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VK3KHB

Transmitting ATV Pictures



If you want to transmit (Tx) Television beyond the boundaries of your property you'll need to hold a amateur radio operators "certificate of proficiency" a license that ensures you understand your obligations and have the abilities to undertake the many technical tasks required. It is not hard, you may already know much of what you'll need. See the Wireless Institute of Australia website for full details on how to become a amateur radio enthusiast (ham).



Overview:
The basic components of an ATV transmitter system.
1. Camera
2. Suitable audio video transmitter.
3. High gain yagi antenna and suitable coax.

In the Melbourne metro area, horizontal polarization is 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"

Cameras:
Cameras are everywhere now days, so long as you have one with a composite video and audio out you have a source for the ATV transmitter. If you don't have a camera then the output of a PVR, VCR or DVD player can be used, just make sure the content is suitable and be aware of copyright infringements...

Transmitters: (Analogue)
Referring to the "you build it" kits which are designed to operate directly to satellite receiver. These kits will be either on 444.25Mhz UHF TV or 1250 - 1283Mhz satellite TV and need to produce around 2 - 20+ Watts output to the antenna.

Transmitters: (DiGiTaL)
There are several DVB-S semi-completed kits available and these may work with VK3RTV. It is best to check with the active ATVers what is known to work. Currently the MR-SYSTEMS DVB-S module is the preferred system to get, however there are cheaper systems becoming available al the time.

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)
Domestic TV transmitters (AV repeaters) available at all good electronic and electrical stores will do the same job, but they won't work with the ATV repeater or go as far. Current domestic versions operate at very low power (<1-5 mWatts) at 50-200Mhz (VHF) or 530-560Mhz (UHF) or at 2.4-2.5Ghz (UHF) or 5.8Ghz (SHF) and their signal usually contains several harmonic frequencies as well as some frequency drift over time or with temperature. Only good for with-in your property.

Antennas:
Antenna for use with the ATV repeater or simplex operation are of a "you build it" type. Domestic TV antenna are only for reception and are not usable to transmit on. Commercially available transmitting antenna are obtainable, they can be expensive and are frequency dependent...

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.

In-House Antennas:
The domestic transmitters (AV repeaters) use vertical polarization via telescopic antennas, these can be modified to fit a connector for use with coax and external horizontal antennas, but their power levels are so low that an expensive amplifier and filter network is needed. So for ATV use outside of your backyard (~30Meters) these become very uneconomical...

Transmission Line Coax:
Connecting "transmitter / receiver" to the "antenna" is very important!
The coax feed line must be suitable for use at 1 Ghz frequencies or most of your signal power will be lost in the transmission line as heat.

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:
RG-8 or 213/214 is around 9 - 10 dB/30m (10watts in for ~2watts out)
Belden 9913 and RG-8 "foam dielectric" 3 - 6dB/30m (10W in ~4W out)
Hard-line coax 2 - 4dB/30m (10W in ~6W out)

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.



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VK3RTV - Repeater Input Frequency (The "uplink")
VK3RTV ATV repeater changed from AM to FM analogue inputs in the late 1990's and in October 2009 added a DVB-S Standard Definition Digital Video Broadcast. This digital DVB-S (satellite) input is on 1255 with analogue FM inputs also on 1250, 1283 MHz & 10.410 Ghz. (may change as experiments with adjacent frequencies continue...)

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.


73's for now
Gary, VK3KHB


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