#### EMF Calculator

Dave

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

Andy G4JNT

Just don't transmit with anyone standing in front of the dish - and if you feel you must, write down that that would be the case.   No need to rope off
You wouldn't have anyone standing there for 6 minutes while you're working anyway, would you.

Andy

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

Dave

Thanks Andy,
Great. That makes it all seem a lot easier.
73
Dave G4GLT.

On 22 Apr 2021, at 09:59, Andy G4JNT <andy.g4jnt@...> wrote:

﻿
Just don't transmit with anyone standing in front of the dish - and if you feel you must, write down that that would be the case.   No need to rope off
You wouldn't have anyone standing there for 6 minutes while you're working anyway, would you.

Andy

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

Roger Ray

I suggested before a simple pir to turn off the tx, if someone is standing in front of the dish. You would not be communicating then anyway.
Seems like a good idea particularly to stop someone looking down the waveguide, while you are sorting another problem.
I am intending to do some real life measurements against icnirp levels with a Narda s3 personal monitor, especially for the bands above 10GHz. As it will be near field the calculator is not a lot of use, even if it worked above 10G.
Roger
G8CUB

On 22 Apr 2021, at 10:08, Dave via groups.io <davidnewman55@...> wrote:

﻿Thanks Andy,
Great. That makes it all seem a lot easier.
73
Dave G4GLT.

On 22 Apr 2021, at 09:59, Andy G4JNT <andy.g4jnt@...> wrote:

﻿
Just don't transmit with anyone standing in front of the dish - and if you feel you must, write down that that would be the case.   No need to rope off
You wouldn't have anyone standing there for 6 minutes while you're working anyway, would you.

Andy

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

SAM JEWELL

Being a little unwary of computer EMF models/calculators for ICNIRP, I have been doing some measurements on my 23cm EME system, using a calibrated portable spectrum analyser and an antenna with a known antenna factor. The results have been interesting and do actually bear out some of the calculated numbers.
I felt that I would be happier knowing actual W/m2 levels at points of public access near my premises, on the bands above 1GHz, rather than relying completely on calculated values in a highly complex environment, where vegetation, fences and relative access height are all factors that affect the real levels.

73 de Sam, G4DDK

On 22 Apr 2021, at 14:06, Roger Ray via groups.io <g8cub@...> wrote:

﻿I suggested before a simple pir to turn off the tx, if someone is standing in front of the dish. You would not be communicating then anyway.
Seems like a good idea particularly to stop someone looking down the waveguide, while you are sorting another problem.
I am intending to do some real life measurements against icnirp levels with a Narda s3 personal monitor, especially for the bands above 10GHz. As it will be near field the calculator is not a lot of use, even if it worked above 10G.
Roger
G8CUB

On 22 Apr 2021, at 10:08, Dave via groups.io <davidnewman55@...> wrote:

﻿Thanks Andy,
Great. That makes it all seem a lot easier.
73
Dave G4GLT.

On 22 Apr 2021, at 09:59, Andy G4JNT <andy.g4jnt@...> wrote:

﻿
Just don't transmit with anyone standing in front of the dish - and if you feel you must, write down that that would be the case.   No need to rope off
You wouldn't have anyone standing there for 6 minutes while you're working anyway, would you.

Andy

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

SAM JEWELL

That should, of course read wary. Not unwary!
Sam

Sent tomorrow from my time machine

On 22 Apr 2021, at 14:30, SAM JEWELL via groups.io <jewell@...> wrote:

﻿Being a little unwary of computer EMF models/calculators for ICNIRP, I have been doing some measurements on my 23cm EME system, using a calibrated portable spectrum analyser and an antenna with a known antenna factor. The results have been interesting and do actually bear out some of the calculated numbers.
I felt that I would be happier knowing actual W/m2 levels at points of public access near my premises, on the bands above 1GHz, rather than relying completely on calculated values in a highly complex environment, where vegetation, fences and relative access height are all factors that affect the real levels.

73 de Sam, G4DDK

On 22 Apr 2021, at 14:06, Roger Ray via groups.io <g8cub@...> wrote:

﻿I suggested before a simple pir to turn off the tx, if someone is standing in front of the dish. You would not be communicating then anyway.
Seems like a good idea particularly to stop someone looking down the waveguide, while you are sorting another problem.
I am intending to do some real life measurements against icnirp levels with a Narda s3 personal monitor, especially for the bands above 10GHz. As it will be near field the calculator is not a lot of use, even if it worked above 10G.
Roger
G8CUB

On 22 Apr 2021, at 10:08, Dave via groups.io <davidnewman55@...> wrote:

﻿Thanks Andy,
Great. That makes it all seem a lot easier.
73
Dave G4GLT.

On 22 Apr 2021, at 09:59, Andy G4JNT <andy.g4jnt@...> wrote:

﻿
Just don't transmit with anyone standing in front of the dish - and if you feel you must, write down that that would be the case.   No need to rope off
You wouldn't have anyone standing there for 6 minutes while you're working anyway, would you.

Andy

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

Dave

The thing that I don’t understand is what is done with the assessments. Are they stored at home  in case they are asked for or do they have to be submitted to Ofcom and updated when necessary.
Dave G4GLT.

On 22 Apr 2021, at 14:39, SAM JEWELL via groups.io <jewell@...> wrote:

﻿That should, of course read wary. Not unwary!
Sam

Sent tomorrow from my time machine

On 22 Apr 2021, at 14:30, SAM JEWELL via groups.io <jewell@...> wrote:

﻿Being a little unwary of computer EMF models/calculators for ICNIRP, I have been doing some measurements on my 23cm EME system, using a calibrated portable spectrum analyser and an antenna with a known antenna factor. The results have been interesting and do actually bear out some of the calculated numbers.
I felt that I would be happier knowing actual W/m2 levels at points of public access near my premises, on the bands above 1GHz, rather than relying completely on calculated values in a highly complex environment, where vegetation, fences and relative access height are all factors that affect the real levels.

73 de Sam, G4DDK

On 22 Apr 2021, at 14:06, Roger Ray via groups.io <g8cub@...> wrote:

﻿I suggested before a simple pir to turn off the tx, if someone is standing in front of the dish. You would not be communicating then anyway.
Seems like a good idea particularly to stop someone looking down the waveguide, while you are sorting another problem.
I am intending to do some real life measurements against icnirp levels with a Narda s3 personal monitor, especially for the bands above 10GHz. As it will be near field the calculator is not a lot of use, even if it worked above 10G.
Roger
G8CUB

On 22 Apr 2021, at 10:08, Dave via groups.io <davidnewman55@...> wrote:

﻿Thanks Andy,
Great. That makes it all seem a lot easier.
73
Dave G4GLT.

On 22 Apr 2021, at 09:59, Andy G4JNT <andy.g4jnt@...> wrote:

﻿
Just don't transmit with anyone standing in front of the dish - and if you feel you must, write down that that would be the case.   No need to rope off
You wouldn't have anyone standing there for 6 minutes while you're working anyway, would you.

Andy

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

Andy G4JNT

They're just stored at home
Which means any I bother to do may well will be written on the back of a used envelope in green ink

Everyone's all taking this far too seriously.

Andy

On Fri, 23 Apr 2021 at 10:23, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:
The thing that I don’t understand is what is done with the assessments. Are they stored at home  in case they are asked for or do they have to be submitted to Ofcom and updated when necessary.
Dave G4GLT.

On 22 Apr 2021, at 14:39, SAM JEWELL via groups.io <jewell=btinternet.com@groups.io> wrote:

﻿That should, of course read wary. Not unwary!
Sam

Sent tomorrow from my time machine

On 22 Apr 2021, at 14:30, SAM JEWELL via groups.io <jewell=btinternet.com@groups.io> wrote:

﻿Being a little unwary of computer EMF models/calculators for ICNIRP, I have been doing some measurements on my 23cm EME system, using a calibrated portable spectrum analyser and an antenna with a known antenna factor. The results have been interesting and do actually bear out some of the calculated numbers.
I felt that I would be happier knowing actual W/m2 levels at points of public access near my premises, on the bands above 1GHz, rather than relying completely on calculated values in a highly complex environment, where vegetation, fences and relative access height are all factors that affect the real levels.

73 de Sam, G4DDK

On 22 Apr 2021, at 14:06, Roger Ray via groups.io <g8cub=yahoo.co.uk@groups.io> wrote:

﻿I suggested before a simple pir to turn off the tx, if someone is standing in front of the dish. You would not be communicating then anyway.
Seems like a good idea particularly to stop someone looking down the waveguide, while you are sorting another problem.
I am intending to do some real life measurements against icnirp levels with a Narda s3 personal monitor, especially for the bands above 10GHz. As it will be near field the calculator is not a lot of use, even if it worked above 10G.
Roger
G8CUB

On 22 Apr 2021, at 10:08, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

﻿Thanks Andy,
Great. That makes it all seem a lot easier.
73
Dave G4GLT.

On 22 Apr 2021, at 09:59, Andy G4JNT <andy.g4jnt@...> wrote:

﻿
Just don't transmit with anyone standing in front of the dish - and if you feel you must, write down that that would be the case.   No need to rope off
You wouldn't have anyone standing there for 6 minutes while you're working anyway, would you.

Andy

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

Stephen Tompsett

Having done assessments for my home systems it would appear that at VHF/UHF/SHF most people should have little or no problem as long as the antenna is mounted at or above roof level, even when running full legal power...

This probably also applies for the higher HF bands using dipoles/yagis, where it may be possible to approximate the antenna as a point source, but the calculator appears to be totally inappropriate for 'long wire' and similar antennas.

On 23/04/2021 10:27, Andy G4JNT wrote:
They're just stored at home
Which means any I bother to do may well will be written on the back of a used envelope in green ink

Everyone's all taking this far too seriously.

Andy

On Fri, 23 Apr 2021 at 10:23, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:
The thing that I don’t understand is what is done with the assessments. Are they stored at home  in case they are asked for or do they have to be submitted to Ofcom and updated when necessary.
Dave G4GLT.

On 22 Apr 2021, at 14:39, SAM JEWELL via groups.io <jewell=btinternet.com@groups.io> wrote:

﻿That should, of course read wary. Not unwary!
Sam

Sent tomorrow from my time machine

On 22 Apr 2021, at 14:30, SAM JEWELL via groups.io <jewell=btinternet.com@groups.io> wrote:

﻿Being a little unwary of computer EMF models/calculators for ICNIRP, I have been doing some measurements on my 23cm EME system, using a calibrated portable spectrum analyser and an antenna with a known antenna factor. The results have been interesting and do actually bear out some of the calculated numbers.
I felt that I would be happier knowing actual W/m2 levels at points of public access near my premises, on the bands above 1GHz, rather than relying completely on calculated values in a highly complex environment, where vegetation, fences and relative access height are all factors that affect the real levels.

73 de Sam, G4DDK

On 22 Apr 2021, at 14:06, Roger Ray via groups.io <g8cub=yahoo.co.uk@groups.io> wrote:

﻿I suggested before a simple pir to turn off the tx, if someone is standing in front of the dish. You would not be communicating then anyway.
Seems like a good idea particularly to stop someone looking down the waveguide, while you are sorting another problem.
I am intending to do some real life measurements against icnirp levels with a Narda s3 personal monitor, especially for the bands above 10GHz. As it will be near field the calculator is not a lot of use, even if it worked above 10G.
Roger
G8CUB

On 22 Apr 2021, at 10:08, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

﻿Thanks Andy,
Great. That makes it all seem a lot easier.
73
Dave G4GLT.

On 22 Apr 2021, at 09:59, Andy G4JNT <andy.g4jnt@...> wrote:

﻿
Just don't transmit with anyone standing in front of the dish - and if you feel you must, write down that that would be the case.   No need to rope off
You wouldn't have anyone standing there for 6 minutes while you're working anyway, would you.

Andy

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

```--
Stephen Tompsett (G8LYB)
Tel: 01788 578940
Mob: 07956 855816```

Martyn G3UKV

My thoughts EXACTLY. Invisible ink even better.
73 Martyn G3UKV

On 23/04/2021 10:27, Andy G4JNT wrote:
They're just stored at home
Which means any I bother to do may well will be written on the back of a used envelope in green ink

Everyone's all taking this far too seriously.

Andy

On Fri, 23 Apr 2021 at 10:23, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:
The thing that I don’t understand is what is done with the assessments. Are they stored at home  in case they are asked for or do they have to be submitted to Ofcom and updated when necessary.
Dave G4GLT.

On 22 Apr 2021, at 14:39, SAM JEWELL via groups.io <jewell=btinternet.com@groups.io> wrote:

﻿That should, of course read wary. Not unwary!
Sam

Sent tomorrow from my time machine

On 22 Apr 2021, at 14:30, SAM JEWELL via groups.io <jewell=btinternet.com@groups.io> wrote:

﻿Being a little unwary of computer EMF models/calculators for ICNIRP, I have been doing some measurements on my 23cm EME system, using a calibrated portable spectrum analyser and an antenna with a known antenna factor. The results have been interesting and do actually bear out some of the calculated numbers.
I felt that I would be happier knowing actual W/m2 levels at points of public access near my premises, on the bands above 1GHz, rather than relying completely on calculated values in a highly complex environment, where vegetation, fences and relative access height are all factors that affect the real levels.

73 de Sam, G4DDK

On 22 Apr 2021, at 14:06, Roger Ray via groups.io <g8cub=yahoo.co.uk@groups.io> wrote:

﻿I suggested before a simple pir to turn off the tx, if someone is standing in front of the dish. You would not be communicating then anyway.
Seems like a good idea particularly to stop someone looking down the waveguide, while you are sorting another problem.
I am intending to do some real life measurements against icnirp levels with a Narda s3 personal monitor, especially for the bands above 10GHz. As it will be near field the calculator is not a lot of use, even if it worked above 10G.
Roger
G8CUB

On 22 Apr 2021, at 10:08, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

﻿Thanks Andy,
Great. That makes it all seem a lot easier.
73
Dave G4GLT.

On 22 Apr 2021, at 09:59, Andy G4JNT <andy.g4jnt@...> wrote:

﻿
Just don't transmit with anyone standing in front of the dish - and if you feel you must, write down that that would be the case.   No need to rope off
You wouldn't have anyone standing there for 6 minutes while you're working anyway, would you.

Andy

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

 Virus-free. www.avg.com

Robin Szemeti - G1YFG

Just ignore it, you won't get anything sensible or useful out of that spreadsheet for microwaves. The calculations don't work for parabolic reflectors at close range.  They assume a point source.  Do the calculations for 2W on a 10M  50dBi dish and it will tell you the safe distance as being 18m ... it should be (fairly) obvious that with 2W spread over a 10m dish, you could sit in the dish itself without harm for months.

To get a more sensible result, you should probably use the gain of the feedhorn, and look at the safe distance from the feedhorn, not the reflector ... assume a 10dBi gain on the feedhorn, and you get the safe distance as 0.2m ... so providing you don't spend too long transmitting while a member of the public has their head between the feedhorn and hte dish, you should be fine.

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

--
Robin Szemeti - G1YFG

Andy G4JNT

Robert, you didn't quote your frequency, but back-calculating I reckon your 10m dish is on 3.4GHz to give 50dBi
In which case the beam width is around 0.5 deg.
SO it's quite reasonable to suppose that at 18m in that narrow beam you are getting a few milliwatts per sq. cm.

You can work it out from first principles.  It's only basic physics.  But for high powers in that narrow beam the safe distance could well be some way off.   But they do need to be standing in it for 6 minutes - and even the Moon moves faster than that.

I agree for large dishes the feedhorn is the antenna.   Since a large dish will probably have a feedhorn designed for 10% spillover, use a value of -10dBi as the antenna gain  - unless you really believe someone will sit in the dish :-)

Andy

On Fri, 23 Apr 2021 at 19:53, Robin Szemeti - G1YFG <robin@...> wrote:
Just ignore it, you won't get anything sensible or useful out of that spreadsheet for microwaves. The calculations don't work for parabolic reflectors at close range.  They assume a point source.  Do the calculations for 2W on a 10M  50dBi dish and it will tell you the safe distance as being 18m ... it should be (fairly) obvious that with 2W spread over a 10m dish, you could sit in the dish itself without harm for months.

To get a more sensible result, you should probably use the gain of the feedhorn, and look at the safe distance from the feedhorn, not the reflector ... assume a 10dBi gain on the feedhorn, and you get the safe distance as 0.2m ... so providing you don't spend too long transmitting while a member of the public has their head between the feedhorn and hte dish, you should be fine.

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

--
Robin Szemeti - G1YFG

Andy G4JNT

Of course, the far field of that dish is at 2km, so that comment about a few mW / sq. cm at 18m isn't valid, and it'll still be spread out over a larger area.

Andy

On Fri, 23 Apr 2021 at 20:06, Andy Talbot <andy.g4jnt@...> wrote:
Robert, you didn't quote your frequency, but back-calculating I reckon your 10m dish is on 3.4GHz to give 50dBi
In which case the beam width is around 0.5 deg.
SO it's quite reasonable to suppose that at 18m in that narrow beam you are getting a few milliwatts per sq. cm.

You can work it out from first principles.  It's only basic physics.  But for high powers in that narrow beam the safe distance could well be some way off.   But they do need to be standing in it for 6 minutes - and even the Moon moves faster than that.

I agree for large dishes the feedhorn is the antenna.   Since a large dish will probably have a feedhorn designed for 10% spillover, use a value of -10dBi as the antenna gain  - unless you really believe someone will sit in the dish :-)

Andy

On Fri, 23 Apr 2021 at 19:53, Robin Szemeti - G1YFG <robin@...> wrote:
Just ignore it, you won't get anything sensible or useful out of that spreadsheet for microwaves. The calculations don't work for parabolic reflectors at close range.  They assume a point source.  Do the calculations for 2W on a 10M  50dBi dish and it will tell you the safe distance as being 18m ... it should be (fairly) obvious that with 2W spread over a 10m dish, you could sit in the dish itself without harm for months.

To get a more sensible result, you should probably use the gain of the feedhorn, and look at the safe distance from the feedhorn, not the reflector ... assume a 10dBi gain on the feedhorn, and you get the safe distance as 0.2m ... so providing you don't spend too long transmitting while a member of the public has their head between the feedhorn and hte dish, you should be fine.

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

--
Robin Szemeti - G1YFG

Robin Szemeti - G1YFG

Exactly,  if you assume the dish is evenly illuminated, (I know it's not)  but a 10m dia dish has an area of  ~785,000 cm^2 ... so with 2W  thats just 2 uW for cm^2 ... yes, it might be 0,5 degree divergence, but it starts off as a very wide beam! ... as the dish surface is already well below the limit, you don't need to calculate further.

On Fri, 23 Apr 2021 at 20:10, Andy G4JNT <andy.g4jnt@...> wrote:
Of course, the far field of that dish is at 2km, so that comment about a few mW / sq. cm at 18m isn't valid, and it'll still be spread out over a larger area.

Andy

On Fri, 23 Apr 2021 at 20:06, Andy Talbot <andy.g4jnt@...> wrote:
Robert, you didn't quote your frequency, but back-calculating I reckon your 10m dish is on 3.4GHz to give 50dBi
In which case the beam width is around 0.5 deg.
SO it's quite reasonable to suppose that at 18m in that narrow beam you are getting a few milliwatts per sq. cm.

You can work it out from first principles.  It's only basic physics.  But for high powers in that narrow beam the safe distance could well be some way off.   But they do need to be standing in it for 6 minutes - and even the Moon moves faster than that.

I agree for large dishes the feedhorn is the antenna.   Since a large dish will probably have a feedhorn designed for 10% spillover, use a value of -10dBi as the antenna gain  - unless you really believe someone will sit in the dish :-)

Andy

On Fri, 23 Apr 2021 at 19:53, Robin Szemeti - G1YFG <robin@...> wrote:
Just ignore it, you won't get anything sensible or useful out of that spreadsheet for microwaves. The calculations don't work for parabolic reflectors at close range.  They assume a point source.  Do the calculations for 2W on a 10M  50dBi dish and it will tell you the safe distance as being 18m ... it should be (fairly) obvious that with 2W spread over a 10m dish, you could sit in the dish itself without harm for months.

To get a more sensible result, you should probably use the gain of the feedhorn, and look at the safe distance from the feedhorn, not the reflector ... assume a 10dBi gain on the feedhorn, and you get the safe distance as 0.2m ... so providing you don't spend too long transmitting while a member of the public has their head between the feedhorn and hte dish, you should be fine.

On Thu, 22 Apr 2021 at 09:52, Dave via groups.io <davidnewman55=icloud.com@groups.io> wrote:

Hello,
I have done my calculation for 10ghz portable using 5 watts and a 48cm dish with waveguide feed.
I hadn’t realised just how good waveguide is as regards losses are concerned. My duty cycle is quite low.
The separation distance given at the end is 1.8 metres. I presume that the only reliable way of achieving this is to
rope off a radius of 1.8m in the favoured directions. Any comments?
73,
Dave G4GLT

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Robin Szemeti - G1YFG

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Robin Szemeti - G1YFG

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