Message-ID: <393E3A1A.7E14@xtra.co.nz>
Date: Thu, 08 Jun 2000 00:03:38 +1200
From: "vernall" <vernall@xtra.co.nz>
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Formula
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Walter Blanchard wrote:
> 
> I found the following in an article recently.
> 
> Quote:
> 
> "The intensity E (known as the field strength) of a transmission
> at a distance D from a source transmitting P watts of RF power via
> a half-wave dipole in a free, unobstructed space, can be estimated
> using the formula:
> 
> E=(7*sqrt (P))/D.
> 
> Thus, for a 2 watt transmitter, the theoretical field strength in free
> space at one metre distance is approximately 10 V/m and at 100 metres
> distance 0.1 V/m"
> 
> This formula is new to me and I can't find it in any
> textbooks I have.  Since it ignores frequency shouldn't
> it be power flux per unit area and not field strength?
> Can anyone tell me where it comes from?
> 
> Walter G3JKV.

In free space, field strength for a given radiated power is independent
of frequency.  This is basically the case for a point source radiator
surrounded by an imaginary sphere, and the power flux density is
constant for any given same area of the imaginary sphere.  This is not
to say that the received power using a reference antenna is independent
of frequency: it is not.  For the likes of half wave dipoles, receiving
aperture has a -6 dB per octave relationship with increasing frequency. 
So field strength is independent of frequency, but received power is -6
dB/octave.

Whatever, at LF we hardly ever encounter "free space" around our
antennas.  Free space is a concept for VHF and higher frequencies as far
as ground-based radio stations are concerned.

73, Bob ZL2CA