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Date: Wed, 17 Aug 2011 15:26:15 +0200
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Subject: Re: LF: Ferrite RX antennas
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Hello Markus, Jim, LF,

Tnx for suggestions. Have to think about that later. I want to go on in=20
small steps now.

I added a 100 pF vari-cap which allows to resonate in the desired range=20
(up to 137.8 kHz and down to 136.3).
As a first test a added a small winding, just 3 turns, 3 cm far for the=20
rods end, matched to 50 Ohm. So now it works as a passive antenna and=20
can be fed to my RX 50 Ohm input.
This is the complete LF RX arrangement, suitable to see and hear on 137=20
wideband: http://dl.dropbox.com/u/19882028/LF/LF-P%20RX%20RIG.JPG

The ferrite antenna is just as broad a the netbook now. Of course the=20
distance to the netbook must be increased later. It still has no=20
electric shield but a suitable housing to protect the Litz winding,=20
necessary to go on with tests on various locations.

The DCF-39 strength is 50 dB S/N in 1 Hz while the antenna (3 dB=20
bandwidth =3D 280 Hz) is tuned to 137.0 kHz (cannot tune to 138.83). The=20
band noise within the passband is 10 dB above the soundcards noise but=20
this may be different in a quiet location on a quiet day.

This looks all promising to me. I'll try the BF862 as a preamp soon.

Will do further tests with a test signal in the passband and compare=20
this to my 1m diameter single turn loop. And i will try my 50 Ohm preamp=20
in front of the RX.
Looking forward to the first QSOs!

73, Stefan/DK7FC

Am 17.08.2011 10:59, schrieb Markus Vester:
> Stefan, Jim,
> you could increase the signal bandwidth without compromising SNR by=20
> connecting a low impedance preamplifier. This technique has been used=20
> widely and successfully in magnetic resonance imaging ("preamp=20
> decoupling"). The preamp is designed to have an input impedance that=20
> differs largely from the noise-optimum source impedance, so that you=20
> can preserve the noise match but create an intentional signal mismatch.
> In practice, you would still want to use a low-noise FET connected to=20
> the high impedance point of a parallel resonant antenna. Normally the=20
> gate input impedance (megohms) is higher than the noise optimum (tens=20
> of kiloohms), so you would have no preamp damping at all. The trick=20
> is then to lower the input impedance by lossless feedback, which has=20
> neglegible effect on the noise parameters.
> One configuration is a compromise between common source and common=20
> gate circuit configuration ("Zwischenbasisschaltung"), which can be=20
> realised by inserting an additional negative-feedback winding in the=20
> source-to-ground path. This is similar to the old regenerative audion,=20
> but with the feedback coil polarity reversed. Another configuration is=20
> parallel feedback from drain to gate, by intentionally increasing the=20
> Miller capacitance.
>
> Best regards,
> Markus (DF6NM)
> -----Urspr=C3=BCngliche Mitteilung-----
> Von: James Moritz <james.moritz@btopenworld.com>
> An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
> Verschickt: Mi, 17 Aug 2011 1:13 am
> Betreff: Re: LF: Ferrite RX antennas
>
> Dear Stefan,
>
> Looking good so far...
>
> A Q of 486 is certainly reaching the point where it becomes inconvenien=
t -
> but remember that it is easy to reduce Q (probably connecting a preamp,
> putting it in a container, etc, will reduce Q a bit anyway), and that t=
he
> higher Q is, the better the SNR. So I would test it as an antenna with =
a
> preamp - if there is more SNR than you need, you could experiment with =
some
> damping resistance.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>

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</head>
<body bgcolor=3D"#ffffff" text=3D"#000000">
Hello Markus, Jim, LF, <br>
<br>
Tnx for suggestions. Have to think about that later. I want to go on in
small steps now. <br>
<br>
I added a 100 pF vari-cap which allows to resonate in the desired range
(up to 137.8 kHz and down to 136.3).<br>
As a first test a added a small winding, just 3 turns, 3 cm far for the
rods end, matched to 50 Ohm. So now it works as a passive antenna and
can be fed to my RX 50 Ohm input.<br>
This is the complete LF RX arrangement, suitable to see and hear on 137
wideband: <a class=3D"moz-txt-link-freetext" href=3D"http://dl.dropbox.co=
m/u/19882028/LF/LF-P%20RX%20RIG.JPG">http://dl.dropbox.com/u/19882028/LF/=
LF-P%20RX%20RIG.JPG</a><br>
<br>
The ferrite antenna is just as broad a the netbook now. Of course the
distance to the netbook must be increased later. It still has no
electric shield but a suitable housing to protect the Litz winding,
necessary to go on with tests on various locations.<br>
<br>
The DCF-39 strength is 50 dB S/N in 1 Hz while the antenna (3 dB
bandwidth =3D 280 Hz) is tuned to 137.0 kHz (cannot tune to 138.83). The
band noise within the passband is 10 dB above the soundcards noise but
this may be different in a quiet location on a quiet day.<br>
<br>
This looks all promising to me. I'll try the BF862 as a preamp soon.<br>
<br>
Will do further tests with a test signal in the passband and compare
this to my 1m diameter single turn loop. And i will try my 50 Ohm
preamp in front of the RX.<br>
Looking forward to the first QSOs!<br>
<br>
73, Stefan/DK7FC<br>
<br>
Am 17.08.2011 10:59, schrieb Markus Vester:
<blockquote
 cite=3D"mid:8CE2AD319756ED7-1C20-14370@Webmail-m104.sysops.aol.com"
 type=3D"cite"><font color=3D"black" face=3D"arial" size=3D"2">
  <div>Stefan, Jim,</div>
  <div>=C2=A0</div>
  <div>you could increase the signal bandwidth without compromising SNR
by connecting a low impedance preamplifier.=C2=A0This technique has been
used widely and successfully in magnetic resonance imaging ("preamp
decoupling").=C2=A0The preamp is designed to have an input impedance that
differs largely from the noise-optimum source impedance, so that you
can preserve the noise match but create an intentional signal mismatch.</=
div>
  <div>=C2=A0</div>
  <div>In practice, you would=C2=A0still=C2=A0want to=C2=A0use a low-nois=
e FET
connected to the high impedance point of a parallel resonant antenna.
Normally the gate input impedance (megohms) is=C2=A0higher than the noise
optimum (tens of kiloohms), so you would have no preamp damping at all.
The trick is=C2=A0then to lower the input impedance by lossless feedback,
which has neglegible effect on the noise=C2=A0parameters. </div>
  <div>=C2=A0</div>
  <div>One configuration is a compromise between common source and
common gate circuit configuration ("Zwischenbasisschaltung"), which=C2=A0=
can
be realised by inserting an additional negative-feedback winding in the
source-to-ground path.=C2=A0This is similar to=C2=A0the old=C2=A0regenera=
tive audion,
but with the feedback coil polarity reversed. Another configuration is
parallel feedback from drain to gate, by intentionally increasing the
Miller capacitance.<br>
  <br>
Best regards,</div>
  <div>Markus (DF6NM)<br>
  </div>
  <div
 style=3D"font-family: arial,helvetica; color: black; font-size: 10pt;">-=
----Urspr=C3=BCngliche
Mitteilung----- <br>
Von: James Moritz <a class=3D"moz-txt-link-rfc2396E" href=3D"mailto:james=
.moritz@btopenworld.com">&lt;james.moritz@btopenworld.com&gt;</a><br>
An: rsgb_lf_group <a class=3D"moz-txt-link-rfc2396E" href=3D"mailto:rsgb_=
lf_group@blacksheep.org">&lt;rsgb_lf_group@blacksheep.org&gt;</a><br>
Verschickt: Mi, 17 Aug 2011 1:13 am<br>
Betreff: Re: LF: Ferrite RX antennas<br>
  <br>
  <div
 style=3D"margin: 0px; background-color: rgb(255, 255, 255); font-family:=
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 id=3D"AOLMsgPart_0_ef62e7a6-5539-4a6b-85ec-e39aa4a8f27a">
  <pre style=3D"font-size: 9pt;"><tt>Dear Stefan,

Looking good so far...

A Q of 486 is certainly reaching the point where it becomes inconvenient =
-=20
but remember that it is easy to reduce Q (probably connecting a preamp,=20
putting it in a container, etc, will reduce Q a bit anyway), and that the=
=20
higher Q is, the better the SNR. So I would test it as an antenna with a=20
preamp - if there is more SNR than you need, you could experiment with so=
me=20
damping resistance.

Cheers, Jim Moritz
73 de M0BMU=20


</tt></pre>
  </div>
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