat when using the radar? If I'm missing something here please let me know. Maybe some of the new CD25 purchasers getting radar can please fill me in on what the factory or your dealers have to say on this.

Thanks,

Pete in NY
 
I hab ma radarr mounted right on the rufe. Don't need no arch. Da onliest times I get hedakes is when I press my nose to the dome an lissen to the thingie go round and round. Sum say it causes brane dammage, but I don't tink so.
 
Okay, time for a serious answer to a serious question, at least as I understand it.
I was a microwave radio tech. in the Army circa 1969. We worked on and repaired 10 to 1000 watt transmitters.
According to our army electronic instructors, if you stand in front of a unidirectional 10 watt transmitter while working on it, you will get a headache after a while if you have your head about 1 ft. away from it. I can recall several of our instructors telling us about this because they had done it. Obviously they were not brain dammaged. Also keep in mind that this is a uni (one) directional transmitter that is constant. Radar sweeps around horizontaly in all directions so it is intermittent. Also keep in mind that transmitted power rapidly dissipates the further you get away from the transmitter. Thus we are not all fried from 1000 watt FM and AM radio station transmitters.
Someone correct me if I'm wrong, but I believe my radar is 28 watt. That's kinda weak compared to the 600-750 watts used for microwave cooking.
Having said that, I mounted my radar directly to the roof. The main reason was so that I could get the boat under in the 11 ft doors of my shop. You can see a picture of where I located my dome in my photo album. The dome is right over the overhead electronics shelf. The excess cable is stored on the shelf. The passengers in the cabin are protected because they are below the direct line of the intermittent swipe of the radar. Out on the cockpit there is over 8 ft. separating you from the dome. That allows the power to dissipate substantially. I'm not worried about it.

The above is merely my opinion and should under no circumstances be accepted as fact.
 
Larry - I believe your radome takes 28 watts of battery power to run it. The output from the transmitter is probably 2k watts. But, as the document I loaded into the document section points out, what can hit your head at the back of the boat is considerably less.

My radome is sitting on a pedestal about 15" above the cabin roof.

This is very interesting. I think the concern is very valid, but suspect that any warnings printed in the instructions are more for liability than reality. The manufacturer probably tells you not to eat the thing, too.
 
This quote was taken from the Aussie file that Dan provided.
Safe Practice
Exposures to microwave radiation above the recommended limits are most likely in the immediate vicinity of the antenna when it is stationary. When the antenna is rotating, as required for normal operation of the radar, average exposure is below the recommended limits, even at points as close as one metre. Some radars are designed to prevent microwave emission unless the antenna is rotating.Other models,however,lack this safety feature.In these cases some care is required on the part of the operator to prevent unnecessary exposure of personnel.Operating procedures should be adopted which ensure that no microwave radiation is emitted except when the
antenna is rotating.
A rule of good practice which applies to all small
marine radars is that equipment should be turned off
when not required,particularly in areas of high popu-
lation density (e.g.at dockside).
Click to expand...

In simple terms, don't put your face closer than 1 meter directly in front of the dome.

Mike, I don't know where the mfg. gets the 2kw from. Heck, to transmit 1000 watts of power we needed 3 phase 220 volt power and a klystron that would bend the hands of your wristwatch if you got it even in close proximity. These little radar units that we have are only transmitting 28 watts. It says so right on their website.
 
Well shoot, I think I've discovered the solution to our countries energy problems. These little radar units that operate on 12 volt batteries and use 28 watts, but convert that to 2 kilo watts of power output....
Ya gotta realize that I've been out of the electronics field for over 35 years, but somthing doesn't add up here. I'm confused.
 
Yep - it really does put out 2kw. The Raymarine specs list Power Consumption at 9w/28w, and Peak Output at 2kw nominal. You can see the specs at the Raymarine site by clicking on two pages, but the same ones are listed here all together.

Sure you needed all that to xmit 1000w back in the 60's. You also needed a room as big as my house to hold a computer that would do a fraction of what the handhelds do today. I don't know what a klystron is, but I had a physical exam last week and mine must be fine because the Doc didn't say anything about it.

I think them microwaves are different critters. There must be some mechanical advantage to them magnetrons that the klystrons didn't have. Maybe hydraulics or something.

Dan, my Furuno uses a 5a fuse.
 
My guess is that the 28 watt / 2000 watt paradox differential is due to the following:

28 is all the power that is taken in and the output cannot exceed that, in fact, would be 28 watts continuous minus the power required for the rotation.

Where we get 2000 watts is in the way the beam strength is measured.

The beam is probably intermittent, an alternating on/off radiation that is mostly off, but for the very short time the impulse is on, it measures 2000 watts peak output.

A continuous 2000 watt beam would have power requirements that a small vessel would not be able to supply. (2000 watts/12 volts = 167 amps!)

Fish finders have the same type of phenomena, where they generate a high frequency sound beam of 200-500 watts or more, all the while drawing only about 1 amp or 12 watts of power.

Thinking about it, it only requires a short high energy blast of radiation to get a reflected signal that a sensitive detector can pick up for the display unit's computer to turn into a scanned image of what's out there.

An analogy:


2000 watts continuous = 2000 watt light bulb (rated beam peak strength)

20 watts continuous = 20 watt light bulb (antenna draw)

2000 watts intermittent = strobe light (actual beam, intermittent, can be seen just as far as the 2000 watt continuous bulb)

Looking at the 2000 to 20 watt ratio, it means that the beam is actually on for about 1/100 of the time. This allows for the lower power requirement, for the beam to go out and be reflected back and be detected, and for the generation unit to power up for the next pulse.

HTH. Joe.
 
Thank you Joe, what you said makes a great deal of sense. There's a huge difference between continuous power and intermittent (pulse power) when it comes to doing damage to living cells. Kind of like comparing apples to oranges.
Soooo..., if we converted the 2kw of pulse power out into continuous power out, it would be transmitting 28 watts. (apples to apples)-minus the power used for rotation.

That's not so bad for the little salmon guys. :lol:
 
Joe is spot on. Radar "power" is measured as peak power that is transmitted as a short, 2-5 microsecond pulse, depending on the radar. Then it doesn't transmit for a relatively LONG time, while the signal goes out and returns and the magnetron circuitry builds up a charge for the next pulse. The pulse repetition rate determines range. Closer range, more pulses per second, higher *average* power, better resolution. Some of our radars take advantage of the increase number of short range pulses by speeding the antenna rotation up so closer targets get painted more often, thus become easier to process and see. In any case, average power is what the fuse worries about, and that's really low.

Personally, I don't worry too much. Ask an old Dermatologist and he'll tell you us military types who were often painted with pretty high power stuff while working on or around them typically exhibit symptoms very similar to folks who are overexposed to the sun's rays. Skin cancers start popping up about 20-25 years after exposure.

You can always line your favorite fishing cap with tin foil though. :lol:
 
Amazing how science and technology advance and change our world. I've heard "stories" of how medical students several generations in the past would sit on x-ray machines for a while before going out on dates so as to become a little less fertile (!!!) I think this was a few years before the links with cancer and mutations were fully understood. Kind of like putting your b's in the microwave for a treat! That ought to put the radar issue in perspective!!!. :smileo Joe.
 
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
 
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
 
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
 
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.
 
tpbrady":1b7t2tlp said:
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.
Click to expand...

I think you'll find the 2 or 4kw power rating does not include antenna gain. It's the peak power generated during the very few microseconds the transmitter is actually emitting a pulse. Since the pulse is short and the time between pulses is very long in comparison, the average power is surprisingly low considering that it has to both reach a target and reflect enough energy to be detected. A 2kw Furuno draws less than 5-6 amps at 12 volts while in operation yet can illuminate a target 16 miles away. Yes, there is considerable gain in the antenna as well, but it's never used in determining a radar's peak power output.

In the case of a microwave, the pulse is very long and the pulse repetition rate (PRR) is very high in comparison since the object is to pour that raw energy into a hot dog only 6" away from the emitter, not sit and wait for a return echo between pulses.


Don
 
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