How High to mount radar dome on CD25?

Pete in NY

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

Helen O · Posted: Sun Mar 20, 2005 7:31 pm Post subject:

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.

dogon dory · Posted: Sun Mar 20, 2005 9:32 pm Post subject:

Sorry, Folks - Post Deleted By Author

Helen O · Posted: Sun Mar 20, 2005 10:13 pm Post subject:

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.

dogon dory · Posted: Mon Mar 21, 2005 1:21 am Post subject:

Sorry, Folks - Post Deleted By Author

TyBoo · Posted: Mon Mar 21, 2005 1:43 am Post subject:

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.

Helen O · Posted: Mon Mar 21, 2005 2:35 am Post subject:

This quote was taken from the Aussie file that Dan provided.

dogon dory · Posted: Mon Mar 21, 2005 2:58 am Post subject:

Sorry, Folks - Post Deleted By Author

Helen O · Posted: Mon Mar 21, 2005 3:13 am Post subject:

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.

TyBoo · Posted: Mon Mar 21, 2005 3:31 am Post subject:

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.

Sea Wolf · Posted: Mon Mar 21, 2005 8:17 am Post subject:

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.

Helen O · Posted: Mon Mar 21, 2005 9:28 am Post subject:

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. Laughing

dogon dory · Posted: Mon Mar 21, 2005 9:34 am Post subject:

Sorry, Folks - Post Deleted By Author

Sneaks · Posted: Mon Mar 21, 2005 10:03 am Post subject:

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. Laughing

Sea Wolf · Posted: Mon Mar 21, 2005 10:34 am Post subject:

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!!!. Smile

Joe.