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*Subject*: R: [amsat-bb] Satellite Builder Question*From*: "i8cvs" <domenico.i8cvs@xxxxxx>*Date*: Thu, 15 May 2003 03:39:30 +0200

----- Original Message ----- From: Bob Bruninga <bruninga@usna.edu> To: John M. Franke <j.m.franke@larc.nasa.gov> Cc: <amsat-bb@AMSAT.Org> Sent: Wednesday, May 14, 2003 2:55 PM Subject: Re: [amsat-bb] Satellite Builder Question > I asked about how to calculate field strength given TX power: > > On Thu, 8 May 2003, John M. Franke said to See the WEB page: > > > http://www.astron.nl/craf/conv.htm > > Which explains the relationship between power and field stength is > > (P*G)/(4pi*d^2) = (E^2)/(120*pi) Where G is gain, d is distance > > And notice that wavelength does not enter into it. That is where I was > getting all messed up, because the maximum voltage that can be induced in > a 1 meter length of wire depends on frequency. But since the question was > not "what voltage will be induced in a wire", but "what is the field > strength", then the above gives the answer without respect to frequency. > > Thus, my 2W transmitter on our spacecraft generates 7.7 Volts per meter at > one meter no matter what the frequency. > > Thanks to all who responded. > > Bob > Bob, The above equation shoves that : (P*G)= the EIRP of your antenna when the power P is expressed in watt and G is the isotropic gain of the antenna expressed in factor (not dB) (4pi*d^2) = the geometrical surface of a sphere. If we express in meters the distance d from the sphere center than the sphere surface is computed in square meters or (4pi*d^2) = square meters Now put your antenna in the center of the sphere and radiate your EIRP in one arbitrary direction at a distance d from the center. (P*G) / (4pi*d^2)= the power density in watt/m^2 collected by the sphere surface at a distance d from the antenna along its radiation axis or in other words it is the power in W collected in to a square meter of free space at a distance d from the antenna along the direction of radiation. The second term of the equation means that (120*pi)= 377 ohm 377 ohm is the caracteristic impedance of the free space that must be considered as a transmission line having infinite lenght and so by the ohm law the second term of the equation means that (E^2) / 377 = W/m^2 where W is the power density collected by the surface of one square meter of free space at a distance d from the antenna and again by the ohm law the "Field Strenght" or in other words the voltage that can be measured across one side of free space of the above discussed square meter is: E = SQR (377*((P*G) / (4pi*d^2) expressed [ V/m] As you pointed out correctly the frequency is not involved in calculation of the "Field Strenght" because the power collected in to a square meter of free space having impedance of 377 ohm is not frequency depended. Wery often we state that the attenuation at the same distance from the antenna increases increasing the frequency but this is correct only because we compute the attenuation in dB considering the capture area of a receiving isotropic antenna wich area decreases increasing the frequency. Of coarse the same power density in W collected in to a square meter of free space,as you pointed out correctly gives the same Field Strenght" E measured in V/m no matter the frequency. Best 73" de i8CVS Domenico > ---- > Sent via amsat-bb@amsat.org. Opinions expressed are those of the author. > Not an AMSAT member? Join now to support the amateur satellite program! > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org ---- Sent via amsat-bb@amsat.org. Opinions expressed are those of the author. Not an AMSAT member? Join now to support the amateur satellite program! To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**References**:**Re: Satellite Builder Question***From:*Bob Bruninga

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