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Author Topic: 440mhz RF Hazard  (Read 669 times)
AC0DQ
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« on: July 29, 2009, 02:21:07 PM »

Folks...If there is 100w ERP at the antenna at 440mhz...what is the hazard?  What is a safe dwell time in front of antenna, say at 1 meter?  Thanks! AC0DQ/Ken
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N3OX
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« Reply #1 on: July 29, 2009, 02:43:29 PM »

http://hintlink.com/power_density.htm

For 100W ERP you can either put in the actual power and the antenna gain, or the 100W ERP and 0dBi... seems to make no difference, except that for a directional antenna, the hazard is significantly less in some other direction.

You should figure out the average power correctly.  If FM, the average power and the PEP power are the same, so 100W is appropriate, and you're not in compliance with someone 1 meter away in an uncontrolled environment, but you are in compliance in a controlled (i.e. you or another ham who realizes that RF is being emitted any time it is on and so is "controlling" their own exposure) environment.

As far as "dwell time" goes, it's on the 16th page of this PDF file:

http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet65/oet65.pdf

I don't think you have much to worry about regulation-wise unless people who don't know about the RF are going to be that close to the antenna.


73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KB1LKR
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Posts: 1899




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« Reply #2 on: July 29, 2009, 07:56:54 PM »

If you do not already have a copy, see also: http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet65/oet65b.pdf

Bulletin 65b covers amateur radio applications in particular, vs the broader Bulletin 65.
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AC0DQ
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« Reply #3 on: July 30, 2009, 10:09:22 AM »

Exactly the stuff I needed gentlemen!  We're good to go now.
Thanks!
73, Ken, AC0DQ
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W6RMK
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« Reply #4 on: July 31, 2009, 09:49:56 AM »

One caveat on those online calculators.. they generally assume you are in the "far field" of the antenna.  1 meter from a 440MHz antenna (wavelength of 70cm) is probably not in the far field.  

Note also that if you exceed the "safe harbor" power in the FCC rules for amateur stations (and described pretty well in OET 65B), you need to actually do the assessment/analysis required (or measure).

Watch out for the "multiple transmitter" rule.. the power output safe harbor only applies if you have one transmitter on at a time (in ANY band, not just ham transmitters.. That cellphone or wireless access point is also a transmitter. If the second transmitter is more than 5% of the total RF radiated power, then you have to include it in your analysis.)
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KB1LKR
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« Reply #5 on: July 31, 2009, 01:18:10 PM »

Though 5% of 50W (the threshold for assessment for 10m & and VHF) is 2.5W, and 5% of your 100W is 5W, well over the output of part 15 devices, though a Part 95 radio (GMRS, CB, etc.) could be an issue.

I'm not sure I'd want to spend *any* time only 1m from a 100W ERP UHF source!
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KB9CRY
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« Reply #6 on: July 31, 2009, 01:44:20 PM »

You can calculate that yourself.  Google RD exposure calculation for the Texas Univ online calculator.
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KB9CRY
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« Reply #7 on: July 31, 2009, 01:44:45 PM »

I meant to type RF Exposure online calculator.
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