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Re: LF: Re: LF RX loop

To: [email protected]
Subject: Re: LF: Re: LF RX loop
From: Stefan Schäfer <[email protected]>
Date: Thu, 16 Jun 2011 23:42:20 +0200
In-reply-to: <01a101cc2c6a$8c31a100$1502a8c0@Clemens04>
References: <[email protected]> <01a101cc2c6a$8c31a100$1502a8c0@Clemens04>
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Hello Clemens,

I measured the Q by coupling to a smaller loop that was connected to my DDS VFO. First i found the resonance frequency, then i in- and decreased the frequency until û was 0.707x u max, the standard way. I measured the voltage across the C.
Soon i will have a higher Q. Looking forward to that.

Tnx es 73, Stefan/DK7FC

Am 16.06.2011 23:15, schrieb Clemens Paul:
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" <[email protected]>
To: <[email protected]>
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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