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Message-ID: <27519D99CCF748A7850251478AFE6EEF@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4DF8A827.9070106@iup.uni-heidelberg.de> <01a101cc2c6a$8c31a100$1502a8c0@Clemens04> <4DFA78BC.1020106@iup.uni-heidelberg.de> <4DFB5AEB.70607@legal-medicine.de> <4E05E69B.2060803@iup.uni-heidelberg.de>
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Date: Sun, 3 Jul 2011 14:50:24 +0100
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Subject: LF: Re: LF RX loop
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Dear Stefan, LF Group,

A bit late, but better than nothing...

> One more question: Does it make sense (regarding good SNR) to further
> try to improve the Q or does it just make frequency adjustment
> complicated? If the dynamic range of my soundcard can handle the signal
> of DCF39 and DCF49 and DLF and i should have no problem, right? The
> background noise should be limited by the band noise only of course. Is
> there a straight dependency of gain and Q?
>

For a given size of loop, the signal power delivered to the optimum load 
impedance is proportional to the Q. Under this condition, the bandwidth is 
inversely proportional to the Q. At 137kHz, I estimate the band noise field 
strength is about 1uV/m in a 300Hz bandwidth in quiet conditions, For a 1m 
diameter circular loop, this will give an induced EMF of 2.3nV. The EMF 
across the loop terminals at resonance will be this value multiplied by the 
Q, 0.4uV. If the loop is matched to a 50ohm receiver input (a transformer 
with 3:1 turns ratio would be about right), the voltage at the RX input will 
be about 0.07uV. For receivers which maintain good sensitivity at LF, this 
would be several dB above the receiver noise floor, so actually no 
preamplifier would be needed. But many receivers do not have good LF 
sensitivity, so often a preamp will be needed. The preamp in the "bandpass 
loops" article will work well here.

With the loop loaded in this way, the Q will be about 176/2, i.e. 88. This 
will give significant filtering of nearby utility and broadcast signals - 
e.g. 138.8kHz -4.5dB, 135.4kHz -10dB, 153kHz -25dB. This would certainly be 
helpful if dynamic range was a problem.

Cheers, Jim Moritz
73 de M0BMU