# Path Loss over distance...

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• Subject
• Author
• Posted on
• Path Loss over distance...
• Thorbjørn Jørgensen
• 02-26-2006 Hi
I have tried to calculate the theoretical maximum range of a certain WLAN
card. The information that I have retrieved is that the maximum trandmit
power is 20dB and the receive sensitivity of the WLAN vard is -94dB (Cisco
350), which gives a maximum path loss of 114dB. Using the Friis Transmission
Formula this implies a maximum range of 5000m. Since the maximum range
stated on the internet for wlan is in the range of 500 to 1200m (400feet),
there must be somthing missing in my calculation. What I really want, is the
scenario whare the WLAN is on a open field with no interference and no
obsteclas, and be able to estimate the theoretical range...

I hope that someone can help me to point out which element that would
improve my range calculation...

If there exist another more relevant newsgroup please let me know.

Regards
Thorbj�rn Re: Path Loss over distance...
It seems from your calculation that you are already including a portion
of the effective area term in the Friis Transmission formula of
wavelength^2/(4*Pi).  You also need to include the gain of the
transmitting antenna and the gain of the receiving antenna into your
calculation.  Assuming half-wave dipoles for each side (transmitting
and receiving), which have a gain of about 2.15 dB, this would put you
in the range that you are seeking.

Regards,
JHazen Re: Path Loss over distance...
Hi I did assume that the emitted power from a WLAN station could not exceed
20dB totally, hence the calculation did not consider how this emission was
divided between the different parts, but I might have misunderstud something
about that. Since you are talking about a gain in both ends it will increase
the range that I have calculated, right? The range that I calculated was vay
to long, so I need to find some attenuation in the path between the sender
and the receiver, or some thing that I have not considered in the
transmission technique...

Btw, the caltulation of the range was done in the followring way:

Transmit power 20dB;
PathLoss = 10*log((lambda/(4*pi*Dist))^2)

Dist = lambda/(sqr(10^(PathLoss/10))*4*pi)

With a PathLoss of -114dB it gives a maximum distance of 4985m, which I
think is way to much even in a theoretical calculation...

Regards...
Thorbj�rn Re: Path Loss over distance...
Hi again I have found severeal sources that mentions a "Fade Margin" on the receiver
side, which will make the range calculation closer to reality. The sources I
have found mentions Fade Margins from 6-20dB which is quite a big span, and
reduces the maximum range to 500-2500m. Is there any rules that makes it
possible to determine the fade margin? Links or sources are very welcome...

Regards
Thorbj�rn Re: Path Loss over distance...
Hi,

You're correct that the gain would increase the distance.  It occurred
to me last night after I wrote my comment what your problem really is.
Your transmit power, 20 dB, is an error.  If you transmit 20 dB (100
Watts) at 2.45 GHz, the local regulating bodies will probably shut you
down.  It should be read 20 dBm instead, which is 20 dB with respect to
1 mW, or 100 mW.  This would be 30 dB below what you were calculating
distance.

You're correct that the fade margin is included in the Friis
Transmission formula, but usually this has to do with loss due to
vegetation, rain, buildings, and the height of your antenna--which
affects how far it can transmit over the horizon.  Only the last one
would apply to you, because you said you wanted to calculate in an open
field.  There are several books out there that help you calculate this
margin, but mine (_Radiowave Propagation for PCS_) is at work right
now. 