Message-ID: <01a101cc2c6a$8c31a100$1502a8c0@Clemens04>
From: "Clemens  Paul" <cpaul@gmx.net>
To: <rsgb_lf_group@blacksheep.org>
References: <4DF8A827.9070106@iup.uni-heidelberg.de>
Date: Thu, 16 Jun 2011 23:15:32 +0200
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Subject: LF: Re: LF RX loop
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Stefan,

just to be sure how you have measured Q.
If you are feeding the loop with a 50 Ohm source and want
to measure the unloaded Q of the loop as per the formula
Q = f0/B you would have to look where the SWR rises to 2.618
(not 5.8).
This translates to a voltage drop at the terminals from e.g. 224mV to 200mV
(-0.969dB) for a 0dBm power source (i.e. power reduction is  20%, not 50%).

73
Clemens
DL4RAJ
.

----- Original Message ----- 
From: "Stefan Schäfer" <schaefer@iup.uni-heidelberg.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Wednesday, June 15, 2011 2:40 PM
Subject: LF: LF RX loop


> LF,
>
> Some questions about RX loops for LF.
> Yesterday i found a shortcut between my loop ends. Thus i haven't found a 
> resonance when varying the C. It is a single turn circular loop using copper 
> tube with 10mm diameter (about 1m). I calculated the L with a formula of 
> ON7YDs LF info pages and found a C of about 470 nF which agreed very well. It 
> was easily possible to resonate the loop with an induced signal by a smaller 
> loop connected to my VFO and measuring the C volatage with an oscilloscope. I 
> found the Q of the loop is just 8! Is this normal? Which Q do others achieve, 
> with a similar antenna for 137 kHz?
> I'm using PP capacitors which should be well suitable (WIMA MKP10) 
> http://www.wima.de/EN/WIMA_MKP_10.pdf
>
> It is fascinating to null out s signal, which i have done the first time now. 
> I can reduce the signal of DCF39 by 25 dB by turning the loop. I assume there 
> are different paths the signal comes from, this is why i cannot eliminate it 
> completely, right?
>
> 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. The secondary number is now about 25 turns. The resonance frequency is 
> not affected by the load at the output of the transformer.
>
> Now DCF39 appears at up to 45 dB above noise in 3 Hz which. But it should be 
> at least 20 dB higher i think. Furthermore the background noise does not 
> change at all when i connect or disconnect the antenna. Thus i think it makes 
> sense to build a preamp. I found a suitable circuit on the pdf of Jim/M0BMUs 
> loop article which looks well for me. Looking forward to this amp performance!
>
> 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!
>
> I'm optimistic to get much proper reception of UK and PA and HB9 CW signals on 
> 137 soon, when beeing outside the city, steering the LF TX by the 70cm link 
> :-)
>
> Vy 73, Stefan/DK7FC
>
>
>
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