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Very good...
73, Joze s52ab
----- Original Message -----
Sent: Thursday, September 29, 2011 9:30
PM
Subject: Re: LF: Ferrite RX
antennas
LF, Jim, Markus,
I finally finished my preamp for the /p
ferrite antenna. It works excellent and allows to stepless adjust the gain.
The design is very simple, just using a single BF862 like in my separated VLF
E field probe. Amp gain is a good 20 dB. The JFET is not directly connected to
the tuned winding (signal would be to large then with this FET) but is
decoupled with a second untuned winding of just 6 to 10 turns! I found that
this is optimal in my situation.
In the experimental stage i took a
cardboard tube to experiment what could be the best distance to the tuned
winding but then i found that the signal levels strongly depend on the
distance of the center of the rod. So i made the final decoupling coil
movable. Now i use a PVC ring that exactly fits on the ferrite rod's outher
PVC tube (made on a turning machine). Signal levels can be reduced by > 20
dB if the output winding is moved to the end of the rod. This will help to
prevent RX overloading if the antenna points to DCF or DLF. So, the best S/N
can easily be obtained :-)
During the construction i had some trouble with
oscillations of the FET that were not so easy to avoid but later i took a
longer cable (3m, that is necessary anyway, to get some distance to the
netbook) and all works fine now!
BTW i can receive DLF at 153 kHz at 40
dB S/N in 1 Hz just with that untuned loop, i.e. 6 turns of 4 cm diameter
enam. Cu wire on the FET, without the Ferrite rod!
I will further
investigate about a anti-serial winding between ground and source, to increase
the antenna bandwidth...
A quick circuit and picture is here: http://dl.dropbox.com/u/19882028/LF/DK7FC_LF_p_active_ferrite_antenna.JPG
So,
looking forward to new CW skeds, probably tomorrow i will go out and try the
new RX...
73, Stefan/DK7FC
Am 18.08.2011 01:16, schrieb mal
hamilton:
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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