Re: LF: LF RX loop

[Daniele Tincani <danieletincani@yahoo.com>]

Hello Stefan,

 

as far as I know, you should avoid loading a resonant loop too much so I would say that a preamp with an high input impedance is better (so no 50 ohms load at the transformer output in my opinion).

Another important factor is balancing, which allows better rejection of common mode currents and deeper nulls.

 

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From: Stefan Schäfer <schaefer@iup.uni-heidelberg.de>
To: rsgb_lf_group@blacksheep.org
Sent: Wed, June 15, 2011 2:40:07 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