His parents are very supportive and are glad he has become interested in a
hobby where he is receiving the help and assistance and information from other
hams and not doing something else he could be. He got interested, read about
it and got his license almost entirely on his own and is now asking a lot of
questions. His mother is just thinking and asking questions. I was in the
process of preparing a presentation on RF Safety to the club and had read the
OET65 some time ago and was just going over the Supplement B again when this
whole thing came up and Ryan came to our club. I showed her OET65 and Sup B
and OET56 and the URLs where she could get copies of them and my scratch sheet
from my station with references to pages and figures where the figures came
from. I explained the effect of variation of frequencies, power levels, and
antenna gain and the definition of and difference between controlled and
uncontrolled and she is now comfortable with his intended installati
on. I also explained the better installation of getting it up in the air if
possible. She is going to give the task of research and evaluation to Ryan and
monitor the process but with my explanation to her, is now comforted that there
will not be a problem and knows the safe distances. for his equipment. Nice to
run into intelligent, questioning , non-confrontational, supportive parents
dedicated to their kids once in a while.
Try this: ask the next ham you see if he/she has any documentation (scratch
paper or ANYTHING) showing they have done or do not need to do the evaluation
or even thought about it. Most likely you will get a "what"? Don't be
surprised. I know, you signed or will have to sign when you get or renew your
license that you have read and understand the regulations.
Excuse the soap box please,
Tnx es 73, de Jim KG0KP
----- Original Message -----
From: Jim Lux
To: Jim Miller ; TOWERTALK Reflector
Sent: Thursday, June 29, 2006 12:14 PM
Subject: Re: [TowerTalk] Antenna gain
At 08:29 AM 6/29/2006, Jim Miller wrote:
What is the gain of a J-Pole, dBi or dBd? About 3 dBi? More? Less?
A J pole is basically an endfed dipole, so I'd expect the gain to be in the
2-3 dBi range..
Talk about stepping DOWN from towers and beams. BUT this IS an antenna
question.
A new ham who just had his 12th birthday at field day and received a 2
meter radio now has a J-Pole antenna given to him until he gets something
better. The question about RF exposure comes from his parents.
How much power is he running? I assume he's done the analysis (aka "routine
evaluation") laid out in the OET Bulletin 65B?
(http://www.fcc.gov/oet/rfsafety). Following the stuff in the bulletin (such
as doing the worksheet in the back) will go a lot towards assuaging parental
concerns: more so than a casual "well, it looks ok to me".
In particular, take a look at Table 4b on page 24 (page 29 in the pdf). For
an FM rig, PEP = rig output power.
Looks like for 50 Watts, and a 3 dBi antenna, it's 1.4 or 3.2 meters,
depending on controlled or uncontrolled.
Make sure he understands the difference between controlled and uncontrolled
exposures. You only get to use controlled if everyone is aware of the exposure,
etc.. This may or may not be the case for family members, especially where the
control op is young. {i.e. there's a big difference between me (as dad)
operating the radio, being aware of who's in the house, where they are, what
the exposure rules are, etc.; and my 12 year old daughter, who may get
concentrated on radio'ing, and ignoring where her pesky 9 year old sister is)
Those numbers are based on a farfield approximation. Lest you think that
"hey, my duty factor is only 20%, so I can cut those numbers by a factor of
sqrt(5)", you've got to start thinking about the near field. As Balzano (1981)
says: "In the close proximity of sources, the safety critera are no longer
valid because the value of |E|^2 is not directly related to power flow, but,
rather, to the level of electric energy stored around the radiator."
The upshot is that unless you're actually measuring the field, you should use
conservatism and lots of worst case assumptions. Don't bolt that jpole to the
table the radio is on. Put it up on the roof where people will be 10 ft
away.
In the 2 m band, you're looking at a field strength limit of 61.4 V/m
(controlled) or 27.5V/m (uncontrolled). Compare this to measurements made by
the FCC/EPA for a "
Whip on auto roof" at 146.5MHz with
100W:
22-75 V/m at 2 m from
antenna
15-30 V/m In
vehicle
90 V/m in
Rear seat
This is for an antenna that's nominally about 1 dBi. That jpole is higher gain:
more energy stored in the near field and higher power densities in some
directions in the far field. Note that even 2 m away, you're still above the
uncontrolled exposure limit (granted, nobody is going to talk for 6 (or 30)
minutes straight, but....)
Jim Lux, W6RMK
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