-----Original Message-----
From: [email protected] [ mailto:[email protected]]
Sent: Thursday, January 13, 2005 7:01 AM
To: [email protected]; [email protected]
Subject: RE: Question on RF conversion
Lisa,
Rather than giving you one formula I will give you an algorithm (easier for me
to remember).Assuming that the Walkie Talkie is really radiating
5 watts and we have an isotropic antenna:
1) Chose a distance. Let's say the distance is 1 meter. This is far-field at
430 MHz.
2) The area of the 1 meter radius sphere is 4(PI)(r^2) = 12.6 square meters.
3) 5 watts covers the 12.6 sq meters. The power density per square meter is
5/12.6 = 0.4 watts/sq m.
4) Using P = (V^2)/R, solve for V which is V = (PR)^0.5. R is 377 ohms and P
is 0.4 watts.
5) V = 12 volts/meter at 1 meter distance from the Walkie Talkie
Assuming a dipole pattern the antenna gain is 2.1 dB and the E-field will be
15V/m at 1 meter.
Dave Cuthbert
Micron Technology
Dave's image of the power propagating outward through the surface of a sphere
is my favorite way of explaining why and how field strength (and power
density) decays with distance. Most non-technical people can comprehend an
antenna emitting a certain amount of power, and then, if you wave your arms
around and get them to imagine a big sphere around that antenna, then they can
grasp that all the antenna's power has to pass through the surface of that
sphere.
If they imagine the radiating power to be like suffused light, shining equally
strong in all directions, you have just given them the model of an isotropic
radiator. Now, if you tell them that the antenna has the ability to move, or
focus the radiating power, robbing some directions, and concentrating the
energy in a certain direction, you have just moved them to the concept of
directional gain.
The next step may need a piece of paper. Remember how all the RF power
radiates out from the point source of the antenna in the center of the sphere?
The power passing through a unit area (like one square meter) spreads as it
moves outward, passing through an infinite number of imaginary spheres. If you
think of a second sphere, at double the first sphere's radius, then the
radiating wave has to spread out to double the surface length on that second
sphere. But since the wave still has the same power, that power is now
stretched over double the distance. (In 2 dimensions, this can represent the E
or H field strengths. How about that, double the distance, half the field
strength. That's 1/r! In 3 dimensions, the area of sphere 2 is 4 times the
area of sphere 1, so the same power is spread over an area that's now four
times larger. And how about that, double the distance, and you get 1/4 the
power density. That's 1/rr!)
So with just a little geometry and some arm waving, you can show a lot about
RF propagation.
Ed
Ed Price
[email protected] WB6WSN
NARTE Certified EMC Engineer & Technician
Electromagnetic Compatibility Lab
Cubic Defense Applications
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (Fax)
Military & Avionics EMC Is Our Specialty
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