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Message-ID: <07BFB60B5E834C13864621EBED0D4D51@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Mon, 5 Sep 2011 20:21:26 +0100
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Subject: Re: LF: Loop vs. ferrite antenna tests...
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Dear Daniele, LF Group,

The high-impedance buffer type of loop preamp as I used with my ferrite rod 
is probably the best choice for your experiments - it has a number of 
advantages:

Low noise - for loops that have an equivalent parallel resistance at 
resonance of a few kilohms or more, the noise added by the FET buffer is 
negligible (noise figure <1dB). Almost any practical values of ferrite rod L 
and C will meet this criterion.

High gain - the output from the circuit is nearly equal to the EMF induced 
in the loop multiplied by the Q of the loop, which will typically mean 40 - 
50dB voltage gain. This increases the rather small induced signal to a level 
that most reasonably sensitive receivers can hear.

Good linearity - the unity-gain buffer has inherently large negative 
feedback, and strong-signal handling is good.

Simplicity - a small number of cheap components, instability is unlikely.

If you use this type of circuit (and you have a reasonably sensitive 
receiver), simply connecing the input to the terminals of the resonant loop 
will almost guarantee that you will be able to detect any signal that is 
above the thermal noise of the loop (and above the external noise level of 
course), without having to do detailed design of the preamp or matching 
arrangements. So it is good for playing with different designs of loops and 
rods.

The main drawback is that the antenna bandwidth is defined by the loop Q, 
and can be very narrow. But if your loop is to be tunable anyway, this does 
not matter much, and the narrow bandwidth will help prevent receiver 
overloading.

Cheers, Jim Moritz
73 de M0BMU