Txpower calibration

G.W. Haywood ged
Fri Feb 3 00:38:44 PST 2006

Hi there,

On Thu, 2 Feb 2006 19:18:15 +0530 Sukanto wrote:

> I am calibrating Netgate minipci card with hostap driver [...]
> CR31(txpower control register) [...]
> how this register is indicative of true txpower that is transmiited?

Short answer - it isn't.

Transmitted power depends very strongly on many more factors than the
power produced by the RF output device.  For example if you have poor
impedance matches between the output device and the antenna and/or its
feeder cable, most of your output power will never reach the ether.

The registers in the card can only be broadly indicative of the output
power, which for a given set of values will vary _very_ greatly with
temperature, from one card to another, with degradation of components
through the action of moisture and chemical processes, with time, with
power supply characteristics (the output voltages will be affected by
temperature and current drain, which in turn may change with data rate
and amplifier gain - and temperature - to name but a few).

See for example http://www.jubileegroup.co.uk/JOS/radio/diurnal.gif
which shows the approximate signal strength seen by a receiver located
750m from from an access point which is mounted in a black plastic box
on a mast outdoors (in England).  The graph covers a period of about a
month in summer last year.  Temperatures in the box in daytime were up
to about 35C, down to about 10C at night.  The receiver was indoors at
an approximately constant temperature.  Signal strengths often varied
by more than 6dB during days when the sun was shining.


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