ATV on 1255 MHz
The television set
The standard in amateur television on 1255 MHz is the standard used in satellite broadcasting, except for large DX where excursion is reduced.
Thus the receiver will be a commercial receiver, preferably Bi-standard PAL-SECAM (in Europe) but amateurs use more and more PAL standard.
It is necessary to avoid the too sophisticated TV sets which pass on a blue screen when the signal weakens, they make very difficult the search of weak signals.
The satellite receiver (tuner)
Any satellite receiver covers at least 950 MHz to1750 and now 2050 MHz; that covers the amateur band, you have only to tune it on 1255 and to put the frequency in memory.
That is enough to receive a local station.
Attention, the satellite receivers provide a voltage to the antenna intended to feed the satellite head of the parabola, if the 1255 antenna is uninterrupted by a capacitor, there will be the short-circuit: it is necessary either to prohibit the output of the tension to the antenna, or to put a condenser somewhere.
For better results, a preamplifier is essential; there were many descriptions these last years.
The preamplifier must be placed the more close possible to the antenna, it is fed directly by the cable thanks precisely to the power provided by the tuner.
The standard is frequency modulation, with one or even several sub carriers; at least one for the sound, the whole area from Marseille to Narbonne was standardized with sub carrier sound at 6,5 MHz; 16 dB below the picture carrier.
The levels are the same as on ASTRA (a european broadcasting satellite), when one puts 1Vcc video to the input of the transmitter, the correspondent must obtain exactly 1Vcc video at the outpout of its tuner.
For the sound, when one puts .5 Vcc audio to the input of the transmitter, the correspondent must obtain exactly .5Vcc audio at the output of his tuner.
There were many descriptions of transmitters these last years, in general, the 1255 is produced by a VCO followed by a 256 divider.
The VCO is frequency modulated by the vidéo+sound sub carrier, sound carrier itself is modulated in FM; the signal is post amplified by MMIC and attacks a hybrid module 1255 1,5W which can attack a hybrid module 18 W as on the repeater F5ZGN placed at the top of the Aigoual mount.
Video qualities are impressive, the limitation in quality with these transmissions is that of the sources (camera, video tape recorders) and that of the screens of the television sets, but not in the transmitting and receiving chain.
In television, the least decibel counts, one can receive television locally with quad elements (*) in front of a reflector, it's what we use at the Aigoual mount with F5ZGN, but it is 1650 m high.
The maximum must be done at the antenna if we want better results: 23 elements ATV , better 35 elements, and if possible two in phase. With more elements, the directivity of the antennas makes the trafic more difficult.
Don't be too full of hope however, in television it is better to live on a hill, because ATV contacts will be possible only if the contact in telephony is S9++.
(*) in French
Return to the welcome page
© 1999-2011 A. Ducros F5AD