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| To: | <[email protected]> |
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| Subject: | Re: LF: Loop vs. ferrite antenna tests... |
| From: | "James Moritz" <[email protected]> |
| Date: | Mon, 5 Sep 2011 20:21:26 +0100 |
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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 |
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