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(1981) says: "In the close proximity of sources, the safety critera are no
longer valid because the value of |E|^2 is not directly related to power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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