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Stefan
Excellent performance for a small ferrite rx antenna.
You mention that the loop is better than the ferrite
antenna possibly because physically the loop has a larger signal capture
area.
73 es gl
de mal/g3kev
----- Original Message -----
Sent: Wednesday, August 17, 2011 9:30
PM
Subject: Re: LF: Ferrite RX
antennas
Mal, LF,
Back from /p from JN49IV75OR
Mal, you
are the sceptic on ferrite antennas, so pse fasten your seat belt before you
take a look on this picture: http://dl.dropbox.com/u/19882028/LF/G3KEV_on_ferrite_antenna.png
Signal
was audible as well: http://dl.dropbox.com/u/19882028/LF/G3KEV_audible_on_ferrite_antenna.WAV
QRB
is 796 km.
Critical for a fast CW QSO but it is evening and QRN was high.
The ferrite antenna was just 20 cm above the ground/soil and 30m RG58 in
between. I used the 50 Ohm preamp during that test.
Well visible in
the picture: Sferics are strongest arround 137.7, due to the high antenna Q.
There was a frequency offset of about 10 Hz but never mind. Below 137.5 kHz i
assume this is WSPR by PA0A?
A comparison between the 1 m diameter
single turn loop was done. Both antennas were adjusted to minimise DCF39. My
own test signal out of 53.8 km was up to 55 dB S/N in 1 Hz on the loop without
a preamp, audible of course. On the ferrite antenna + preamp it was 55 dB S/N
too! But there must have been QSB due to a skywave component. Hasn't been the
best time for that test.
Both antennas can't be accurately resonated at
138.8 kHz, and have different Q. The ferrite antenna has about twice the Q of
the loop. When using the loop + preamp, the ADC gets heavyly overloaded.
Without a preamp the ferrite antenna performs about 15 dB worse than the loop.
This should be solved with a single JFET amp stage.
Will do further
tests soon but this ferrite antenna seems to become a really useful
alternative to any other large antenna. It is very well /p and
/m.
Looking forward to further improvements and first QSOs. Glad to
have realised that project in a few days after starting the discussion
:-))
If someone follows the discussion and thinks about building such
an antenna too, just ask if you have questions that weren't discussed so far.
Maybe this will lead us to new ideas.
Ah, BTW, still no E field shield.
Do the experts really think that this is necessary? I mean, because i will
ever use it in a quiet location!
73, Stefan/DK7FC
Am
17.08.2011 20:02, schrieb mal hamilton:
ok Stefan
Nw CQ 137.7 can u receive me
de mal/g3kev
-----
Original Message -----
Sent:
Wednesday, August 17, 2011 5:55 PM
Subject:
Re: LF: Ferrite RX antennas
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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