And of course a good number of statics.
Pic has been taken with Spectrogram demo version.
by Claudio Parmigiani IZ2FER.
The problem.
We all know very well that
the first step to have good VLF receptions is an "electromagnetically quiet"
place.
Often it is not easy to find
a place with this requirements for continuative monitoring and in case
of portable stations the recordings are necessarily short, let's say one
or two hours at the most.
As Renato use to say, there
are very rare phenomena on the air, without a continuous monitor the probability
to catch them is very low. For this reason I tried to connect my good old
RS4 receiver to my ham aerials, just to see what's going on.
With a big surprise I found
out much more than hum noise.
Currently I have on my rooftop two dual band antennas, a Comet 2x4 MAX, a Diamond X30 and a self-made windom antenna for HF bands, plus a spare cable (open at the top). The length of each cable is about 30 meters.
I am experiencing the best
results connecting RS4 to the HF antenna cable or to the Comet one (only
central pin, not the shielding).
Due to this fact I know that
my antenna is composed only by the cable but I have different behaviors
with different cables.
For example, the open cable
is a generator of noise, like the one connected to the Diamond antenna,
the other two works much better.
My RS4 has some little advantages,
it includes a little filter for 50 Hz hum (better than nothing) and its
power consumption is about 7-8 mA. With a 12V 2Ah Pb-Gel sealed battery
it can operate continuously for several days.
Earth bonding for the receiver
is absolutely necessary.
This is one of the first spectrogram
I got:
|
This spectrogram shows about
11 seconds of recording in the range 0-24 kHz. Man made noise is present
up to 6 kHz, but also I had good RTTY signals and often very good signals
from Alpha Radionavigation system (in this picture in a unusual configuration).
And of course a good number of statics. Pic has been taken with Spectrogram demo version. |
| Background noise
level has great variations during the day. In this b/w pictures you can
see a "lucky strike", I cought a CW transmission from Russian RKS station.
Also presents some carriers just under 16 kHz (typically 15625 Hz) , most
probably caused from neighbor's TV set.
These kind of signals disappears during night time. Pic has been taken with Spectrogram demo version. |
 |
The solution.
By the way, a system like
that is not good for frequencies under 6 kHz.
If your purpose is to monitor
RTTY or slow CW activities it should be ok, but if you want to listen some
statics/tweeks/whistlers something has to be made to clean up the spectrum
from the rubbish.
Consider also that designing
and building a hardware filter is not easy, and often take a long time
for adjust.
When a unexpected source
of noise is found (i.e. TV set) we need a flexible system to cut it out
of the spectrum without building new pieces of hardware.
Even more, a receiver equipped
with all this hardware will be more noisy (and complicated).
Renato suggested me to use
Spectrum Lab, by Wolf DL4YHF. It includes a powerful set of real time filters,
delay lines, echo lines, local oscillators, mixers and all you need to
design filters by yourself.
The program can be found
at Wolf's homepage .
Actually I am not very skilled
in filter design but thanks to the information included in the user's manual
I have been able to clean up all the stuff.
Here there are some examples:
 |
In the image on the right
it is possible to see the man made noise spreading up to 10 kHz.
This pic has been taken early in the morning, during the day the noise is even worst, due to industrial and home activities. It's easy to record elevators, TV sets, switching power supplies for halogen lamps, electric mixers, drills and so on.. This picture shows 1'30'' screen shot, full frequency span. |
 |
Settings of "Spectrum Components"
have been configured as suggested in the user manual, in the chapter related
to the "black box":
Set "Delay Time = 0.02 sec",
"Input Factor = -0.7", "Feedback Factor = 0", "Bypass Factor = 0.7"
This removes 50 Hz (1/0.02sec) and all harmonics of 50 Hz by adding the signal delayed by 20 ms to itself. This picture shows 1'30''
screen shot, full frequency span.
|
 |
The image on the right has
been taken using my loop antenna (Bikeloop) and Easyloop receiver.
As you can see, there are very few statics, also the carriers on the higher part of the spectrograms are caused by local man-made noise. This picture shows 1'30''
screen shot, full frequency span.
|
 |
The picture on the left has
been taken under the same conditions of the previous, but now the filter
for 50 Hz and harmonics has been applied.
The gaps in the spectrogram are caused by filter, but there are not big benefits, probably because the interference are complex harmonics of 50 Hz, added with other spurious emissions. This picture shows 1'30'' screen shot, full frequency span. |
As you can see in the examples,
the use of receivers for electrical field seems to achieve better results.
Magnetic aerials needs to
be put far away from relevant metal masses, but often this is not possible.
On the other side, the results
obtained with digital filters and electrical field receiver are impressive.
Plus, it could be a nice
and challenging idea to play with VLF using your existing aerials.
When using cables connected
to VHF/UHF antenna (or provided with BALUN) just remember to disconnect
earth from the shielding and leave shielding unconnected. This is because
most VHF/UHF antennas have an induction coil who shorts radiating element
and shielding (to eliminate static charges).
The coil is tuned for VHF
frequencies and up, and it eliminates all the static and signal we want
to receive. The same happens with BALUN.
Spectrum Lab also allow users
to program periodic actions, such as image capture.
In this way it is possible
to monitor continuously what's going on, in particular during the night
time.
It has also many other interesting
features, please refer to the extensive manual for use them properly.
Conclusions.
My opinion is that, under
certain conditions, VLF recordings are possible even in big cities
or in locations not "VLF-compliant".
A good 50 Hz hardware filter
is suggested anyway. Hum is present almost everywhere and the filter will
always find something to do.
The rest of "dirty job" can
be done by DSP.
To avoid intermodulations
caused by local strong signal, i.e. close broadcasting transmitters, a
simple low pass filter with frequency cutoff about 50-100 kHz could be
needed, depending on your location.
Of course receiver has to
be constructed in the right way, it has to be quiet, not noisy. In few
words it has to be a good receiver.
DSP can help in many circumstances
but cannot increase the performances of a poor receiver.
That's all folks !
I hope, as usual, that these
few lines could help someone else to get involved in the wonderful world
of VLF...!
Many thanks to Renato for pushing me to try this program and thanks also to Wolf for Spectrum Lab.
Feedbacks/comments are welcome!
Write me: iz2fer@tiscali.it