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RF Loss in Air - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

 Post subject: RF loss in air
Posted: Thu Aug 25, 2005 5:56 pm 
Does anyone know the formula to calculate the RF power loss in air from a transmitting antenna to a specified distance away from the antenna? Lets say I am transmiting 100 watts into a unity gain antenna, 400MHz. What will be the power level (in watts, dB, or uV) 100 meters away.

Kirt Blattenberger
 Post subject: Path Loss Equations
Posted: Thu Aug 25, 2005 10:30 pm 
Site Admin
User avatar

Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
Greetings tbob:

Here is the path loss equation you are looking for...

...and here is a 1-way calculator for you... ... -1-way.htm

...and here is a 2-way (radar) calculator for you... ... -2-way.htm

- Kirt Blattenberger :smt024
RF Cafe Progenitor & Webmaster

 Post subject:
Posted: Fri Aug 26, 2005 5:12 am 
Site Admin
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Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello tbob,

The Frii's formula which is given in linear terms can be also calculated in dB:

Lpath=32.5+20log (F)+20log (D)-Gtx-Grx


F- frequency in MHz
D- Distance in Km

Best regards,

- IR

 Post subject:
Posted: Fri Aug 26, 2005 11:41 am 
Thanks to both of you for your help. I'll have to visit this forum more often.

Bob - RF Test Engineer

Posted  11/12/2012

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