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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