Return to KLUBNL.PL main page

rsgb_lf_group
[Top] [All Lists]

Re: LF: A question of calibration

To: <[email protected]>
Subject: Re: LF: A question of calibration
From: "Malcolm Harman" <[email protected]>
Date: Fri, 28 Sep 2007 16:53:11 +0100
References: <000a01c80132$aae29df0$83e41a52@enigma> <[email protected]> <000601c801ae$38cc27a0$0fee1a52@enigma> <[email protected]> <001d01c801c6$a918b420$0fee1a52@enigma> <[email protected]> <000001c801dd$a0ecc1d0$7900a8c0@athlon1200> <[email protected]>
Reply-to: [email protected]
Sender: [email protected]

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" <[email protected]>
To: <[email protected]>
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






<Prev in Thread] Current Thread [Next in Thread>