RadioBanter - View Single Post

December 13th 03, 10:55 AM

On Fri, 12 Dec 2003 16:41:06 +0000,
said...
Hi all,

I wanted to build an RF relative field strength meter, so set about
searching on the Web for any existing designs. Those I turned up
weren't particularly impressive, so I decided to start from scratch
and design my own. I've just completed that this afternoon. I've
allowed for 0.25mV input to give rise to FSD on the microammeter.
Question being, however, is that going to be sensitive enough? Does
anyone have any idea what the field strength in microvolts or
millivolts is from a half Watt transmitter at about 6 feet away? I
guess I should have posed this question *before* designing it, but who
among us can honestly say they haven't designed something without
knowing what the spec is? :-)
Anyway, ballpark figures gentlemen, please.

p.

E = sqrt(30PG)/d Volts/meter - Field Strength

P = Tx power
G = Tx antenna gain - and don't forget the gain of the meter's
dipole.

S = PG / (4.pi.R^2) Watts/meter^2 - Power density

R = distance

S = E^2/377 (E field^2)/(Z of free space in the far field)

Where the far-field starts is antenna dependant, but it is
acceptaed the the boundary is where the inverse-square-law for
power density above becomes invalid, that is, as you approach the
antenna, the rate of change of S decreases and S is no longer
inverse-square-law dependant.

For large aperature antennas (dishes, dipoles, etc.) this seems to
work out to

R = 2L^2/lambda

lambda is wavelength and L is length of antenna

HTH,
Mike