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References: <4DF8A827.9070106@iup.uni-heidelberg.de>
Date: Wed, 15 Jun 2011 06:31:34 -0700 (PDT)
From: Daniele Tincani <danieletincani@yahoo.com>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: LF RX loop
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Hello Stefan,=0A=0Aas far as I know, you should avoid loading a resonant lo=
op too much so I would =0Asay that a preamp with an high input impedance is=
 better (so no 50 ohms load at =0Athe transformer output in my opinion).=0A=
Another important factor is balancing, which allows better rejection of com=
mon =0Amode currents and deeper nulls.=0A=0A=0A=0A=0A______________________=
__________=0AFrom: Stefan Sch=E4fer <schaefer@iup.uni-heidelberg.de>=0ATo: =
rsgb_lf_group@blacksheep.org=0ASent: Wed, June 15, 2011 2:40:07 PM=0ASubjec=
t: LF: LF RX loop=0A=0ALF,=0A=0ASome questions about RX loops for LF.=0AYes=
terday i found a shortcut between my loop ends. Thus i haven't found a =0Ar=
esonance when varying the C. It is a single turn circular loop using copper=
 =0Atube with 10mm diameter (about 1m). I calculated the L with a formula o=
f ON7YDs =0ALF info pages and found a C of about 470 nF which agreed very w=
ell. It was =0Aeasily possible to resonate the loop with an induced signal =
by a smaller loop =0Aconnected to my VFO and measuring the C volatage with =
an oscilloscope. I found =0Athe Q of the loop is just 8! Is this normal? Wh=
ich Q do others achieve, with a =0Asimilar antenna for 137 kHz?=0AI'm using=
 PP capacitors which should be well suitable (WIMA MKP10) =0Ahttp://www.wim=
a.de/EN/WIMA_MKP_10.pdf=0A=0AIt is fascinating to null out s signal, which =
i have done the first time now. I =0Acan reduce the signal of DCF39 by 25 d=
B by turning the loop. I assume there are =0Adifferent paths the signal com=
es from, this is why i cannot eliminate it =0Acompletely, right?=0A=0AI use=
d a high mu toroid (ferroxube, blue material) to transform the primary side=
 =0A(=3Dthe loop) to 50 Ohm. I have done this by varying the secondary turn=
 number =0Auntil i achieved a maximum voltage at a 50 Ohm load at a given i=
nput signal. The =0Asecondary number is now about 25 turns. The resonance f=
requency is not affected =0Aby the load at the output of the transformer.=
=0A=0ANow DCF39 appears at up to 45 dB above noise in 3 Hz which. But it sh=
ould be at =0Aleast 20 dB higher i think. Furthermore the background noise =
does not change at =0Aall when i connect or disconnect the antenna. Thus i =
think it makes sense to =0Abuild a preamp. I found a suitable circuit on th=
e pdf of Jim/M0BMUs loop article =0Awhich looks well for me. Looking forwar=
d to this amp performance!=0A=0AIs it important to terminate the loops tran=
sformer output with a 50 Ohm load in =0Athat case? On an oscilloscope i fou=
nd that the level of DCF39 is higher when =0Ahaving no R connected to the o=
utput but the waveform looks much better / =0Acleaner!=0A=0AI'm optimistic =
to get much proper reception of UK and PA and HB9 CW signals on =0A137 soon=
, when beeing outside the city, steering the LF TX by the 70cm link :-)=0A=
=0AVy 73, Stefan/DK7FC
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<html><head><style type=3D"text/css"><!-- DIV {margin:0px;} --></style></he=
ad><body><div style=3D"font-family:arial, helvetica, sans-serif;font-size:1=
0pt"><DIV>Hello Stefan,</DIV>=0A<DIV>&nbsp;</DIV>=0A<DIV>as far as I know, =
you should avoid loading a resonant loop too much so I would say that a pre=
amp with an high input impedance is better (so no 50 ohms load at the trans=
former output in my opinion).</DIV>=0A<DIV>Another important factor is bala=
ncing, which allows better rejection of common mode currents and deeper nul=
ls.</DIV>=0A<DIV>&nbsp;</DIV>=0A<DIV style=3D"FONT-SIZE: 10pt; FONT-FAMILY:=
 arial, helvetica, sans-serif"><BR>=0A<DIV style=3D"FONT-SIZE: 13px; FONT-F=
AMILY: arial, helvetica, sans-serif"><FONT face=3DTahoma size=3D2>=0A<HR SI=
ZE=3D1>=0A<B><SPAN style=3D"FONT-WEIGHT: bold">From:</SPAN></B> Stefan Sch=
=E4fer &lt;schaefer@iup.uni-heidelberg.de&gt;<BR><B><SPAN style=3D"FONT-WEI=
GHT: bold">To:</SPAN></B> rsgb_lf_group@blacksheep.org<BR><B><SPAN style=3D=
"FONT-WEIGHT: bold">Sent:</SPAN></B> Wed, June 15, 2011 2:40:07 PM<BR><B><S=
PAN style=3D"FONT-WEIGHT: bold">Subject:</SPAN></B> LF: LF RX loop<BR></FON=
T><BR>LF,<BR><BR>Some questions about RX loops for LF.<BR>Yesterday i found=
 a shortcut between my loop ends. Thus i haven't found a resonance when var=
ying the C. It is a single turn circular loop using copper tube with 10mm d=
iameter (about 1m). I calculated the L with a formula of ON7YDs LF info pag=
es and found a C of about 470 nF which agreed very well. It was easily poss=
ible to resonate the loop with an induced signal by a smaller loop connecte=
d 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?<BR>I'm using PP capacitors which should be well suita=
ble (WIMA MKP10) http://www.wima.de/EN/WIMA_MKP_10.pdf<BR><BR>It is fascina=
ting to null out s signal, which i have done the first time now. I can redu=
ce the signal of DCF39 by 25 dB by turning the loop. I assume there are dif=
ferent paths the signal comes from, this is why i cannot eliminate it compl=
etely, right?<BR><BR>I used a high mu toroid (ferroxube, blue material) to =
transform the primary side (=3Dthe loop) to 50 Ohm. I have done this by var=
ying the secondary turn number until i achieved a maximum voltage at a 50 O=
hm 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.<BR><BR>Now DCF39 appears at up to 45 dB above noise in 3 Hz wh=
ich. But it should be at least 20 dB higher i think. Furthermore the backgr=
ound 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. Loo=
king forward to this amp performance!<BR><BR>Is it important to terminate t=
he loops transformer output with a 50 Ohm load in that case? On an oscillos=
cope i found that the level of DCF39 is higher when having no R connected t=
o the output but the waveform looks much better / cleaner!<BR><BR>I'm optim=
istic to get much proper reception of UK and PA and HB9 CW signals on 137 s=
oon, when beeing outside the city, steering the LF TX by the 70cm link :-)<=
BR><BR>Vy 73, Stefan/DK7FC<BR><BR></DIV></DIV></div></body></html>
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