Author Topic: Dbm to Distance  (Read 9173 times)

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Offline airman00Topic starter

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Dbm to Distance
« on: January 03, 2009, 10:24:56 PM »
Hi,
I'm trying to calculate the approximate distances at each dbm for this transmitter - http://www.sparkfun.com/datasheets/Wireless/General/MO-SAWR.pdf

I looked online but cannot really find a good calculator or equation.
The transmitter uses 315mhz and all the dbm values can be found in the datasheet I linked above .

Any calculations or helpful links would be appreciated
Thanks,
Eric
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Offline Soeren

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Re: Dbm to Distance
« Reply #1 on: January 04, 2009, 09:58:40 AM »
Hi,

The file appears to be broken here or my PC needs a reboot (been a few weeks), so I cannot read it, but given the exact same parameters (the only way you can get useable results), use the fact that when you halve the distance, the signal will be 4 times as strong, double the distance and it will be 1/4 as strong (strength changes with the square of the distance change, just like light and sound).

Whether you measure in dBm, µV or whatever the same applies.
Regards,
Søren

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Offline izua

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Re: Dbm to Distance
« Reply #2 on: January 04, 2009, 10:01:57 AM »
if you have a transmitter inside an open field, and you emit from 20m, and you take the same transmitter, same conditions, and put it inside a concrete building, and emit from 20 meters, through walls, you will get different dbm power values.
but you will get different power readings even in open field, for the same transmitter, at different times of day or while navigating around a 20m radius with the receiver.

ergo, the best way to determine these values are thorugh direct measurments, formulas can't apply here.
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Offline airman00Topic starter

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Re: Dbm to Distance
« Reply #3 on: January 04, 2009, 11:12:52 AM »
ergo, the best way to determine these values are thorugh direct measurments, formulas can't apply here.
I was hoping you wouldn't say that. Anyway to calculate approximate values?
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Offline Admin

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Re: Dbm to Distance
« Reply #4 on: January 09, 2009, 07:04:48 AM »
Quote
Anyway to calculate approximate values?
Not really. Its actually even more complicated than izua said.

Antennas are directional and vary in intensity in a 3D manner. Height from the ground matters, for example.

Then there is the effect from large conductive things, like refrigerators.

And lets not forget about humidity ;D

Basically the quoted value in the datasheet is a rough estimate . . . they don't even tell you if its average, worst case, or best case.

Ok so I lied, there is a way to calculate it. There is multi-thousand dollar multiphysics industrial level simulator software you can use. Its like the electrical engineers version of CFD, but predicts electrical fields and stuff. Usually used for MEMS design . . .

Offline pomprocker

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Re: Dbm to Distance
« Reply #5 on: January 09, 2009, 11:21:45 AM »
Your best off overshooting with what you need

 


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