LF:Interesting details of PA0LQ's operation
Sat, 19 Jun 1999 09:01:35 -0400
> Dick Rollema wrote:
> 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
> 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.
> 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
> 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.