radar dome position

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simonbertie

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We are consiering positioning our new radar on the existing 6ft frame at the stern our sadler 34. (wind gen on already as well as gps/ vhf aerials) The model is a jrc 1500 mk2. Can anyone give me the pros /cons of mast v frame especially in terms of performance. Frame seems easier to install and keeps weight down.
Ta, simon

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Ours is on a goalpost gantry and is fine, a bit lower than on the mizzen on our last boat but since the ranges used normally are from 6mls downwards it doesn't matter. It is also easier to reach for service/repair. Make sure though the beam (allowing say 20 degs beam depth up/down) cannot zap the GPS aerial, in our case the scanner is on a raised pole, part of the structure, and above the side perch where the GPS aerials are.

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On my 34 the scanner is on a Scanstrut pole on the stern. Performance seems perfectly adequate. I bought the pole from yet another 34 owner, who told me that he had initially sited the scanner on the mast, then moved it to the pole, without being able to detect any change in performance.

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I had a Westerly Discuss 33 with the scanner mounted on the mast. Yes the set, a Forune CRT model was supposed to have a 24 mile range but it could never pick up anything, not even the Isle of Wight cliffs for instance until well within that range. Other considerations include not having to play around with the connections when the mast is lowered. Secondly, the jib can snag on a mast mounted scanner

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The following may be usefull: Radar horizon can be calculated from the formula.
R=2.2X sqroot antenna height in meters. Where R is the horizon range. This will be greater than the visable horizon at eye level due to the curvature of the radar beam. As a previous post has noted be careful not to place the antenna where it can "Zap" the GPS, active antenna used with navaids can be destroyed by radar pulses. Other antenna for radios, TV etc can also pick up radar pulses and if in close proximity you can at worst blow the electronics in the equipment, interference could be the least problem with a strong "BUZZ" each time the radar swings through that arc.
When siting the antenna also be aware of possible blind arcs caused by stucture on the boat.
Also from a personal safety point the antenna should be at sufficient hieght to ensure there is no Radiation Hazard (RADHAZ) to you or your crew! The documentation for the set should contain recommended safe distances etc. If not check with the maker or his agent. You are dealing with microwave frequencies and you don't want to cook your eyeballs (Or any others!).

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I would check on the vertical beamwidth of this radar (scanner) and site it high enough so that the beam cannot radiate a crew members head (particularly when he is forward at the bow) - a simple piece of trigonometry. I would suggest a safe height to be 4 metres for a ten metre boat, allowing some margin for overspill e.t.c.
I've always thought that the spreader mounted radomes are mounted unnecessarily high (allowing more movement of the scanner than necessary when the boat rolls), and that pole mounted radomes are mounted too low from a safety aspect; and that gantry mounted radomes are dangerous - unless you have very high gantry.
I personally don't like the idea of 2KW of X-band power squirted onto the back of my head - even if it is only for a few microseconds at a time.
Just for interest you can calculate the average power from your set wth the formula:

Average Power = Peak power X pulse width X pulse repitition frequency

Even the average power is surprisingly high!!

Alan.

<hr width=100% size=1>Alan Cloke
 
I think your idea of mounting the radar on your frame is perfectly adequate considering pros and cons.
Pros: Less weight high up
Better resolution and wave suppression for closer objects.
Smaller angle of mast obstruction.
No interferece with head sails.
Less of a problem when taking the mast down.
Cons: False echos from the mast in front, try to offset the unit from the centerline.
Radiation?? (see below)
Somewhat shorter horizon. (really not a problem).

Radiation: The 6 ft is a bit low and the beam can hit people in maybe just a foot distance when standing on deck. I'd try to go up a bit like one foot or so. Radars have a vertical angle of around 25 dregrees, so it will point down at about 12 degrees. So some small increase in height gives a big one in distance. The radiation issue is not a big problem though if you believe current understanding that the frequencies of such a radar are low enough to not have any (cell damaging) effects, but just cause heating. In that case think that the power of the field will drop with more than the square of the distance and with your head two or three feeds off the unit, you will receive less radiation than woth a cell phone right next to your head.

Again, this is current knowledge, and we may or may not think differnt about this in some years. So if all other things were equal, it doesn't hurt to be further away from the unit.




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"Haralds" forgot to mention one pro: if any problem arises with the radar, it's much easiere to take the scanneer down and bring it with the cable and dispaly screen to a repaoir shop than wait for a techie to come and climb up the mast. Happened to me in Nova Scotia and I rented a car (cheaper than a taxi...) top go to the shop 25 miles away. I was quite surprised to see the guy working on the radar without any protection; he told me these small ones "didn't have any bite in them". One additional con is that a scanner at only 9-10 ft above the water has some trouble finding a small buoy in 6-8 ft seas.
jon

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radar zaps GPS?

i can see that happening. what about other kit that is likely to be goalpost mounted, eg wind generator, ssb aerial, backup vhf antenna, navtex etc etc.

anyone with technical knowledge have an answer on that? half the circuits on hte boat light up when i transmit on ssb so i'd be a bit worried that it might fry the radar or vice versa.

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Re: radar zaps GPS?

I don't think it is a permanent Zap, just destroys the signal received. There were some radar jamming trials in Poole Bay back in July and when crossing the line of the trial one of our GPS sets lost the signal, another set was not affected. I spoke to Greenham Marine who said microwave transmissions (like between N Sea oil rigs/shore) were known to cause problems with GPS signal strength reception and that the Shipmate aerial was better than most at avoiding this (this was our set that remained unaffected). I believe Radar is microwave frequency so hence the ability to Zap GPS signal reception if the GPS aerial is in line of fire. Raymarine mounting instructions for GPS aerials say keep clear of radar beam.

We also have backup VHF aerial, wind generator, solar panel, radar and 2 GPS aerials on the gantry and SSB on the backstay. I haven't really noticed any problem, the aerials are below the radar beam though and SSB use is infrequent so far on transmit. What happens when you transmit on SSB??

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Re: radar zaps GPS?

I cannot claim technical knowledge in this area, but Scanstrut sell a hoop for mounting antennas, GPS and VHF, immediately above the radar scanner. I have a Nasa Clipper active antenna and a Garmin mushroom above mine, neither is affected by the radar.

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Re: radar zaps GPS?

the ssb signal seems to induce RF oscillation throughout the boat. all the LEDs flash in time with the transmission. it has also fried a CO detector which was close to the coax from the set to the ATU (alarm came on and wouldn't stop)

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Re: radar zaps GPS?

Your SSB aerial, or it's feeder, does not have enough seperation from other equipment. You need to look at your cable runs and aerial position. If your feeder has to cross or run close to any other cables you can get induced interference. If you have to cross a cable run do it at right angles and try and seperate cable runs by about 2 inches, not easy on a small craft I know. Check the position of the aerial and try and reposition it if possible.
I have seen fire detection systems activated on ships, both from transmissions of radars from ships in close proximity and from wave guides running close to detector heads. I have also had more than enough active aerials, GPS etc. destroyed from radars in close company. Also more worrying I have seen a someone receive a shock from a crane hook on a ship when the shack transmitted on an HF transmitter. The crane, a permanent fixture, was sited outside the RADHAZ area but the wire acted as an aerial. I saw the same hook weld itself to a hand rail during the subsequent RADHAZ trials. Addmitedly I am talking about quite high power transmitters on ships but I think it serves to show that aerial positions, feeder runs and ajacent equipment must be considered. With bad positioning the least you will get is interference and the worst is injury or death to personel. Damage to your equipment comes somewhere in between.

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