Mark, W1EOF, wrote:
> 1. What is "EHT overwinding rescued from an old TV set" ???
That's the 'extra high tension' winding on the line output
transformer. Some older TVs have that as a separate winding
which can be slid off the core. Unfortunately modern TVs
are not so hobbyist-friendly and these days the whole EHT coil
assembly tends to be potted along with the EHT rectifier.
But no worries - there are alternatives:-
a) Wind your own.
b) Buy one of those ready made ferrite rod/coil assemblies
that are advertised for receiving time code signals at around
60kHz. Add capacitance across it to tune it down to VLF.
c) Make up a loop antenna on some sort of a wooden frame.
> 2. What was T1's function in the TV set?
Not entirely sure. It's only a small one (2cm across) and
it came out of the scanning circuitry, so I guess it's just
a driver transformer. Again, not critical. I'm sure you
could use a small audio transformer from an old transistor
> 3. Why did you choose BF245B for Q1, and is this critical?
It just happened to be what was handy, and for this application
it is not critical. You could leave out that stage altogether
and put the antenna straight into the opamp.
> 4. Why did you choose NE5534, and is it critical?
More or less any opamp (except the noisy old 741) would do
here. Its main job is to buffer the signal to drive a long
coax back to the house.
My advice would be to disregard the circuit that I use, and
evolve your own to suit.
Here are some additional comments and suggestions for anyone
contemplating a similar project:-
Concentrate on the antenna coil, initially with it connected
directly into the soundcard microphone input via a couple of
metres of screened lead. You'll get a lot of interference,
but it will allow you to get the antenna tuned right, and it
will give good enough reception for you to decide which signals
you want to monitor, etc.
The capacitance of the screened lead significantly affects
the antenna tuning, so choose a length of cable (a few metres
max) and then stick with it. If you use a ready made 60kHz ferrite
rod antenna intended for time code signals, you'll probably find
that the extra capacitance of the screened lead is enough to
pull its resonance down to 15-25 kHz.
Once you have it working nicely into the microphone input,
estimate how much extra gain you would need for it to work
into the soundcard *line* input (this is preferable because it
is less sensitive to local interference in the shack). Then
put together a preamp to give you the required gain when driving
into the 50 ohm load of a terminated coax.
These VLF signals are very strong, so an overall gain from
the preamp of say 10 to 100 will probably be enough, and a
modest antenna is all that is required. Most audio preamps
will work happily at these frequencies, so you might be able
to buy one in kit form. On the whole, this is a much easier
project than, say, a broadband whistler receiver.
I spent about a month fiddling with different combinations of
coils, cables, and preamps before settling on the scheme described
in the web page. It's a great help for tuning the antenna if you
have an LCR meter and a signal generator, but a certain amount of
guesswork and trial/error will make up for any lack of test
equipment. You can use sidd as a spectrum analyser just by
plotting the spectrum output file.
If you have an existing VLF receiver, you need only tee off some
of the signal into the soundcard of a spare PC. Those pesky MSK
signals which up to now have been giving you intermodulation, can
be put to good use!
It's remarkable that exotic deep space things like gamma ray bursts
and magnetars can be detected at home by the amateur with such
modest equipment. My monitor has been running continuously for
only a month, so I've not seen any of those yet. But it's
picked up a couple of solar flares, including a B9 flare on the
Also, I never appreciated the extent to which lightning can
affect propagation, albeit indirectly by upsetting the flow of
particles in the radiation belts.