New F.C.C. Maximum Permissible RF Exposure (MPE) Rules for Dummies MUST READ!

[W1RC]

This goes into effect tomorrow, Monday May 3rd, 2021 and is applicable to ALL radio,amateurs.

I have posted a short article that distills all the technical, legal and bureaucratic verbiage relative to this matter down to one page explaining it in understandable terms and showing you how to calculate the numbers you will need to keep with your station documents in compliance with F.C.C. Maximum Permissible Exposure (MPE) rules and requirements that now affect ALL radio amateurs.

www.near-fest.com

In reality, when all is said and done, it is merely some technical and legal housekeeping that is easier to perform than it is to explain and describe.

73,

Michael Crestohl, W1RC/VE2XL
New England Amateur Radio Festival, Inc.
aka NEAR-Fest......
Join us for our “Grand Homecoming” 
Friday October 15th and Saturday, October 16th 2021.

[K1JJ]

Very good, Mike and Dan!

Dan I watched your video. And Mike, I see a lot on the NearFest site that you wrote up.  Instead of thinking about it for the next two years, I decided to do it now and get it out of the way.

The ARRL site has a good RF calculator link that I included below. It uses dBi for antenna radiation calculations.  I did all of my antennas in less than 30 minutes, made some full report copies and placed one at the operating position next to the FCC rulebook in case of a visit....  Find below a sample of my 75M antenna/radiation description and calculations included in the report to get the general idea of how to do each band...  

Easy as pie.


ARRL site calculator link:   (Lake Washington Ham Club)

http://www.lakewashingtonhamclub.org/resources/rf-exposure-calculator/



My own description and field strength calculations for 75M taken from the report:


75M

The antenna is a pair of wire delta loops (2 elements) on a 40' boom at 190' high. (12 dBi)  I also have a flat dipole (6 dBi) at 90' high.  I used 1500W average power at the antenna to be conservative since the duty cycle of CW, SSB and AM is less than 1500W.  Antenna gain was calculated using NEC4 software. Both of these antennas are at least 230' from the operating position fed with underground 3/4" CATV hardline.  The public street is about 300' away from any antenna. The nearest neighbor's house  is ¼ mile away. This is a safe installation since the minimum uncontrolled environment safe distance is calculated to be 20.4' away from the antenna.

Average Power at Antenna (watts): 1500W
Antenna Gain (dBi):12
Operating Frequency (Mhz):3.8
Include Effects of Ground Reflections -  Yes

Calculate

Results for a controlled environment:
Maximum Allowed Power Density (mw/cm2): 62.3269
Minimum Safe Distance (feet): 9.1455'

For an uncontrolled environment:
Maximum Allowed Power Density (mw/cm2):12.4654
Minimum Safe Distance (feet): 20.4500'   SAFE



All finished and put to bed. Like the feeling you get after finishing your taxes...  

T

[W1RC]

Time for a tune.......how about FeeCee Law!

[W1RC]

The ARRL site has a good RF calculator link that I included below. It uses dBi for antenna radiation calculations.  I did all of my antennas in less than 30 minutes, made some copies and placed one at the operating position next to the FCC rulebook in case of a visit....  Find below a sample of my 75M antenna/radiation description and calculations included in the report to get the general idea of how to do each band...  

Wise move, doing it now.  You won’t have to do it again until you make any changes to your antenna or transmitter.

Our article links to the Lake Washington Club calculator because it is the simplest one but it isn't perfect.  There are some preliminary calculations required to determine “average RF power” which factor in duty cycle, whether RF processing is used for SSB, as well as percentage of time when transmitter is actually transmitting.  The calculator doesn’t do those,  This is outlined in the tables attached to the article.  

The final figures will be “ballpark” and will vary somewhat from everyday use but this whole schlemazel is really only to comply with FeeCee law from which we will no longer be exempt in two years.The reality is that most HF stations running full legal strap will be perfectly safe.  The ones that probably will not be in compliance will be mobiles on 2m running over 50 Watts because the distances from the antenna to other humans will be very close, like the driver in the car beside you.  However this will be for very short duration and probably not matter at all.  Besides you could never accurately recreate those scenarios.

[W1RKW]

the fee cee should focus on the neighborhood 11 meter op running a dirty 10kW

[K1JJ]

The final figures will be “ballpark” and will vary somewhat from everyday use but this whole schlemazel is really only to comply with FeeCee law from which we will no longer be exempt in two years. The reality is that most HF stations running full legal strap will be perfectly safe.  The ones that probably will not be in compliance will be mobiles on 2m running over 50 Watts because the distances from the antenna to other humans will be very close, like the driver in the car beside you.  However this will be for very short duration and probably not matter at all.  Besides you could never accurately recreate those scenarios.

Yep, my thoughts exactly.   I look at it as an exercise in awareness, rather than guessing about it.  

I just use 1500W for all calculations and don't bother with the duty cycle. If it passes 1500W full duty, it will pass any mode conservatively.

BTW, I did some "extreme" calculator tests using 2500W (10KW * SSB 25% duty cycle)  and a 10M Yagi stack giving 15 dBi  and found it needed about 170' of minimum distance. So hams generally don't need to be concerned  running 1500 pep up to about 10M.   But 6M and above may enter the zone of wetness with some "close quarters" QRO operations and large arrays when beaming in certain directions.

Generally 160-40M is of no concern and neither is 20-10M when using modest stations, but 20M and above needs to be figured out case by case when running QRO and high gain arrays in close proximity.   Personally I don't think there is any danger with HF radiation at ham levels since there are so many broadcast engineers and hams from the 1920's to 1950's who lived well into their 90's in a time when shielding was not a common practice.

T

[w9jsw]

A few questions -

Is there a good published resource for "average" antenna gain numbers that take into account height above ground?

Say for instance an inverted V dipole at 40ft peak on 80M would be xx dBi?

Assume that a fan dipole setup for 80/40/20/15 can be treated as 4 separate dipoles?

On a Inverted-L, since you can approach the base of the antenna quite close, it becomes more interesting to understand the MSD zone? Do you also consider the radial field? Is the safe distance from the base or from the edge of the radial field?

John

[K1JJ]

Hi John,

Generally, making a model of the antenna in NEC4/EZNEC or equivalent will answer most of your questions.

I use 6 dBi for standard 1/2 wave dipoles and 12 dBi for 3el Yagis or 2 el wire loops.   11dBi is good for 2el Yagis ,  8.5dBi for two half waves in phase.

Yes, the fan dipole can usually be treated as separate dipoles, but the real model is more complex than that since on some freqs some elements take more power than others and the lobes get like an octopus on the higher bands. NEC4/EZNec it if you have the inclination, but not really necessary.

But be conservative and lean towards making the antenna gain a little higher than it really is and use 1500W full duty cycle and you will probably still be well within radiation limits on HF.

The average height of the antenna (stacked arrays included)  is generally where to figure the distance away starting point.   But again, on 160-10M running 1500W, you will likely always be well into safe limits, so no need to get anal about it.

I know of a guy up in VT running  three stacked 6M Yagis on 60' booms, running a  full 1500W. Now he is someone who has probably already done this exercise and should be concerned when beaming in the direction of his own shack...  

I agree with Dan's video where he says the only really accurate way to measure field strength is with one of those calibrated field strength meters costing $900 as used by the pros. Lots of testing and walking. Waste of money once used and put on the shelf though.



T

[w9jsw]

EZNEC - probably should explore how to use it. Good advice.

What about the vertical?

[K1JJ]

EZNEC - probably should explore how to use it. Good advice.

What about the vertical?

John,

The dBi gain of a 1/4 wave vertical on a perfect groundplane  is 2.1dBi -    (as compared to a vertical isotropic - in free space)  

Your inverted L (1/2 wave total length) will change this with the horizontal leg producing some  higher angle gain in a bi-directional pattern.  EZNEC will show this.

If you wanted to estimate, figure your inverted L  as ~3.7 dBi  broadside to the horizontal wire and you should be close enuff for this reporting purpose.

T



Update:   I snagged this inverted L plot off the web. Notice the inverted L's high angle influence compared to a straight vertical.