Receiving ATV Pictures
Overview
One of the easiest ways to try ATV is by setting up a receive system. The VK3RTV ATV repeater currently has its output on 445.500 MHz using DVB-T2 in HD which can be received on some but not all digital set top boxes. See below for further details about digital requirements. All that is necessary is for the signal to be amplified (as it is MUCH weaker than a normal broadcast TV signal) or an unobstructed (RF) view to the transmitter site. This can be achieved with a small preamp or masthead amplifier and a high gain antenna, or if in direct line of sight you may get a picture with your normal UHF TV antenna. The receive antenna polarity should be the same as the transmitter antenna. Currently to receive VK3RTV the receive antenna should be vertically polarised. 23cm reception involves a bit more equipment, FM ATV is the one of the modes used on 23cm so an FM receiver of which the most common is an analogue satellite receiver. As satellite receivers are designed to have an LNA at the antenna they are inherently deaf so a preamp is needed between the antenna and the receiver. The preamp will also help with cable losses which are greater the higher the frequency. Some of the losses can be reduced by using a better type of feed line. The other most common ATV mode on 23cm is digital. DVB-S or DVB-S2 is the standard most people are using so the receive system is similar to the FM recieve system except the analogue satellite reciever is replaced with a digital satellite receiver cable of receiving DVB-S (or DVB-S2 if that is the mode you are trying to receive). The conversion to HD for VK3RTV has been completed. Uplink signals on 1246,1255 and 1278 Mhz are DVB-S2 or DVB-S with the output on 445.5 Mhz DVB-T2. As before, VK3RTV has a two channel multiplexed output. There are two controllers each with a CHA and CHB input. The 1255 and 1278 Mhz inputs feed CHA and CHB switched to VK3RTV2 and 1246 Mhz inputs feed CHA and CHB switched to VK3RTV1. The system still accepts a DVB-S signal to ensure no station is dis-advantaged if they are not HD. Signal video is taken from the HDMI outputs of the receivers and is then routed via a HDMI switch and a HDMI Splitter. One output of the Splitter feeds the HDTV Encoder and the other goes to the BATC Streamer. DVB-T2 is an advanced HD technology not yet implemented by Australian commercial stations. It is likely that receiving stations will have to re-scan for the signal and remember that no television set or older set top box will decode DVB-T2.
Antennas
The output power from the ATV repeater is not very strong (200W ERP) compared to a broadcast station of many thousands of Watts. If you are trying to receive a station direct then the power they are using will more than likely be even lower. Fortunately, on UHF, small antennas can have a significant amount of gain. The yagi antenna is the most common in use for ATV in Melbourne. Different types of antennas can be used, either commercial or home brew, with both types in use. Remember to use the same polarity as the transmitter (VK3RTV is transmitting a VERTICALLY polarized signal).
 |
 |
| 18 Element UHF TV yagi with preamp. | Home brew 20 Element 23cm yagi. |
Receivers and Preamp's
The type of receiver used depends on whether you want to receive analogue TV ( rarely used except for 23cm and above and mostly FM ) or digital ATV ( normal digital type TV ). To receive the VK3RTV repeater a TV or set top box capable of receiving DVB-T2 in HD on 445.500 MHz is required along with a good UHF antenna of reasonable gain and unless you have a good line of sight view to the repeater a preamp. Not all set top boxes will tune DVB-T2 on 445.500 MHz, and so far none will find / tune it automatically so you have to dive into the menus and do it manually. Here is a link to a number of set top boxes that have been found to work and not work. If you have a set top box or digital TV that works and is not on the list let us know at vk3rtv@hotmail.com. No guarantees are made about the list as models change their specs, design, and firmware all the time.
|
|
|
| Commercial Preamp | Digital set top box | TV / Monitor |
To receive a 1.2 GHz signal which is mostly digital you will need a digital satellite receiver and a preamp, for FM an analogue satellite receiver needs to be used. Analogue satellite receivers are reasonably hard to come by now as most satellite transmissions are digital. Satellites receivers are not very sensitive so a preamp at the antenna is a must. A suitable preamp can be made from kits or you can buy a ready made one.
Antenna Location
The best location for an ATV receive antenna is where there is a clear and unobstructed view of the transmitter. At UHF frequencies, the signals are attenuated by trees, buildings and hills but there are exceptions to these cases. If you can see (RF wise) Mount View (Victoria Police Academy, Glen Waverley) you have an excellent chance of receiving the repeater. If you can't, then some improvements over your standard TV antenna are in order. In general the higher the better and the more "in the clear" the antenna is the better the results will be. A good commercial UHF TV antenna will normally work and a preamp may or may not be required depending where you are. Remember to receive VK3RTV the antenna must be vertically polarised. People in recent times are putting their TV antenna's inside the roof as they are seen as ugly. Unless you are very close to the transmitter chances are that you will receive nothing. Outside antenna's are the ONLY way to go.
Installation and Alignment
Most preamps and antennas are designed to mount directly to standard TV antenna mast using common U bolts and mast clamps. Note: the VK3RTV ATV repeater output is vertically polarized ( The antenna elements are perpendicular to the ground, like the 2nd picture below ).
 |
| Antenna & preamp ( note direction of elements on antenna ) |
The coax cable from the antenna to preamp should be kept as short as possible, in the order of 6 feet or less, and should be of the large diameter, low loss variety. The coax cable out of the preamp should be low loss foil shielded RG-59 or RG-6. It is recommended that for these installations RG-6 be used for its better shielding. This will reduce the chances of strong nearby transmitter signals leaking into your ATV receive system.
Note that one connector on the power supply (or power inserter) for the preamp feeds DC to the preamp as well as receiving the signal from it. Make sure you do not connect this DC side of the power supply to the television or damage to both may occur. The other connector is a DC blocked IF output for the TV set.
Alignment of the antenna is critical, especially for larger antenna's. As a guide, point your antenna at the Victoria Police Academy in Glen Waverley. It can be difficult doing a trial and error or dual person alignment so take your time. It is very handy for the person actually pointing the antenna to be able to see the point of best reception. The easiest way is to watch for a picture then fine your system or ask on 147.400 MHz for someone to put up a picture for you. Once the best antenna position is found, lock the antenna down tightly. Make sure the coax connections are sealed from the elements. If you need any assistance, our ATV group is always willing to help however we can. Contact us via vk3rtv@hotmail.com or on 147.400 MHz.