. QST . un ricevitore di a.m., come nefariously prac-ticed da la maggior parte di noi e chiunque con una testa inhis del cervello concluderebbe che se desiderate un segnale senza alcuna vitamine, se jou desidera la vostra voce annoiato in una cascata di rumore e coagulo-bed con QRM, usi il f.m. Whoa là! Tuttavia, cari amici, w^e sono stati over-guardare un oggetto succosa. Superiore a 52.5 Mc. (e onsome frequenze sotto) ci siamo limitati fino alla versione a banda stretta di f.m. Che cosa se wecrank il guadagno audio fino a darci una modulazione *1146 Prospect Road, Pittsburgh 27, Pa. ^ nella maggior parte delle posizioni non c'è tanto v.h.F. QRM thatthe extra ban
1529 x 1634 px | 25,9 x 27,7 cm | 10,2 x 10,9 inches | 150dpi
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. QST . an a.m. receiver, as nefariously prac-ticed by most of us, and anyone with a brain inhis head would conclude that if you want asignal without any vitamins, if jou want yourvoice droaied in a waterfall of noise and clob-bered with QRM, just use f.m. Whoa There! However, dear friends, w^e have been over-looking a juicy item. Above 52.5 Mc. (and onsome frequencies below) we arent confined tothe narrow-band version of f.m. What if wecrank the audio gain up to give us a modulation *1146 Prospect Road, Pittsburgh 27, Pa. ^ In most locations there is not so much v.h.f. QRM thatthe extra bandwidth would matter much; and in any caseheavily-modulated a.m. transmitters in the region above144 Mc. are already using more space than f.m. with adeviation ratio of 1.0. index of 1.0? Second-order sidebands appear, but they are still relatively weak.^ The happypart is what happens to the first-order sidebands.Now they have some real zip. Put this signalthrough an i.f. ampUfier of 6 or 7 kc. bandwidth, . The 3Q C f.m. exciter is a compact package. Atthe right front is the 6.G7 oscillator, with the modula-tor behind it. Left is the tripler, with the 5763 doublerin the back corner. The voltage-regulator tube is visiblein back of the modulator. Note ventilation holes inthe side of the bottom cover. detect it with a good ratio detector or discrimi-nator-foUowing-a-limiter, and presto — f.m.comes into its own! Rummage through the old QSTs publishedjust before the war and there you will see the ^beginnings of a development in amateur f.m.that was nipped in the bud at Pearl Harbor.Look, for instance, at that fascinating articleby ^Murray Crosbj^ on Bandwidth and Reada-bilit} in Frequency Modulation, page 26 ofMarch, 1941, QST. There we see that for maxi-mum readability of weak signals the deviationratio should be 1.0, and that such an f.m. signalis always more readable than a.m. In fact, atsome weak signal levels this almost-n.f.m.signal is as readable as an equivalent 100 perc
