The ABC’s Transmission Planning Manager, Craig Todd, explains details of the current maintenance work at the Liverpool transmitter site, which is currently causing the shutdown of Sydney ABC services overnight.
He told radioinfo:
702 ABC Sydney Local Radio & Radio National both employ 2 x Nautel Solid State
Transmitters operating in parallel
to produced a combined transmitter power of 50kW each.
The parallel
configuration provides extra redundancy should one transmitter fail.
NewsRadio
employs a single Nautel Solid State Transmitter with an output power of 10kW.
The Liverpool upgrades are associated with modifications to the Antenna Coupling Unit (ACU) which combines the Local Radio and Radio National
transmissions into the antenna.
NewsRadio is not being modified, but for
OH&S requirements must be switched off as well.
The ACU is not correctly matched with the input impedance of the 702 and Radio
National
transmitters, causing some of the transmitter power to be reflected back from
the antenna into the transmitters (known as reflected power).
The transmitters
have a fail safe operating causing them to reduce power to protect themselves.
This mismatch is complicated by temperature variations between summer and winter
causing a drift in this mismatch.
To prevent the power cut-backs the modulation depth and bandwidth was reduced to
allow the ABC to transmit at full power (50kW) but this impacted on our sound
quality. Last year the power was reduced from 50kW to 40kW, and the modulation
depth increased to provide a ‘fuller’ sound. However 702 still sounds ‘flat’
compared to Radio National.
Four weeks ago air-conditioning was installed in the ACU Building to stabilise
the mismatch drift, and this week between midnight and 5.00am the ABC’s service
providers Broadcast Australia along with TVNZ will be re-tuning the ACU in an
attempt to allow 702 & Radio National to transmit at full power, full modulation
and full
bandwidth.
During this time both 702 & Radio National will remain off. The end
result
will hopefully be a fuller crisper sound at full power for both services.
As staff are working on the ACU which is connected to the antenna, the NewsRadio
transmission which operate nearby will be received by the 702/Radio National
antenna and
radiate back into the ACU being re-tuned. For OH&S reasons, NewsRadio is
required to
be switched off as well.
Please note that Electromagnetic (EMC) Surveys were conducted in March 2003, and
demonstrate that the ABC could increase it transmitter power by 100 times (i.e.
5000kW) and still be within Australian Standards at the boundary fence. The
changes at Liverpool do not exceed Australia Standards for AM Radio emissions.