VK3KHB
Melbourne ATV Repeater Service
why the move Ch.35 to 444Mhz

Melbourne ATV uses FM television into the repeater and
AM television or Standard Definition DiGiTaL out...

So why the change AM to FM and DiGiTaL ?
Basically, this move made everything much neater.

The TV repeater output used to be at Ch.35 at 576.25 Mhz way before the commercial, community and government TV stations used the UHF band. Amateur radio operators where quite active as a secondary user from around ~1979 till ~1990's when a shake up of frequency usage was being worked out.

As commercial interests geared up to use this band, the analogue AM TV channel 35 became plagued with interference from regional TV stations and many other problems relating to technical operations were just too difficult or more importantly disheartening to overcome as we had to put up with any interference. So, it became obvious and simpler to surrender the frequency allocation for Melbourne metro area and drop back to 70cm amateur band for TV repeater operations using 426 and 444Mhz. However commercial pressure for more UHF frequencies seen 426Mhz lost, leaving a TV repeater without a input, luckily satellite equipment was coming down in price and the 23cm amateur band could be used as an input. The advantage with satellite TV it uses FM a better modulation technique over AM for recovery of TV signals at low signal levels and 1.2Ghz became the input. Most TV receivers were still analogue AM at this time and it became and still is in 2009 very important to keep 444Mhz as the repeater's output.

The cost of getting into amateur TV has always been a hurdle, so keeping receivers and associated equipment at affordable prices is a key concern. Unlike commercial TV stations, amateur radio stations are not eligible for Government upgrade grants. (I know I tried) It is odd really these commercial TV stations make money with their stations and get grants, while those that experiment and show the way loose resources and get nothing but I'm in Australia the land of the missed opportunity or vested interest! (but I I'm showing my bias and digressing...)

As the repeater had inputs at for AM television at 444.25 Mhz and FM television at 1250 Mhz, tests revealed that FM television was the better method to get a picture into the repeater. Moving the output down to 444.25 Mhz was the most logical move. It was already a reserved frequency, it kept most of the existing equipment at both the repeater and all sending stations in operation.

Only a realignment of the repeater's transmitter and replacing the sending antenna array was necessary. This enabled an increase in real output power at the repeater and less loss in the system. 444.25 Mhz AM was now receivable by most 1990 or later made domestic TV's / VCR's and had minimal impact on all users and viewers.

Using 1250 Mhz FM as the uplink, decreased the adjacent frequency interference and sending stations could see there own transmissions via the repeater much easier. Ghz Antenna are smaller and easier to make, with considerable EiRP gains being made cheaply. Coaxial cables for Ghz or using mast head mounted transmitters connected by UTP are possible with FM kits and amplifiers available at reasonable prices. More testing with an additional 10 Ghz input revealed better picture quality by way of improved signal to noise, however availability, afford ability and ease of use of equipment at this frequency is limiting and has not progressed beyond trials (still ongoing).

As things are about to change to DiGiTaL TV in 2009 and analogue services being switched off sometime shortly after, ATV will be going DiGiTaL too with 1 to 4 Standard Definition TV channels or more; it is just a matter of multiplexing the AV services into the 7Mhz bandwidth. Low resolution High Definition maybe possible but has limited practicable use for ATV experiments a present unless someone wants to try 3D TV or hologram video transmissions...

Talk of a DiGiTaL uplink is also being investigated but costs are limiting at this time Jan.2009.

The official VK3RTV website has the latest about the service...
73's