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Stefan
But what sort of strength do you Receive weak Radio
Amateur signals. That is the real test
Commercial radio stations a different matter with their
Megawatts
de mal/g3kev
----- Original Message -----
Sent: Wednesday, August 17, 2011 2:26
PM
Subject: Re: LF: Ferrite RX
antennas
Hello Markus, Jim, LF,
Tnx for suggestions. Have to
think about that later. I want to go on in small steps now.
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).
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.
This is the complete LF RX
arrangement, suitable to see and hear on 137 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 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.
The DCF-39 strength is 50 dB S/N in 1 Hz while the
antenna (3 dB bandwidth = 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.
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 this to my 1m diameter single turn loop. And i will try my 50 Ohm
preamp 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
connecting a low impedance preamplifier. This technique has been used
widely and successfully in magnetic resonance imaging ("preamp
decoupling"). The 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.
In practice, you would still want to use a low-noise FET
connected to the high impedance point of a parallel resonant antenna.
Normally the gate input impedance (megohms) is higher than the noise
optimum (tens of kiloohms), so you would have no preamp damping at all. The
trick is then to lower the input impedance by lossless feedback, which
has neglegible effect on the noise parameters.
One configuration is a compromise between common source and common gate
circuit configuration ("Zwischenbasisschaltung"), which can be realised
by inserting an additional negative-feedback winding in the source-to-ground
path. This is similar to the old regenerative audion, but
with the feedback coil polarity reversed. Another configuration is parallel
feedback from drain to gate, by intentionally increasing the Miller
capacitance.
Best regards,
Markus (DF6NM)
-----Ursprüngliche
Mitteilung----- Von: James Moritz <[email protected]>An:
rsgb_lf_group <[email protected]>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 inconvenient -
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 the
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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