Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - Those excellent c.w. signals
Any philosopher looking for proof of the adage "History repeats itself" has to go no farther than one of the present c.w. bands. Admittedly there are a few excellent signals that might be considered as representing good 1946 technique, but altogether too many of them are not much of an improvement over the so-called "selfishcited" and "prehistoric" signals of the late 20s and early 30s. In those days signals suffered from chirps, clicks, drift and too much modulation. Fifteen years later, c.w. signals have been improved to the point where their only shortcomings seem to be from chirps, clicks, drift and too much modulation. It shouldn't be necessary to remind any amateur that the present regulations (§12.133) could put a large number of colored tickets in the hands of the c.w. men any time the FCC feels mean and decides the paper shortage is no more. So rather than force the hand of the FCC, let's take a look at some old principles that may have been forgotten.
Better yet, let's take a look at one of the bands before we continue. It doesn't matter much which band, because we'll find those poor signals on any. Tune across the band and just count the number of signals that are a real pleasure to copy - discounting the fist, of course, because that's another story. If you find more than 50 per cent that are really sweet sounding, don't read any farther. But we'll give you odds the percentage doesn't run that high. And when you find one you like, listen closely to see what makes it nice to copy. You will find that it suffers from none of the ailments ascribed above to "prehistoric" signals.
A chirp is a change in frequency during a dot or dash and is most prevalent in transmitters where the oscillator is keyed, although keying a later stage is by no means insurance that a chirp cannot develop. Keyed crystal oscillators will chirp when tuned for maximum output instead of for best keying, and some crystals key better than others. Generally the regenerative crystal oscillators, like the Tri-tet and grid-plate, will key better than straight triode or tetrode oscillators and are somewhat less critical in adjustment. Keyed self-excited oscillators require high-C tank circuits and careful adjustment of the feedback. Either crystal or self-excited oscillators key best when they are capable of oscillating with only a few volts on the plate (and screen) and when they are lightly loaded. If a shunt-connected grid leak is used, an r.f. choke should be used in series with the leak to reduce loading, and the value of the leak should be high.
If a stage following the oscillator is keyed and the oscillator is running all of the time, a chirp can be introduced if the oscillator is sensitive to load changes introduced by keying, if there is insufficient isolation, or by changes in the voltages applied to the oscillator resulting from the fluctuation in line or supply voltage caused by keying. Neutralization or further isolation will cure the first condition, and the use of VR tubes or other means of stabilization will help if voltage regulation is causing the trouble.
Detecting a chirp on one's own signal requires that a stable receiver or monitor be available, and the listening can best be done on 28 Mc., where the chirp will be accentuated by frequency multiplication. If you are running a kilowatt on 7 Mc. and your receiver dial lights blink every time you hit the key, there is a good chance that your receiver is chirping when you try to listen to the harmonic on 28 Mc., and the receiver should be moved to a spot where the line voltage isn't affected. Also, steps should be taken to insure that the receiver isn't overloading while observing the signal, and it may be necessary to short the input terminals of the receiver while making the tests. Run with the audio gain wide open and the r.f. gain backed off. Tune in the signal at a low beat note and, while sending slow dashes, listen closely on both "make" and "break" for any changes in pitch. Try tuning on both sides of zero beat, to see if the signal sounds the same on either side, in case you missed a slight chirp when listening on one side only. This "tuning on both sides" is an important point, and will often show up faults that otherwise would be missed. And do the listening yourself - the other fellow may not want to hurt your feelings!
There is, of course, no standard on the maximum permissible amount of key click, and opinions differ somewhat as to what is desirable or undesirable. It might be said that if you don't interfere with any broadcast receiver and no nearby ham has cut down your antenna in a blind rage, your clicks are not bad, but the signals you like to copy are usually the ones with no objectionable clicks on either "make" or "break." Most of the c.w. men who pay attention to the fine points of good keying say that they like their signals "firm" on "make" and "soft" on "break," with no clicks adjacent to the signal frequency on either make or break.
Clicks are caused by turning the signal on and off too abruptly, and it is generally necessary to slow up this process by introducing some delay. Series resistors and shunt capacitors, necessary to the circuit for other reasons, often introduce some cidental lag but generally it isn't enough for complete shaping. When an oscillator is keyed, the grid leak and grid condenser affect the keying pc when cathode keying is used, by introduce g a shaping characteristic of their own, and for this reason it is generally advisable to key an 1 eillator in the negative or positive plate lead, any lag circuits are to be used.(1) On the other hand, a driven amplifier can be keyed in the cathode or plate circuit with equal response to lag cirruits, because of the constant excitation. Lag circuits generally consist of a small iron-core choke series with the key and a condenser across the key. If a small current is being keyed, the choke 'II be large and the condenser will be small, and vice versa for the keying of a high-current circuit. Transformer primaries and secondaries can be used for the chokes. Making the choke larger will make the "make" softer, and increasing the value of the condenser will soften the "break." Small 2.5 mH r.f. chokes in series with the condenser and placed right at the key will cut down radiation and clicks caused by sparking at the key. A tube keyer(2) is a deluxe method of shaping the signal, and eliminates a large portion of the experimental work necessary with choke-and-condenser combinations. Primary keying has many faithful followers and is an excellent system for reducing clicks, although two stages should be keyed(3) if the signal is to remain T9 and not be too soft.
To check for clicks, tune in the signal on your receiver with the input shorted and the r.f. gain reduced, and turn off the b.f.o. When making a series of slow dashes, a slight thump will be heard on "make" and practically none on "break" if the keying is about right, and no clicks will be heard with the receiver tuned away from the signal. However, if the keying is made too soft it will be difficult to copy the signal when it is weak or when sending at high speeds. Personally - and we have found many who share this opinion - we like a signal that is soft enough to start to run together at around 45 w.p.m. and so is crisp enough for all normal sending.
If the oscillator is being keyed, the introduction of lag will show up any tendency to chirp even ore than if no shaping is used, because the volt-y is rising and falling relatively slowly as the ator is keyed on and off and it is easier to rbserve the chirp. Thus an oscillator that keys e+l with shaping circuits to remove the clicks is one of the most difficult pieces of equipment to achieve, and the difficulty increases with the operating frequency. Good keyed oscillators on 14 and 28 Mc. are a very rara avis indeed.
Drift is one of those things that you're a chump to tolerate unless you are a rare piece of DX, in which case the Ws will put up with anything to get a contact. If your frequency drifts during a QSO the other fellow stands a good chance of losing you and, in this day and age, he can't be blamed for giving you up quite quickly. He has to retinae his receiver all of the time he is trying to copy you, and that isn't fun in our crowded bands that require the crystal filter in most of the time. The cure for drift is obvious, of course - cut down the input to the oscillator until it doesn't drift, or else redesign it so that it stays put. Don't check your drift against your receiver - which may have some drift of its own - check it against a low-drift crystal or some other standard of which you can be sure.
Here is a beautifully controversial subject that has been the axis of many a heated debate. There seems to be one school that will accept only the pure d.c. signal and consider anything else a violation - and then blithely overlook the p.d.c. signal with a 100 cycle chirp and clicks extending 10 kc. either side! The other school will tolerate a small amount of modulation provided the signal has no clicks or chirps. This latter school is well represented by many of the West Coast signals and, in all fairness, it must be admitted that those who adhered closely to their code certainly took up no more room in the band than anyone else and had beautiful signals to copy. Actually the slight amount of modulation was noticeable only when the signals were S8 and S9. Unfortunately, a few eager beavers abused the thing and came up with signals that were modulated so heavily that they were T5 even when they were S3. Here again there is no FCC standard, but any time they want to tag you they can because the definition of AL is "Telegraphy on pure continuous waves." If you must have a few per cent modulation, for sentimental reasons, be sure that it doesn't bother anyone even when you're S9 and you won't have any trouble.
However, if you are plagued with a rough and broad signal, look for too much ripple on your oscillator or final plate supply, and buy up a few µfd. of these excellent condensers that are kicking around in surplus these days. If the oscillator gives a T9 signal and the final supply is well-filtered, you'll have to put in a chopper wheel somewhere along the line to get a modulated signal. Chopper wheel? Oh, that goes back to the early 20s, so we may get to it in a few more years.