[PLUG] 500mW transmitters (was) USB Wifi - My Essentials...

Keith Lofstrom keithl at kl-ic.com
Mon Oct 13 07:50:36 UTC 2008


On Mon, Oct 06, 2008 at 02:32:08PM -0700, Marc Wolsiffer wrote:
> 
> Remember a 500mw is illegal if you're running it in Oregon. 
> 100mw is the legal limit and if found my neighbor saw you with a 500mw
> transmitter you would have investigation going on, she doesn't even hold
> cell phones to her head. The antenna doesn't look like it can support a
> clean 500mw TX anyways. I would go with a directional antenna at a lower TX
> rate and/or a good repeater. DD-wrt can help boot your range a ton.  I set
> have set-up government wifi solutions for a while with RADIUS servers and
> splash pages. Don't fry our children :)

I assume you are joking - perhaps if I understood your syntax I 
would understand your point.

1)  The FCC regulates radios, not the states.  If the states tried,
they would probably be sued by the manufacturers, claiming interference
with interstate commerce.  If Oregon does have such a law, please
provide the Oregon Revised Statute number (they are online);  a few
minutes of looking around produced nothing like a limit.

2)  Cell phones can go up to a watt, especially if they are a long
way from, or shielded from, the base station.   And most people
hold that one watt transmitter right next to their head without
melting their ears off.  A 500mW wireless transmitter is usually
much farther away, and the inverse square law applies.

3)  Antennas do not limit power, at least not until they melt from
resistive losses or dielectric absorption.  Sometimes antennas
couple poorly, have a high standing wave ratio, etc., but with
a proper matching network they not reflect power back to the
transmitter and cause distortion.  Wireless devices are sample
tested for spurious emissions, and you can see the results for
particular devices at the FCC website I pointed at in the last
post.  For the "500mW" Alfa (actually 290mW B and and 90mW G )
with FCCID UQ2 AWUS036H :

https://fjallfoss.fcc.gov/prod/oet/forms/blobs/retrieve.cgi?attachment_id=786770&native_or_pdf=pdf

The Alfa is well within the FCC limits on all specs.  The dipole
antenna on the device has a numeric gain of 1.58, and that short
antenna an efficient emitter at 2.4GHz.  

4)  Again, the main issue with excessive power wifi (as Russell noted
so well) is that your packets are competing with other packets for
the attention of all the other access points you *aren't* talking
to.  Rude and unneighborly.  The additional power won't help you if
the access point you are listening to is wimpy and you can't hear it. 
If there is only one access point around, and it has decent power,
higher power might improve your connection. 

I would only use the Alfa rather than my 100mW Netgear WG511T
if I had trouble reaching the access point.  If nothing else, the
Alfa probably drains my laptop battery faster than the WG511T.

5)  If 500mW transmitters can fry your children, your species is
very small ...

Keith

-- 
Keith Lofstrom          keithl at keithl.com         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs



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