History of the emission in amplitude
modulation
The emission of amateur started about the years 1910.
The " stations " were in general made up with only one tube to make télégraphie.
Some years later, experimenters tested amplitude modulation.
Om's just used a CW transmitter and to insert a high speaker in the wire of antenna. The results were not extraordinary, moreover it was necessary to pay attention not to burn the moustache!!
Many tests made it possible to improve the modulation and to have a good listening.
The power did not exceed 70 Watts .
I would quote only two ways of amplitude modulating.
the grid or screen modulation
The plate an screen modulation
The latter being most effective.
Telegraphy and AM were the principal modulations used until worms the years
60-65 where appeared the first transmitters with single band (BLU). They were
very effective but expensive. Little by little this mode of transmission was
adopted by all to the detriment of AM.
Why?
The modulation in single band
A transmitter with amplitude modulation generates a centre frequency and two bands (higher and lower).
In a transmitter in BLU one removes the centre frequency and one of the band : therefore less spectral obstruction.
What brings to have receivers with a narrower bandwidth thus more sensitive.
To be able to decode the modulation one uses an oscillator which will regenerate the carrier.
This modulation brings the best compréansibility of the messages on weak or disturbed signals.
Moreover from its principle and at equal power, transmitter BLU further carries than a transmitter AM.
The amplitude modulation now
In spite of progress, nostalgic radios amateurs are found regularly on the 80 meters band to traffic in amplitude modulation.
What' use to make AM on 80 meters?
Recent transmitter for amateur. (ICOM, KENWOOD, YEASU,.....).
These materials are especially designed to make BLU.
The carrying AM power is relatively low: 25 to 40 Watts;
Nowadays the noise and the occupation of the 80 meters bandare much more significant than there are a few about thirty années.
Old material for amateur.( HEATHKIT,
GELOSO, HALICRAFTER,COLLINS,...).
These transmitters used for the majority the plate and screen modulation. There
was one or two tubes of type 6146 or 807 on final.
Old military transmitter.
Nowadays It's possible to find this type of transmitters in surplus. The advantage
is to often have material in very good state, providing a comfortable power of
70W to 1KW. These tansmitters are bulky and large consumer of amps!
Personal transmitter
It's possible, with old electronic tubes to mount a AM transmitter.
It is preferable to choose the plate and screen modulation.
That requires a CW transmitter with one or two tubes : type 807, 811, 813,
to see QB3-300, QB4-1100 or TH100, TH250...
Choose an BF amplifier with same power (that gives a good margin of adjustment) and most difficult to find: a modulation transformer.
AM on 80 meters band :
Sur 3550 KHz :
Sunday morning at 9:00
Capitain : F6ACB.
This QSO has existed since 30 years.
Sur 3665.5 KHz :
Saturday morning at 9:00
Sunday morning at 9:30
Capitain : F5OTG.
This QSO has existed since 2 years.
Some French stations equipped with AM transmitters
F6ACB
F5BHJ
F6HSB
F5IST
F5JDG
F5MAF
F5OTG
F5UOO
AM in United States
You can see these sites.
AM
Noth América
The
AM window
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