Message-ID: <023a01cc2d16$defb66d0$1502a8c0@Clemens04>
From: "Clemens  Paul" <cpaul@gmx.net>
To: <rsgb_lf_group@blacksheep.org>
References: <4DF8A827.9070106@iup.uni-heidelberg.de> <A73DB25E1DD9454A910A4747400BE5D7@JimPC>
Date: Fri, 17 Jun 2011 19:49:02 +0200
MIME-Version: 1.0
Subject: LF: Re: Re: LF RX loop
Content-Type: text/plain;
	format=flowed;
	charset="iso-8859-15";
	reply-type=response
Content-Transfer-Encoding: 8bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
X-AOL-SCOLL-URL_COUNT: 0  
x-aol-sid: 3039ac1dc1474dfb93fe5764
X-AOL-IP: 195.171.43.25
X-AOL-SPF: domain : blacksheep.org SPF : none


Jim,

>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.

I would agree.
For 3,14m copper tubing with a diameter of 10mm
you can expect around 10 Milliohm RF resistance on 137kHz.
Given the C with 470nF the loop inductance is 2,872µH whis is 2,47Ohm.
So the loop itself *without C and connections* would have an unloaded
Q of 252.


73
Clemens
DL4RAJ

----- Original Message ----- 
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Wednesday, June 15, 2011 11:39 PM
Subject: LF: Re: LF RX loop


> 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
>
>
>
> -----
> eMail ist virenfrei.
> Von AVG überprüft - www.avg.de
> Version: 10.0.1382 / Virendatenbank: 1513/3705 - Ausgabedatum: 15.06.2011