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Message-ID: <0FB90E3DA47149B98075CEA1021F4C3B@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <o2kfc7eccec1004010402n7f00e1c3l8b88ea090c0fb8bb@mail.gmail.com> <001d01cad190$aa92e590$0901a8c0@lark> <n2kfc7eccec1004010605n95ffca10ia8bacd2ef2049820@mail.gmail.com> <E2055FBF69BE454E804CB4FCB9A5E04C@MJUPC>
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Date: Thu, 1 Apr 2010 16:48:19 +0100
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Subject: LF: Re: E-Field Probe calibration question
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Dear Andy, LF Group,

I haven't done quantitative tests due to the difficulty of using such an 
antenna in the Brookmans Park RF environment, but in some quick checks I 
did, output from an active whip certainly depends on height above ground and 
the presence/absence of a ground plane (not to mention presence of trees, 
other wires, etc.). I suggest you ask the manufacturers how they calibrate 
it, and try to replicate that...

A loop is much less susceptible to these things, it seems. I tried my 
measuring loop (measuring DCF39 during daytime, which is usually a very 
stable field strength here), and there was no more than about 0.1dB 
difference between on the ground and 10m in the air. However, when mounted 
on a metal vehicle, the presence of the vehicle distorts the H field 
locally - I found that my 30cm square loop measured about 2-3 dB higher when 
on the roof of the car, compared to the same height on a tripod. The exact 
amount depended on whether the body of the car was aligned in the direction 
of the signal source (higher apparent FS) or at right angles. Mounting the 
antenna on a 1m mast on top of the car reduced this effect to about 0.2 - 
0.3dB (although it gets even funnier looks ;-)).

The formula given by SM6LKM can be adjusted to give the loop EMF for an 
equivalent E-field strength, assuming E/H = 377ohms:

V = 2.1*10e-8*fNAE

Assuming the loop is small compared to a wavelength, and operating a long 
way below its self-resonant frequency.

Cheers, Jim Moritz
73 de M0BMU