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To All from PA0SE
Harry, PA0LQ, lives on the 12th (top) floor of
an apartment
building. When he wants to operate on LF he sticks out a
fishing
rod from his balcony and lets out some 35 m of antenna wire, feeding
it at the top end.
At first the wire simply hung down.
Because the
distance between wire and building was only 3
meters the efficiency of the
antenna was extremely low and the
wire swaying in the wind caused variable
loading of the
transmitter, leading to QSB.
At the next step the wire at
the bottom end was strung to a
lamppost about 18 m from the building.
Finally Harry attached the end of the wire, now
about 60 m long, to a tree some 40
from the builing. Together with an
increase of transmitter power
from 70 to about 300 W Harry's signals, both
transmitted and
received, over the months have improved some 15 dB, as those
of you
regularly work PA0LQ can confirm.
Harry only uses home constructed
equipment. His receiver
(136 - 10 - 1 kHz) includes an effective noise
blanker.
As Harry has no E-mail/Internet facility he asked me to put
some
details of the noise blanker on the reflector which I do
with pleasure. The
more so as noise blankers seem to be quite
a topic at the moment.
By the way, I will not be on the air myself this
and the
coming weekend as my LF transmitter will be at the Ham Radio
event
at Friedrichshafen where the TX will be on display at
the VERON booth.
PA0LQ wrote:
LF and the
QRM-problem.
At my QTH there sometimes exists a serious QRM
problem. In
general the noise level is already rather high, probably due to
the
continuous burning of over 100 fluorescent lamps on the inner
corridors
of the flat building. I have to live with that. But other noise
sources like light
dimmers, speed regulators of hand tools and the like can
be partially cured,
provided it is an impulse noise related to the mains
frequency. To find
this out, connect an oscilloscope to your receiver's audio
output
and trigger it to the mains (50 Hz) frequency. If the result is
a
still picture of the noise you then can find out if it has either a
50
Hz or 100 Hz relationship.
I have made a noise blanker system based
on
the mains frequency.
The 50 Hz or 100 Hz source can be derived from the
receiver's
mains transformer or otherwise. After clipping it into sharp
pulses
or a square wave it triggers two time-variable monostable MV's
in
sequence in order to obtain the correct phase relationship with
the
noise. Then a third monostable is triggered to provide an adjustable
gating
signal for the series switch in the receiver's signal
path.
The monostables I used were CD4528 CMOS
IC's.
The switching function can then be done by
either a junction-
or a MOSFET.
It is my experience that the series switch
has to be
inserted in the path where the signal level is already rather high.
Otherwise
the switching action produces loud bangs making the cure worse
than
the evil.
If you insert the switch in the audio path to speaker or
headphone, do it prior
to a small bandwidth audio filter, if you use
one.
The procedure is as follows:
Switch the noise
blanker on and set the blanker pulse width
to average.
Carefully adjust
the timing of one or both delay MV's, until
the noise is at minimum. Then
adjust blanker pulse width for an optimum. A slight
readjustment of the time delay then may be
needed.
In this way a loud S9 plus noise can be reduced to some S4
or
less.
Anybody interested for more details please write
to me. There is no
print lay out available as I never make PCB's. Use
VEROBOARD or the like.
My address:
Harry Grimbergen, PAoLQ.
Lijtweg 1202
2341
HE Oegstgeest
The Netherlands.
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