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Re: LF: Field strength calcs.

To: [email protected]
Subject: Re: LF: Field strength calcs.
From: "James Moritz" <[email protected]>
Date: Mon, 07 Jul 2003 12:14:23 +0100
In-reply-to: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
At 17:26 06/07/2003 +0100, you wrote:
When making the signal strength measurements using a loop antenna should
this be resonated or aperiodic?

I made measurements under both conditions but the results are puzzling -
even allowing for my peripatetic decimal point!
Dear Ian, LF Group,

The voltage induced in a loop is:

V = 2.1x10^-8 (fNAE)

where f = frequency (Hz)
        N = number of turns
        A = area (m^2)
        E = field strength (V/m)

Always assuming that the antennas are far enough apart for the loop to be in the "far field" (>1 - 2km seems to be enough)
From the circuit point of view, the loop behaves as a voltage source of
the value given by the formula, in series with the inductance and
resistance of the loop winding. If a high impedance load is connected to
the loop terminals, the measured voltage will be more or less that given by
the formula. If the load impedance is relatively low, the output voltage
will be reduced by the potential divider action of the loop
inductance/resistance and the load resistance. For single-turn loops less
than a few metres in diameter, the inductive reactance is only a few ohms,
so even a 50ohm load on the loop will have little effect on the signal
voltage - this is one reason single-turn loops are popular for
measurements; few variables affect the measurement. A multi-turn loop with
higher inductance can be series-tuned; the reactance of the loop is
cancelled by the capacitor reactance, eliminating this loading effect. A
parallel-tuned loop could be used with a high impedance load - in this
case, the voltage will be stepped up by a factor equal to the loaded Q of
the loop - but since this depends on a number of variables, it introduces
greater uncertainty into the measurements.
If you turn the formula round, you get E by measuring the signal voltage:

E = V/(2.1x10^-8 x fNA)

Then ERP (relative to a dipole) is:

Perp = (Ed)^2 /50 (d is distance in metres, E is fs in V/m, Perp in watts)

Cheers, Jim Moritz
73 de M0BMU

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