Telecom Laboratory Services used this attenuation network in the field along with a level oscillator and a level meter. Together they were used to measure power loss in telephone transmission lines as part of the laboratory's role to monitor the quality of telecommunications lines and ancillary equipment. Easier and faster to use digital electronic instruments would have replaced this attenuation network.

As part of the suite of items from the Telecom testing laboratory this attenuation network is an example of the analogue electrical testing equipment used as part of the quality control of the phone system in Australia in the 1960-1990 period.

An attenuation network is a set of circuit elements to introduce impedances (A.C. resistances) into a transmission line or other circuitry to produce a known loss of power or signal usually measured in decibels. The impedance network is also used to balance the impedance in transmission lines to reduce the loss of power from reflections between the receiver and transmitter. The network reduces the amplitude of the signal by a known amount over a range of frequencies with minimal distortion of its phase. The change in amplitude of the power or signal transmitted between points is measured as a ratio of signal strength over the distance and often expressed as decibels per unit distance (e.g. decibels per metre).

Decibels become an absolute value for power when expressed with respect to a reference power level such as dBm (decibels with reference to 1 milliwatt). A signal received at 1 milliwatt equals power of 0 dBm. Power loss expressed in absolute terms as dBm is often more convenient for measurement purposes than using milliwatts. This attenuation network has a maximum input level of 27 D.B.M.

This instrument was used by Telecom Laboratory Services, The Boulevard, Strathfield, Sydney. Alan Ogier, Telecom lab manager, said the instrument was purchased by Telecom on 7 November 1961 for £94.

'H' attenuation network, 135 ohms impedance, with calibration reports, metal / wood / plastic / paper, made by V M L Pty Ltd, Melbourne, Victoria, Australia, 1961, used by Telecom Laboratory Services, Strathfield, New South Wales, Australia, c. 1961-1990

The operating controls of the attenuation network are fitted into a metal panel screwed into a rectangular wooden box which has a leather handle on the side of the box and a removable wooden cover secured by four metal clips. There are four rotary control switches which are, from right to left, a two position dial indicating '' or '50', a six position dial (scale 0 to 50), an eleven position dial (scale 0 to 10) for units, and an eleven position dial (scale 0 to 1) for tenths. At the right side of the top panel are 3 terminals labelled 'input' with the middle terminal marked 'E'. At the left side are 3 terminals labelled 'output' with the middle terminal marked 'E'. Two terminals are located together at the centre top side of the panel, marked 'C.P.' and 'E.'.

Accompanying written materials:
Photocopy of the layout of the operating controls on the front and rear panel and schematic diagram of the circuitry, one page.
Calibration report dated 11/12/61, one page.
Calibration report dated 15/12/81, three pages.
Repair note dated 10/12/81, one page.

Made: 1961


Used: 1961 - 1990

The top panel of the attenuation network is marked:
''H' ATTENUATION NETWORK' 'DECIBELS' 'V.M.L. PTY. LTD.'
'135 (Ohm symbol) IMPEDANCE' 'MELBOURNE AUSTRALIA'
'MAX. INPUT LEVEL 27 D.B.M.' 'TYPE AT. 6-135 SERIAL No. 740-275'
'ACCURACY:- AUDIO ± ·25% ± ·02 DB.'
'1Mc/s. ± 1% ± ·05 DB.'

Printed and typed information on the paper sticker on the top of the cover of the box from the Telecom MQA Northern Region State Calibration Centre includes 'DATE 01.05.91' and 'THIS UNIT IS NOT CALIBRATED. CALIBRATE ON DEMAND' and 'CALIBRATION DUE: MAY 1993'

2008/187/20

1961

215 mm

200 mm