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Date: Thu, 16 Jun 2011 02:51:41 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: Re: LF RX loop
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Jim, Alan, Scott, Daniele,

Thanks for your comments and suggestions!

I will build the preamp and try to get a higher Q. I have some WIMA 
FKP-1 caps, 2 kV type in that C range and will buy a bigger copper tube 
tomorrow. That makes it even more robust. I would like to have even more 
area but the loop must be placed in the car...
My favorite RX is my homemade RX using the SBL-3 mixer. It seems to work 
quite well now but still needs some more input level. Since it was 
designed to work with the active E field antenna, it has a relatively 
high input Z and so i tried to transform the 50 Ohm output of the loop 
to a higher Z. Maybe i can make the preamp obsolete then, at least on 
the homemade RX. The 706 will definitely need the preamp!

73, Stefan/DK7FC

Am 15.06.2011 23:39, schrieb James Moritz:
> 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