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Message-ID: <A73DB25E1DD9454A910A4747400BE5D7@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4DF8A827.9070106@iup.uni-heidelberg.de>
In-Reply-To: <4DF8A827.9070106@iup.uni-heidelberg.de>
Date: Wed, 15 Jun 2011 22:39:32 +0100
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Subject: LF: Re: LF RX loop
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Dear Stefan, LF Group,

I did some experiments on single-turn loop Q when designing the "bandpass 
loops" (BPloops2.pdf at 
https://sites.google.com/site/uk500khz/members-files/files ). With a 1m x 1m 
square loop made of 15mm copper tubing, I got unloaded Q well over 100 with 
C about 400nF at 137k. This was one unusual example of where the capacitor 
dominates the loss in a tuned circuit - I got a much lower Q using a single, 
lower voltage metallised polypropylene capacitor than when using 4 x 100n, 
1kV capacitors in parallel. I assume this is because higher voltage = 
greater electrode area connected in parallel = less resistance, and more 
capacitors = less resistance from the connecting leads and interface to the 
metallisation. I have noticed similar effects with high-current capacitors 
in QRO PAs. DK7FC's Q of 8 with 470n may be due to this - it implies Rloss 
of about 0.3 ohms, which seems much too high for a few metres of copper 
tubing, so something is definitely wrong here. Obviously, it is also very 
important to have excellent connections between tubing and capacitors in 
order to realise milliohms of resistance. Another possibility is that the 
loop is inductively coupled to something that has high losses - you need to 
keep it a metre or so away from other conducting objects when doing the 
measurements.

>I assume there are different paths the signal comes from, this is why i 
>cannot eliminate it completely, right?

Apart from what others have already said, the null can be degraded  by RF 
currents induced in nearby conductors acting as parasitic antenna elements - 
this includes connecting leads, building structures, cables etc etc. 25dB is 
fairly typical I think.

> I used a high mu toroid (ferroxube, blue material) to transform the 
> primary side (=the loop) to 50 Ohm. I have done this by varying the 
> secondary turn number until i achieved a maximum voltage at a 50 Ohm load 
> at a given input signal.

Whatever the value of unloaded Q, the "maximum power transfer" theorem 
applies, ie. the source resistance of the antenna at resonance should be 
transformed to equal the RX input resistance to achieve the maximum signal 
power delivered to the receiver - this is what you did empirically. In this 
condition, the loaded Q should be half the unloaded Q , so in Stefan's case 
tuning should be very flat with a loaded Q of 4.

> Is it important to terminate the loops transformer output with a 50 Ohm 
> load in that case? On an oscilloscope i found that the level of DCF39 is 
> higher when having no R connected to the output but the waveform looks 
> much better / cleaner!

In my loop designs, I have made a trade-off by increasing the loading 
(reduced loaded Q ), which increases the bandwidth, but reduces the signal 
power delivered to the receiver. If you wanted to obtain maximum loop 
selectivity, you could reduce loading of the loop, which would also reduce 
the power delivered to the RX, so it is your choice... Whatever you do, the 
band noise at the output of a loop like this is only a fraction of a uV, so 
you either need a RX with a low noise figure at 137k (very rare!) or a 
preamp with a low noise figure. Actually, with the preamp in the BPloops 
article, simply connecting the 50ohm input directly across the parallel 
tuned single turn loop is quite a good combination (loaded Q about 15 for my 
loop, a little higher for Stefan's slightly smaller loop, but only assuming 
he can improve the unloaded Q to say 50 or more). The preamp has low enough 
noise to easily hear the band noise with these loops, but beware - some 
receivers have such bad sensitivity at 137k that further gain will still be 
needed.

I am quite pleased with the way the single-turn loops have worked out - they 
work just as well as multi-turn loops, but are almost impossible to de-tune, 
are less susceptible to unwanted capacitive coupling, easy to make 
weatherproof, are mechanically simple and robust.

Cheers, Jim Moritz
73 de M0BMU