Near Field Calculations
 

I'm trying to squeeze a 75m inverted V into my backyard. My
neighborhood is your basic suburban ghetto so there isn't much room
between homes (like 10m behind me and less to either side).
Anyway, am using EZNEC4 to get an idea of how much power I can run. I
am worried about the neighbors exposure level. EZNEC has a serious
disclaimer about using the program to determine compliance. I
understand the vagaries in accurately modeling the near field and I
understand that the author does need law suits. On the other hand, I
want an idea of how much trouble I have and if I need to buy an
accurate field meter (or if I need to move!). Anyway, the disclaimer
makes it sound as if EZNEC4 is worrthless. I assume that it is as
accurate as the inputs and assumptions. Right?

On the other hand, if your worried about exposure levels, what do you
do to bracket the problem?

jimg
jimg
Oregon
USA

Near Field Calculations
 

The purpose of the EZNEC disclaimer is to prevent people from using
EZNEC data as evidence that some level of exposure is or is not safe for
personal exposure, for example in a lawsuit. EZNEC results are allowed
for use in determining compliance with FCC RF exposure standards.

You're absolutely correct that EZNEC results are only as good as the
model and the skills of the person creating it.

I think you'll find that readings you get from an "accurate field meter"
can easily be variable and contradictory, and a truly accurate one is
likely to be very expensive.

If you're worried about exposure levels, the first step would be to try
and determine what level is truly unsafe. To my knowledge, no one has
ever established this, but history has shown us time and again that this
has nothing to do with what a jury is likely to decide. A good place to
start the learning process is
http://www.arrl.org/news/rfsafety/exposure_regs.html. Among other
information, it reveals the relationship among monkeys, bananas, and the
FCC's conclusions about what constitutes a safe exposure level. Good luck!

Roy Lewallen, W7EL

jimg wrote:
I'm trying to squeeze a 75m inverted V into my backyard. My
neighborhood is your basic suburban ghetto so there isn't much room
between homes (like 10m behind me and less to either side).
Anyway, am using EZNEC4 to get an idea of how much power I can run. I
am worried about the neighbors exposure level. EZNEC has a serious
disclaimer about using the program to determine compliance. I
understand the vagaries in accurately modeling the near field and I
understand that the author does need law suits. On the other hand, I
want an idea of how much trouble I have and if I need to buy an
accurate field meter (or if I need to move!). Anyway, the disclaimer
makes it sound as if EZNEC4 is worrthless. I assume that it is as
accurate as the inputs and assumptions. Right?

On the other hand, if your worried about exposure levels, what do you
do to bracket the problem?

jimg
jimg
Oregon
USA

Near Field Calculations
 

thanks. yes, i agree measuring the field directly is very tricky and
the answer changes daily! Anyway, I've read the ARRL reference before
and my own experience and basic conservativism leads me to be very
careful. Anyway, the law. the OET65 tables, and EZNEC clearly will
allow me to run 500W PEP ssb.
I'll run about 250Wpep. This leaves me below 1/10 the uncontrolled
limit (ignoring duty cycle) except directly underneath and in-line
with the doublet (which by chance is difficult to get near).

Thanks for the time to respond.

jimg

The purpose of the EZNEC disclaimer is to prevent people from using
EZNEC data as evidence that some level of exposure is or is not safe for
personal exposure, for example in a lawsuit. EZNEC results are allowed
for use in determining compliance with FCC RF exposure standards.

You're absolutely correct that EZNEC results are only as good as the
model and the skills of the person creating it.

I think you'll find that readings you get from an "accurate field meter"
can easily be variable and contradictory, and a truly accurate one is
likely to be very expensive.

If you're worried about exposure levels, the first step would be to try
and determine what level is truly unsafe. To my knowledge, no one has
ever established this, but history has shown us time and again that this
has nothing to do with what a jury is likely to decide. A good place to
start the learning process is
http://www.arrl.org/news/rfsafety/exposure_regs.html. Among other
information, it reveals the relationship among monkeys, bananas, and the
FCC's conclusions about what constitutes a safe exposure level. Good luck!

Roy Lewallen, W7EL

jimg wrote:
I'm trying to squeeze a 75m inverted V into my backyard. My
neighborhood is your basic suburban ghetto so there isn't much room
between homes (like 10m behind me and less to either side).
Anyway, am using EZNEC4 to get an idea of how much power I can run. I
am worried about the neighbors exposure level. EZNEC has a serious
disclaimer about using the program to determine compliance. I
understand the vagaries in accurately modeling the near field and I
understand that the author does need law suits. On the other hand, I
want an idea of how much trouble I have and if I need to buy an
accurate field meter (or if I need to move!). Anyway, the disclaimer
makes it sound as if EZNEC4 is worrthless. I assume that it is as
accurate as the inputs and assumptions. Right?

On the other hand, if your worried about exposure levels, what do you
do to bracket the problem?

jimg
jimg
Oregon
USA

jimg
Oregon
USA

Near Field Calculations
 

I'll run about 250Wpep. This leaves me below 1/10 the uncontrolled
limit (ignoring duty cycle) except directly underneath and in-line
with the doublet (which by chance is difficult to get near).

Do not ignore duty cycle. It is one of the most important factors involved.
For someone like myself, who listens a lot on CW and does not transmit much,
the duty cycle is something like 1% and this makes a huge difference in the
exposure results.

250 watts PEP (with reasonable compression) and a talk time of perhaps 25%
probably yields a duty cycle less than 10%. (Someone else will probably
have a real formula for PEP x compression x average voice.) The averaging
time is not always clear in the exposure formulas, and this could make a
large difference. What is your duty cycle averaged over 24 hours?

Bill
W2WO