Old Radar / OFCOM EMF calcs.

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IanCC

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I have searched for old posts on this but have drawn a blank. I have just found out about this stuff. Only bought the boat recently and it didn't come with an assessment.

If someone could throw there their eye over this for me i would really appreciate it.

My radar, as per the manual, has :

Peak Output power: 1.5Kw
Pulse repetition rate: 1200Hz
Pulselength: 0.3us.

Then the calculation for max transmission time is:

0.3 x 0.000001=0.0000003 (Pulse length in seconds)
0.0000003x 1200=0.00036 (Transmission time per second measured in seconds)
0.00036*60=0.0216 (Transmission time per minute measured in seconds)
0.0216x6=0.1296 (Transmission time in six minutes measured in seconds)
0.1296/60=0.00216 (Transmission time in six minutes measured in minutes)

Is that correct?

Do I then need to reduce it further because it is rotating?

Thanks in advance.

Ian
 
I'd say yes, it is only irradiating a person when the antenna is pointing at the person. It starts irradiating when the leading edge of the beam first illuminates the target, and continues to illuminate until the the trailing edge of the beam stops irradiating the target.
To estimate the active fraction of a circle, you need to know the beam width of the radar (I guess to the -3dB point) and the width of the target.
You also need to know the range of the target, because the intensity of the radar beam falls off as 1/r^2.
I looked at this a while back, and concluded that if the radar was up the mast (as in a typical yacht), you'd no need to worry.
 
Thanks for that. Is that you saying my calcs as far as they go are correct? I can adjust for rotation.
Mine is on a stick so it is important.
 
Your arithmetic seems correct, you've calculated the total time the radar is transmitting in a 6min period.
You need to workout the fraction of the circle the target will be in, make a range correction and make sure that the transmitting power you are using is the RF power leaving the antenna (in air) not the electrical power being supplied to the transmitter.
 
Thank you for response.

The 6 minutes is for the ofcom spreadsheet.

My horizontal bandwidth is 5.7 degrees and it spins 24 times/minute.
So 5.7x24=137 degrees/minute for a single place. Which as a proportion of a minute is 137/360=0.38. So my original figure of 0.00216 needs to be adjusted down by *0.38 = 0.0008208.

Do you think that is correct?

The 1.5kw is output power.

Why do i need to make a range correction when j am trying to work out what is going on close to the antenna? I understand the pulse length and repetition rate vary with range and have chosen to use a mid distance of 1 to 3 miles.

I really appreciate you engaging with this.

Ian
 
Isn’t the 6 min time on the ofcom for VHF transmission not radar to account for cycle time, the radar cycle time is the rotation speed and beam width?

I don’t think I would like to stand in front of older radar sets, our digital one has 165mw Output so was only 400mm or so safe distance
 
May I ask why you are carrying out the assessment?
If you are worried about the emissions consider a new radar. Modern technology = much lower emissions.
 
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Daverw said:
Nothing really but why not
Click to expand...
No benefit really . I can't do much if anything about location of antennas .
If i did get radar it would be low emission modern equipment.
What more can you do . The mathematics are pointless.
 
Agree for most leisure boats but useful to know that it can in theory be an issue in other areas, as Ham radio operator I run significantly more transmitter power and coukd easily be directly under the antenna,
 
I also think the assessment is about protecting the public so perhaps employees or passengers in the case of a commercial boat.
So that could be your neighbours in the case of your Ham radio
 
Daverw said:
Isn’t the 6 min time on the ofcom for VHF transmission not radar ....?
Click to expand...

No. You use the Duty Cycle of the radar to calculate the the exposure time in 6 minutes. I have to calculate the duty cycle from first principles because my manual doesn't give me that figure.
 
MartynG said:
May I ask why you are carrying out the assessment?
If you are worried about the emissions consider a new radar. Modern technology = much lower emissions.
Click to expand...
It is the law, i like to obey the law. Also I want to be safe. My radar works perfectly and i couldn't afford a new radar.
 
IanCC said:
No. You use the Duty Cycle of the radar to calculate the the exposure time in 6 minutes. I have to calculate the duty cycle from first principles because my manual doesn't give me that figure.
Click to expand...
I do think that you doing the calcs is good as others with older analogue radars will find it very helpful
 
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Thanks, but only if i am correct. Furuno are trying to be helpful. I will post anything i get from them.

Fyi if you go to ofcom excel calculator. It specifically says, for radar, that if duty cycle is 5%, then 5% of 6 minutes is 0.3 minutes. Which is the number you would then plug into the spreadsheet.
 
I would have expected Furuno to have standard tables for this as its a normal commercial requirement and that’s their main customer base, doing emf calcs under H&S is not a new thing
 
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