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Re: LF: A question of calibration

To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: A question of calibration
From: John Andrews <w1tag@w1tag.com>
Date: Fri, 28 Sep 2007 11:37:32 -0400
In-reply-to: <000001c801dd$a0ecc1d0$7900a8c0@athlon1200>
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Andy, all,

Last year, I did a MathCad analysis of the fields from a small vertical and a small magnetic loop at 137 kHz. The program calculated the vector E and H fields, and the magnitude of E/H for distances from 1 to 10 km. For the vertical, there are three terms to consider for the E field, and two for the H field. The situation is reversed with the loop, with three terms for the H field, and two for the E field.
Comparing the calculated |E/H| values with the "far field" value of 377 
ohms, the error was about 12% at 1 km and 1% at 3 km for both types of 
antenna. The errors at 1 km were in opposite directions, as one would 
expect, favoring the E field for the vertical, and the H field for the 
loop.
If you grant a measurement error of 1%, then it would be best to take 
readings no closer than 3 km from a small 137 kHz antenna, which would 
be lambda/(0.73). If higher errors are permissible, then you can move 
in, but I'd have trouble recommending the 1 km figure at that frequency.
As Andy points out, there is at least one other far-field concept that 
doesn't apply to us LF hobbyists. That only refers to large antennas, 
where the height of a vertical, for example, is significant compared to 
the distance. A corollary occurs with multi-element arrays, where the 
directional pattern is distorted if the spacing between the towers is 
significant compared to the distance.
John Andrews, W1TAG


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