Help with calcs for the safety rules QO-100

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AdrianH
Posts: 78
Joined: Thu Jul 18, 2019 2:46 pm

Help with calcs for the safety rules QO-100

Post by AdrianH » Mon Mar 15, 2021 12:37 pm

I am not wishing to start a debate over the proposed risk assessments required for Amateur use, so please can we keep that out of this thread.

I need to relocate my dish from it's present position, it's not the best thing to look out the patio doors and see my dish standing within 2 metres. So with the advent of better weather ahead, I am looking at where I can place it, so consideration of the new proposals might as well be made at the same time. One seems to require some better method then the proposed Ofcom spreadsheet, but I have used that for safe distances at 2410 MHz.

So I am hoping that within this thread I can get a better understanding of how things may be calculated and presented, which in turn may help others as well as myself.

I have a 1.05 metre dish (actual 1.05 wide 1.15 tall elliptical off set dish) From an on-line calculator the dish has a gain of 27 dB at 2410 MHz 0.7 efficiency, ( compared to an isotropic ).

From a bandwidth on-line calculator the 3 dB beam-width is 7.5 degrees (+/-3.75) and is 9 dB down at 13 degrees beam-width (+/- 6.5). It does not go less than this due to errors getting to be too much and also side-lobes can be a concern.
Using a checked Bird Thru line my r.m.s. power is 33 Watts.

So putting everything into dBm for ease at this point. 33 Watts is +45.2 dBm(i) The direct beam gain would be 27 dB so that would be a power level of 72.2 dBm
Now considering the 9 dB down beam-width as the best I can get from a calculator at +/- 6.5 degrees = 63.2 dBm

72.2 dBm would be equivalent to 16.6 KW Safe distance is 18.39 metres.
63.2 dBm would be equivalent to 2.09 KW Safe distance is 6.53 metres.


So I turned to a simple CAD drawing package to get ideas down into my head.
satcalcs.jpg
satcalcs.jpg (70.44 KiB) Viewed 1408 times
My centre of the dish is at 1.5 metres from the ground, the elevation to the sat is 23.5 degrees, so I have drawn lines for that and also the 9 dB down beam-width. I have also assumed a person at 1.85 metres tall.

At first I was way off on my ideas, but included here so others do not make the mistake I did. The first idea gave me a safe distance of over 23.5 metres which was more than standing in the direct beam, so I had a rethink: -

In the second half of the image, I am basically saying that if the top of the person is not within the -9 dB beam-width the safe distance is going to be 6.53 metres, and even at that distance I could actually have 1.57 metres clearance from the -9 dB beam to the head of a person, but can not back that up with any calculations.

How to account for 10 minute periods rather than 6 minutes of constant transmit I am not sure.

I would appreciate comments to see if anyone thinks I am on the correct or wrong track with this.

Adrian

references:
https://www.rapidtables.com/convert/pow ... to_dB.html
https://www.rapidtables.com/convert/pow ... _Watt.html
https://www.everythingrf.com/rf-calcula ... tenna-gain
https://www.satsig.net/pointing/antenna ... ulator.htm

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G1LPS Terry
Posts: 161
Joined: Mon Mar 10, 2008 10:33 pm

Re: Help with calcs for the safety rules QO-100

Post by G1LPS Terry » Mon Mar 15, 2021 1:01 pm

Was having a similar conversation last night with Rob M0DTS and he pointed me to this calculator.

VK3UM Electro Magnetic Radiation Calculator (EMRCalc)
http://www.vk5dj.com/doug.html

Haven't played with it much yet but sofar it has put some of my concerns to bed.
There's a print option for you to save a copy to show you've looked at compliance.

Terry

G8GKQ
Site Admin
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Re: Help with calcs for the safety rules QO-100

Post by G8GKQ » Mon Mar 15, 2021 1:56 pm

Thanks for the posts Adrian and Terry, we realise a lot of BATC members will be considering this in the next few months.

BATC is working to issue some guidelines, which will be able to be adjusted to apply to most typical QO100 DATV uplinks, but they are not ready just yet.

More information to follow

Dave, G8GKQ

AdrianH
Posts: 78
Joined: Thu Jul 18, 2019 2:46 pm

Re: Help with calcs for the safety rules QO-100

Post by AdrianH » Mon Mar 15, 2021 2:09 pm

Hello Terry;

Had to go and find a windows PC, but had a quick play with it on both the dish and 14.2 MHz with a wire dipole, and on the dish part not sure about the mast height, can not get that to be below 2.5 Metres which would be above head height anyway, so anything less than this or floor mounted I am uncertain about.

I could not get it to match the Ofcom calc on 14.2 MHz either, so I am possibly driving things wrong on the program, will need to read more.

I think one would still need written words with explanations etc to go with any printed RA, if meant to be understood by a lay-person, (that includes me).

Adrian

Just seen Dave's post, so a quick comment.
I would have expected the BATC and others to be working on this, but as we all got emails from Ofcom recently it has put it into the minds of many, including me. I would guess for many the issue is ground mounted large dishes in gardens/yards etc. I do not have a large area to use, but can maintain 7 metres to a facing boundary, just have to stop the Mr's from walking in front of it :)

g0mjw
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Joined: Sat Sep 20, 2014 9:15 am

Re: Help with calcs for the safety rules QO-100

Post by g0mjw » Mon Mar 15, 2021 2:36 pm

I can confirm the microwave committee has been working on this. Hold fire. We don't have to do this immediately - though to be fair, we should already be considering RF safety whenever we install antennas. Practical advice will appear soon.


Mike

AdrianH
Posts: 78
Joined: Thu Jul 18, 2019 2:46 pm

Re: Help with calcs for the safety rules QO-100

Post by AdrianH » Tue Mar 16, 2021 5:45 pm

I think moving the dish will be sooner than any deadlines.

Adrian

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