Message-ID: <001301c801e7$ad554960$28e41a52@enigma>
From: "Malcolm Harman" <malcolm.fr263@ntlworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <000a01c80132$aae29df0$83e41a52@enigma> <46FC071D.2090202@w1tag.com> <000601c801ae$38cc27a0$0fee1a52@enigma> <fc7eccec0709280258m35a361dfy75caad028a834d57@mail.gmail.com> <001d01c801c6$a918b420$0fee1a52@enigma> <fc7eccec0709280550x2be11f58k7195c7bd0ffd8fb9@mail.gmail.com> <000001c801dd$a0ecc1d0$7900a8c0@athlon1200> <46FD1FBC.7080008@w1tag.com>
Date: Fri, 28 Sep 2007 16:53:11 +0100
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Subject: Re: LF: A question of calibration
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John.

Scaling your figures to 500kHz that gives a distance of about 820 metres for 
1% error, which ties in very nicely with my 955 metres, give or take.

Malcolm
(G3NZP)

----- Original Message ----- 
From: "John Andrews" <w1tag@w1tag.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Friday, September 28, 2007 4:37 PM
Subject: Re: LF: A question of calibration


> 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
>
>