Message-ID: <4DFA78BC.1020106@iup.uni-heidelberg.de>
Date: Thu, 16 Jun 2011 23:42:20 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <schaefer@iup.uni-heidelberg.de>
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References: <4DF8A827.9070106@iup.uni-heidelberg.de> <01a101cc2c6a$8c31a100$1502a8c0@Clemens04>
In-Reply-To: <01a101cc2c6a$8c31a100$1502a8c0@Clemens04>
Subject: Re: LF: Re: LF RX loop
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Hello Clemens,

I measured the Q by coupling to a smaller loop that was connected to my=20
DDS VFO. First i found the resonance frequency, then i in- and decreased=20
the frequency until =FB was 0.707x u max, the standard way. I measured th=
e=20
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 =3D 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=20
> 200mV
> (-0.969dB) for a 0dBm power source (i.e. power reduction is  20%, not=20
> 50%).
>
> 73
> Clemens
> DL4RAJ
> .
>
> ----- Original Message ----- From: "Stefan Sch=E4fer"=20
> <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=20
>> found a resonance when varying the C. It is a single turn circular=20
>> loop using copper tube with 10mm diameter (about 1m). I calculated=20
>> the L with a formula of ON7YDs LF info pages and found a C of about=20
>> 470 nF which agreed very well. It was easily possible to resonate the=20
>> loop with an induced signal by a smaller loop connected to my VFO and=20
>> measuring the C volatage with an oscilloscope. I found the Q of the=20
>> loop is just 8! Is this normal? Which Q do others achieve, with a=20
>> similar antenna for 137 kHz?
>> I'm using PP capacitors which should be well suitable (WIMA MKP10)=20
>> http://www.wima.de/EN/WIMA_MKP_10.pdf
>>
>> It is fascinating to null out s signal, which i have done the first=20
>> time now. I can reduce the signal of DCF39 by 25 dB by turning the=20
>> loop. I assume there are different paths the signal comes from, this=20
>> is why i cannot eliminate it completely, right?
>>
>> I used a high mu toroid (ferroxube, blue material) to transform the=20
>> primary side (=3Dthe loop) to 50 Ohm. I have done this by varying the=20
>> secondary turn number until i achieved a maximum voltage at a 50 Ohm=20
>> load at a given input signal. The secondary number is now about 25=20
>> turns. The resonance frequency is not affected by the load at the=20
>> output of the transformer.
>>
>> Now DCF39 appears at up to 45 dB above noise in 3 Hz which. But it=20
>> should be at least 20 dB higher i think. Furthermore the background=20
>> noise does not change at all when i connect or disconnect the=20
>> antenna. Thus i think it makes sense to build a preamp. I found a=20
>> suitable circuit on the pdf of Jim/M0BMUs loop article which looks=20
>> well for me. Looking forward to this amp performance!
>>
>> Is it important to terminate the loops transformer output with a 50=20
>> Ohm load in that case? On an oscilloscope i found that the level of=20
>> DCF39 is higher when having no R connected to the output but the=20
>> waveform looks much better / cleaner!
>>
>> I'm optimistic to get much proper reception of UK and PA and HB9 CW=20
>> signals on 137 soon, when beeing outside the city, steering the LF TX=20
>> by the 70cm link :-)
>>
>> Vy 73, Stefan/DK7FC
>>
>>
>>
>> -----
>> eMail ist virenfrei.
>> Von AVG =FCberpr=FCft - www.avg.de
>> Version: 10.0.1382 / Virendatenbank: 1513/3705 - Ausgabedatum:=20
>> 15.06.2011=20
>