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|@ DIR: /ATV|
@ FILE: TXATV
Transmitting ATV Pictures:
The basic components of an ATV transmitter system.
2. Suitable audio video transmitter.
3. High gain yagi antenna and suitable coax.
Horizontal polarization is usually the standard.
The ATV transmitter can be a "you build it kit" or
a "cable TV modulator" or a modified "domestic
wireless video surveillance monitoring system"
For VK3RTV use, only the analogue 1250 - 1283Mhz kit or a DiGiTaL DVB-S system will work. 426.25 is no longer available and 444.25 (446.5 Digital) is VK3RTV's output frequency. Using 444.25 / 446.5Mhz is permitted but you will be overridden or interfere with by VK3RTV's output.
In-House Transmitters: (Analogue)
If you have the skills to cut, drill and bend metal tube you can build your own easily. ATV antenna are usually a horizontally polarized Yagi but can be vertical.
Transmission Line Coax:
A low loss ~4-6dB/30m coax like Belden 9913 or RG-8 "foam dielectric" are good at 1 GHZ and relatively cheap. Hard-line coaxes (such as LDF4-50) are much better, but are not cheap. The connectors and connection tool can also be expensive ~$70+ per connector ~$100 for the tool. (circa. 1990's)
At ~1Ghz coax losses for:
For every 3dB of loss... Power presented to the coax arrives at the other end at 50% (half) less than in-putted. So 10 Watts at the transmitter injected into 30 meters of 3 dB loss / 30 meters coax arrives at the antenna with around 5 Watts to radiate into space. That means 5 watts is lost in the coax as heat!
It becomes very important to keep losses to a minimum by either using low loss coax feed-line and connectors or keeping distance between antenna and transmitter as short as practicable. Mounting the transmitter on the mast of the antenna for Ghz frequencies is possible but fiddly to remotely control DC power, AV signals and monitor RF output power.
VK3RTV - Repeater Input Frequency (The "uplink")
From most areas of Melbourne, amateur stations get a full quieting (no noise) "perfect picture" with a modest 18 element, 2 meter long yagi and using 10 to 30 watts of power for analogue operation. While the DiGiTal uplink has studio quality (576 x 720pixel - SD) perfect picture with stereo sound with ~1 to 10 watts.
Most amateur stations have found DiGiTaL to be a vast improvement over the old analogue AM / FM TV system where a large yagi (2 - 3+ meter boom) and power levels in the order of 10 to 50+ Watts were needed for similar picture quality. With the installation of a digital up-link another quantum leap is obtained in overall system performance and with the added change over to a digital down-link DVB-T Terrestrial, aTV now matches the latest commercial broadcast service standards.
Viewers only need a continuous RF (170-870Mhz) coverage "set top box" to receive all TV signals. 230-470Mhz is skipped over by most receivers, a manual scan is required and a "aTV" signal must be available for the receiver to store the channel.
The detected channel number will be different for each make of receiver as "aTV" is an "out of band" service, although the detected network ID should be VK3RTV1, VK3RTV2, etc. Only other amateurs with DVB-T transmitters and using 446.5Mhz will cause a different network ID to appear.
"aTV" transmitting antenna
2 meter boom, 70 cm (420 - 450 MHz) or a
1 meter boom, 23cm (1250-1290 MHz)
looped yagi antenna.
From most areas of Melbourne line of site to Mt. Dandenong and ~3 Watt of power "in to an antenna" of ~10dB gain or more will trigger the repeater. The antenna should be horizontally polarized and the use of low loss coax is highly recommended.
Liaison frequency for ATV operations is usually on the 2 meter band at 147.4Mhz FM simplex, but users of the TV service can be anywhere in the amateur radio bands (HF - SHF) their images usually give call signs and contact details.