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Message started by xband on May 29th, 2011 at 1:37pm

Title: RF Radiation 150-200 W BUCs
Post by xband on May 29th, 2011 at 1:37pm

Does anybody know how much radiation would someone get when working near a live VSAT Antenna with 150-200 W BUCs ? (Advantech or other)

(Working in a dome behind and close to the sides of the dish)

Thanks !

Title: Re: RF Radiation 150-200 W BUCs
Post by USN - Retired on May 29th, 2011 at 4:20pm
Without a very sensitive vicinity RF meter, I'd say the amount would be between negligible and immeasurable. You probably get more radiation off the radium in your watch dial. And if you have a smartphone, you probably have more RF in your pocket than you can detect from the dish.


Title: Re: RF Radiation 150-200 W BUCs
Post by xband on May 29th, 2011 at 4:40pm
I was under the impression that you must get something, I see that the permissible dosage for Cellular Telephony is 10 mW/cm2, for a 200 W BUC wouldn't that be x20 times more ?

Im only worried as recently I spend 1 hour working in a live dome and when I came out I was feeling very very hot together with a headache (It wasn't from the complexity of the job I'm certain...!)

Title: Re: RF Radiation 150-200 W BUCs
Post by Eric Johnston on May 29th, 2011 at 7:38pm
I would regard it is prohibited to get any body part between the feed horn and the dish or in the cylindrical beam (same diameter as the dish) towards the satellite.

Anywhere in the vicinity of the feed horn aperture is really dangerous.  

In addition, I, personally, don't stand just behind the side of the dish where I might see the feed/sub reflector if I looked in that direction.

The power from the transmitter is spread across the dish with small amounts spilling over the edges. The power per unit area is tapered across the surface, such that at the edges of the dish the power per unit area is normally 10 to 16 dB lower than at the centre of the dish.

Avoiding complex calculation and making the simplifying and conservative assumption that the power is evenly spread across the whole dish.  A 3m diameter dish, for example, has an area of 70685 sq cm. If the 200 watts were spread evenly then the power per sq cm at the dish edge (coming from the feed/subreflector) would be 3 mW per sq cm. You could measure the actual level (expect something rather lower than 2 mW/sq cm) with a RAHAM detector held just beside the dish edge.  Most teleports will have such hand held meters for use checking leakage at joints along transmit waveguides etc.

Microwave energy causes body heating.  If you put your hands in front of a 1kW electric fire you will get the idea.  The blood in your hands and sweat takes the heat away to some extent.  The particular hazard with microwaves is that it is not visible like a fire or the sun  (you don't see a bright glowing red/orange light at the feed horn) and the way microwaves warm the body below the surface rather than just on the surface.  The eye lens has no cooling blood flow through it and is particularly sensitive to being heated. So, never look down a transmit waveguide.

Mobile phones even if just 1 watt radiate that power almost adjacent to the human body so the heating power per sq cm can be quite high.  There is also the added irritation of the TDMA power pulsing like a bright flashing light - no wonder some people claim to find mobile phones causing headaches!

If you got hot and a headache it might well have happened even if the transmitter was off.  Ventilation is important.  Beware of possible oxygen deficiency in any enclosed space. Ships have suitable %oxygen meters for use before entering spaces where rusting iron may exist.

Best regards, Eric.

Title: Re: RF Radiation 150-200 W BUCs
Post by USN - Retired on May 29th, 2011 at 8:51pm

wrote on May 29th, 2011 at 4:40pm:
permissible dosage for Cellular Telephony is 10 mW/cm2.....for a 200 W BUC wouldn't that be x20 times more ?

Only if you held it up to your head. I've worked with up to 10KW systems, and am still here to tell you about it. It's all about knowing where to work - when. If your terminal doesn't have such safety guidance posted, I suggest you take that on as your next personal project.


Title: Re: RF Radiation 150-200 W BUCs
Post by xband on May 29th, 2011 at 10:16pm
Eric and Greg, thank you so much for the replies

Due to the nature of my work I spend a lot of time inside C-band domes, most of the time while the I-Direct is in the network as in vessels like Cruise Ships the officers do not allow any downtime.

So next time... rx enable off... to be safe just in case

Thanks !

Title: Re: RF Radiation 150-200 W BUCs
Post by Oasis Networks on May 30th, 2011 at 9:58am

Couple of notes regarding this issue.

Several years ago, I contacted the Ministry of Health and asked them for their instructions and recommendations regarding RF radiation hazards - if they have some kind of standards, thumb rules and etc (like do not stand more than x minutes, y meters away from a system that radiates at z power). To my surprise, even though we have so much RF radiation all around us, there are no rules, no really any regulation, just some isolated recommendations here and there. I talked with the person responsible for this field, and she told me, after an investigation she did, that actually she doesnt know of any formal such regulations anywhere - not in the USA and not in Europe either. Maybe things have changed since then, this was like 5 years ago.

Another small story - I remember working on a very important system, high level customer and the site had too many problems when I was asked to fix it. the 2.4m dish with the 8W C-band BUC was transmitting and because of the sesitive situation with the customer....I had to work on the feed itself and I decided to work when the system is on. I remember how bad I felt, I really had a headache and felt everything is turning around me. Then I went downstairs to found out they had power outage for the last hour...the system was not transmitting while I was there. It was just my imagination and worries that made me feel like that :)

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