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Author Topic: RF Safety Calculator  (Read 1448 times)
KB1TJY
Member

Posts: 257




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« on: January 11, 2013, 09:12:07 AM »

With respect to the RF Safety Calculator as found at http://hintlink.com/power_density.htm

If you have a vertical antenna, in my case a Hustler 6BTV, what value would you use for the field "The antenna gain in dBi"?

Thanks,

Lyman
KB1TJY
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WB2WIK
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Posts: 20611




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« Reply #1 on: January 11, 2013, 09:33:27 AM »

You could use 2.2 dBi for this (that's 0 dBd, and probably "worst case"). 
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KB1TJY
Member

Posts: 257




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« Reply #2 on: January 11, 2013, 10:05:31 AM »

You could use 2.2 dBi for this (that's 0 dBd, and probably "worst case"). 
Many thanks!

Lyman
KB1TJY
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KB1TJY
Member

Posts: 257




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« Reply #3 on: January 11, 2013, 07:21:44 PM »

It states at http://hintlink.com/power_density.php that:

"If you wish to estimate the power density at a point below the main lobe of a directional antenna, and if the antenna's vertical pattern is known, recalculate using the antenna's gain in the relevant direction."

How does one know the value of that gain with a vertical? Is it within the manufacturers specifications? Mine is a Hustler 6BTV.

Thanks again.
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WB6BYU
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Posts: 13336




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« Reply #4 on: January 11, 2013, 08:50:06 PM »

That might be the case if, for example, you had an 11-element yagi at 60', and
wanted to claim that most of the power was passing over the neighbor's house
rather than through it.  In that case you would use the vertical radiation
pattern for the antenna (either from the manufacturer, by modeling, or by
direct measurement) to determine the actual gain towards the target area.

But that isn't an issue with a common vertical, unless it is quite high in the air.

For a ground mounted vertical, the main radiation (at least at the distances of
interest) will be along the ground, straight into the ground floor of any nearby
buildings.  You could, I suppose, argue that the electromagnetic field off the top
of the antenna was lower than at ground level, in case you were going to mount
a bird house on top and were calculating the exposure level for the occupants.

For a common 1/4 wave vertical (and the Hustler is certainly close enough
for this purpose) the actual signal level will depend on the ground losses, etc.,
because those affect the efficiency, and therefore the actual radiated power.
But you're safe if you assume 2.2dBi gain (the figure for a dipole), since it is
unlikely that you will exceed that in practice.  If you find that you are one or
two feet short on clearance, then it may be worthwhile taking more precise
measurements, which probably will result in a smaller exclusion zone, but if
the dipole numbers give you enough clearance, there is no sense in doing
extra work.
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K2DC
Member

Posts: 1374


WWW

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« Reply #5 on: January 12, 2013, 03:51:15 AM »

Good advice from all here.  Before retiring, I did many of the radiation safety calculations for radar installations, and best practice is always starting with a conservative estimate.  If and when you find yourself with an exclusion zone that's difficult to deal with, you can look into mitigating factors like gain measurements, pattern effects, scattering, etc.  2.2 dBi is certainly higher than you're likely to get from a typical ground-mounted vertical, but it's as good a place to start as any.

73,

Don, K2DC
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KE3WD
Member

Posts: 5689




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« Reply #6 on: January 12, 2013, 08:19:32 AM »

I squared R. 
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KB1TJY
Member

Posts: 257




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« Reply #7 on: January 13, 2013, 11:08:58 AM »

That might be the case if, for example, you had an 11-element yagi at 60', and
wanted to claim that most of the power was passing over the neighbor's house
rather than through it.  In that case you would use the vertical radiation
pattern for the antenna (either from the manufacturer, by modeling, or by
direct measurement) to determine the actual gain towards the target area.

But that isn't an issue with a common vertical, unless it is quite high in the air.

For a ground mounted vertical, the main radiation (at least at the distances of
interest) will be along the ground, straight into the ground floor of any nearby
buildings.  You could, I suppose, argue that the electromagnetic field off the top
of the antenna was lower than at ground level, in case you were going to mount
a bird house on top and were calculating the exposure level for the occupants.

For a common 1/4 wave vertical (and the Hustler is certainly close enough
for this purpose) the actual signal level will depend on the ground losses, etc.,
because those affect the efficiency, and therefore the actual radiated power.
But you're safe if you assume 2.2dBi gain (the figure for a dipole), since it is
unlikely that you will exceed that in practice.  If you find that you are one or
two feet short on clearance, then it may be worthwhile taking more precise
measurements, which probably will result in a smaller exclusion zone, but if
the dipole numbers give you enough clearance, there is no sense in doing
extra work.
Excellent, thank you.

73's,
Lyman
KB1TJY
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KB1TJY
Member

Posts: 257




Ignore
« Reply #8 on: January 13, 2013, 11:09:37 AM »

Everyone, thank you for the excellent input. As always, greatly appreciated!

73's,

Lyman
KB1TJY
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