Mal,
I know. But anyway one can compare the SNR levels between different
antennas.
I'll set up a beacon now on 137.73 kHz, testing in QRSS-3.
Maybe you want to call CQ or so. If i can receive you, i will send a
capture. But i still cannot answer. Am in Darmstadt now, not in
Heidelberg. The UHF link for transmitting works just in a range of 5 km
and is disabled now.
I expect that i need a preamp and will not get the necessary sensitivy
now. Anyway i can compare the antennas and check how many dBs are
missed.
RX QRV in half an hour. Beacon starting at 17:30 UTC
73, Stefan/DK7FC
Am 17.08.2011 18:18, schrieb mal hamilton:
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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