RF Exposure limits

Mon Aug 5 22:39:48 EDT 1996

Hans Brakob wrote:
> 
> (B)   Limits for General Population/Uncontrolled Exposure
> ____________________________________________________________
> 
> Frequency     Electric Field          Magnetic Field
> Range         Strength    Strength    Power Density
> 
> (MHz)         (V/m)       (A/m)       (mW/cm2)    (minutes)
> ____________________________________________________________
> 
> 0.3-1.34      614         1.63        (100)*          30
> 1.34-30       824/f       2.19/f      (180/f2)*       30
> 30-300        27.5        0.073       0.2             30
> 300-1500      --          --          f/1500          30
> 1500-100,000  --          --          1.0             30
> ____________________________________________________________
> f = frequency in MHz
> * = Plane-wave equivalent power density

before we all go running off to dust off old text books and slide
rules, or to buy frequency sensitive voltmeters can someone give
us a plain language breakdown in amateur terms about what these
mean?  maybe a table with some typical antennas at typical heights?
in particular maybe some monobanders at 30, 60, and 100' at a distance
of 50', 100', 1/4mi?  and some low band ants like an inverted v and
ground mounted vertical might be nice.  probably the most restrictive
would be the vhf/uhf yagis, so some general examples of them would
probably be good.  i'm sure we can all scale them for different gains
of yagis and power levels, and its probably only the main lobes of
the yagis that will matter so it shouldn't be too big of a project for
someone with the proper program.

some basic questions that are not obvious from the tables need to 
be answered also... like what is the time period, 6 or 30 min/day??
or that long at a time, then time to 'cool off'???  also, are all
the field strengths and power densities really the same, or would 
different limits be reached first in different circumstances, like
near the base of a 160m vertical would you reach the e field limit
first or are they just different ways of stating the same power level?

-- 
ky1h at berkshire.net   or   robbins at berkshire.net
http://www.berkshire.net/~robbins/ky1h.html

>From k6sti at n2.net (Brian Beezley)  Mon Aug  5 23:36:51 1996
From: k6sti at n2.net (Brian Beezley) (Brian Beezley)
Date: Mon, 5 Aug 1996 15:36:51 -0700
Subject: RF-Exposure Calculations
Message-ID: <199608052236.PAA03753 at ravel.n2.net>

I ran some quick near-field calculations for 1500 watts into a 40-meter
dipole 30 feet above perfectly conducting ground using AO.

The electric field gets larger as you get away from ground (the field
roughly doubles when you double the measurement height above perfectly
conducting ground).  In contrast, near ground the magnetic field is almost
independent of measurement height.  I picked a spot 10 feet above ground
directly underneath the antenna as a conservative, worst-case exposure
geometry for pedestrians.  Here are the numbers:

Uncontrolled Limits:  824/f V/m =>  824/7 =>  118 V/m at 7 MHz
                     2.19/f A/m => 2.19/7 => 0.31 A/m at 7 MHz

Uncontrolled limits:         118   V/m   0.31 A/m  (peak or RMS?)
Fields 10' under 40m dipole:  12.4 V/m   0.17 A/m  Peak fields

Unfortunately, the information posted on the contest reflector doesn't say
whether the standard specifies peak or RMS fields.  It does say the fields
are averaged over a 30-minute period for an uncontrolled environment.
Unless you're into long-winded RTTY QSOs, you can decrease the field
strengths calculated above roughly by a factor of 4 for CW (50% keying, 50%
TX/RX), and somewhat more for SSB depending on your speech-compression level.

The electric field calculated above is about 20 dB below the worst-case RTTY
limit but the magnetic field for this particular case is uncomfortably close. 

NOTE:  Near-field calculations like these ignore some important, real-world
factors:

1.  Lossy ground will increase the electric-field strength for horizontal
antennas.

2.  Structures, terrain, and incidental conductors can create hot spots many
dB above calculated values.

Things are much better 100 feet away (say, at your neighbor's house):

Uncontrolled limits:                 118   V/m   0.31  A/m
Fields 10' up, 100' away, broadside:   2.0 V/m   0.013 A/m
Fields 10' up, 100' away, off ends:    4.7 V/m   0.011 A/m

I'll try some more antennas and frequencies and report back.  But at this
point I'd say if I were a lawyer, I could really raise havoc with this new
rule on behalf of someone who wanted to force an active contester off the
air.  Even with high permanent antennas, I could ask a lot of nasty
questions about key-down tests of temporary low wires, fields from radials
that come near neighbors' houses, coupling into power and telephone lines,
etc.  Put yourself in the place of an angry neighbor and use your imagination.

I hope MFJ and AEA come out with affordable, accurate, calibrated
field-strength meters that hams can use to defend themselves against
allegations arising from this new FCC rule.  The commercial meters are
really expensive.  K6JYO, an ex-rep for Narda, tells me the appropriate
field-strength meter costs $6000!

Brian Beezley, K6STI
k6sti at n2.net

>From k6sti at n2.net (Brian Beezley)  Tue Aug  6 00:07:24 1996
From: k6sti at n2.net (Brian Beezley) (Brian Beezley)
Date: Mon, 5 Aug 1996 16:07:24 -0700
Subject: 10m RF Calcs
Message-ID: <199608052307.QAA05779 at ravel.n2.net>

Here are calculated near-fields for 1500 watts key-down into a 6-element 10m
Yagi on a 33-foot boom.  This antenna has a calculated free-space gain of
9.24 dBd.

Uncontrolled exposure limits:  28.9 V/m  0.077 A/m (peak or RMS?)

Antenna 30' high, measurement
  point 6' above ground and
  40' in front of antenna:     35.5 V/m  0.080 A/m  peak field

Antenna 60' high, measurement
  point 6' above ground and
  80' in front of antenna for 
  E and 90' in front for H:    17.8 V/m  0.038 A/m  peak field

These numbers are not encouraging because a real-world measurement could be
many dB worse.

Brian Beezley, K6STI
k6sti at n2.net

>From ke6ber at tiac.net (Alfred J. Frugoli, KE6BER/1)  Tue Aug  6 00:26:56 1996
From: ke6ber at tiac.net (Alfred J. Frugoli, KE6BER/1) (Alfred J. Frugoli, KE6BER/1)
Date: Mon, 5 Aug 1996 19:26:56 -0400
Subject: TopBand Antena Question
Message-ID: <v01540b00ae2bf73c3e96@[206.119.237.35]>

In the book Simple, Low-cost Wire Antennas by William Orr (W6SAI) and
Stuart Cowan (W2LX) there is mention of a "Twin-Lead Marconi for 80 or 160
Meters" (if you happen to own the book it's on p 103-4).  The antenna looks
much like an inverted L, but it uses twin lead for a good legnth of the
antenna.  The dimensions are 109 feet of twin lead and 17 feet of single
wire for the low segment of 160M.  One conductor of the twin lead is
connected to the center of the coax, the other is connected to the shield.
The twin lead is shorted at the far end from the base of the antenna and
terminates in the 17 feet of single wire.  The twin lead acts as a kind of
linear impedance matching transformer, and the base is said to be a good
impedance match for 50 ohm transmission line.

I have a couple of questions:

1) The book makes no mention of weather or not it matters what the ratio of
vertical/horzontal is.  Will this matter?  (the fig in the book shows them
to be about equil, which is what my situation would be)

2) Is this antenna better/worse than using a regular inverted L with some
kind of matching network (L net?) at the base?

I do not have any modelin software at this point, so can't do any of those
neat things myself at this point.  Also, this antenna is so attractive
becuase I have 1000 feet of surplus 300 ohm twinlead that I got for free
sitting in the shed, which would cut down my cost for parts some.  Thanks
for your info.  Please reply directly and I'll summarize if there is
interest.

Al, KE1FO (ex. KE6BER), ke1fo at contesting.com, ke6ber at tiac.net
                        http://www.tiac.net/users/ke6ber