[Fwd: Q at LF]
Mon, 04 Jan 1999 19:56:37 -0500
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From: "Mike Dennison" <email@example.com>
Organization: Radio Society of Great Britain
Date: Mon, 4 Jan 1999 11:00:38 -0000
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Subject: Q at LF
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I read the article by Hal, WB3KDU, (vol XXV no2) with interest.
However, I note he makes a mistake often seen in articles about
LF. He supposes that the antenna Q will be extremely high,
quoting commercial experience and theory. Certainly, a practical
amateur LF antenna will have a small bandwidth but this is not the
fractions of a Hertz often seen in theoretical articles. It is not even
several Hertz. A good practical antenna will have a 3dB bandwidth
of a few hundred Hertz, whilst an effective antenna might be 1kHz
Why? Losses, mainly resistance! In the case of a loop, wire
resistance is important. A practical LF loop must have wire thin
enough to support (and cheap enough to buy), and N4ICK's 0.8
ohms is good. In the Marconi case a practical earth will have
several ohms (often many tens of ohms) resistance. But there are
other losses. Ironically some of these will be in the very matching
systems which WB3KDU is aiming to replace.
Whilst a low-loss system is important for efficiency, do not assume
that all losses are bad news - most contribute to the necessary
bandwidth of the antenna.
73 to all US LF enthusiasts.
Mike, G3XDV (IO91VT)