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LF: Re: LF RX loop

from [James Moritz] [Permanent Link][Original]
To: <[email protected]>
Subject: LF: Re: LF RX loop
From: "James Moritz" <[email protected]>
Date: Sun, 3 Jul 2011 14:50:24 +0100
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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
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