The Musa connector (Multiple Unit Steerable Antenna) is a type of coaxial connector, originally developed for the manual switching of radar signals. It had a characteristic impedance of 50 Ω, and was adopted for use in the emerging broadcast industry.
By the time the first 'high definition' television first appeared in 1936, the Musa connector was used as standard, unlike many popular types of coaxial connector it is engaged and disengaged by a straight push-pull action, making it ideal for patch bays.
Used in telecommunications and video, the connector has performed well until the end of standard definition (525 or 626 line) analogue television. However, in Europe the Belling Lee coax connector did replace the Musa connector in a lot of consumer products from the 1950s to the 1990s.
With the advent of modern digital high definition signal now being broadcast, the mismatch between the original 50 Ω connector and the standard 75 Ω, used in almost every device in the broadcast industry, has become apparent, due to the high bandwidth and fast edge times of the signals. There is no satisfactory workaround to the impedance issue in analogue systems, as impedance matching transformers do not pass DC, which is usually important. However, in low performance applications, where the electrical length of the mated connector pair is short compared to the wavelength of the highest frequency in the final output, the mismatch can sometimes be ignored, because reflections at the impedance mismatch between cable and connector at one end are in antiphase to those from the other end of the connector, and if sufficiently close in time (propagation delay through the connector) will substantially cancel. Many digital systems may use Manchester encoding, or some other method of achieving DC symmetry in the signal waveform, and therefore could use matching transformers, a very expensive and complex method of avoiding the simpler approach of changing to a more suitable type of connector with 75 Ω impedance.
It may be thought that the use of 75 Ω impedance as the standard for video broadcast equipment is unfortunate, however the converse is true, and better wideband performance is obtainable at 75 Ω than in the other common cabling standard of 50 Ω, due to differing cable losses, and certain grades of cable are cheaper, due to their extensive use in consumer TV aerial systems. Additionally, a 75 Ω system consumes less power than one operating at 50 Ω, for the same voltage swing, which is beneficial in terms of battery life in portable equipment, and in energy conservation in all equipment, so the historical choice of 75 Ω has fortuitously turned out to be advantageous in the latest technology, which could not have been foreseen in 1929 when standard cable impedances were first established.
There is no fundamental reason why a video connector with similar properties to MUSA but with 75 Ω impedance can not be devised, however this has not happened, which may suggest that market demand for such a connector is low. Often, 75 Ω BNC connectors are used, however these are not freely pluggable like MUSA connectors, due to the bayonet locking action.
MUSA connectors typically deployed solid Sterling-silver contacts, silver-plated brass bodies and screws.
- Ohm is where the art is - Publication: IBE - International Broadcast Engineer; Date: Tuesday, April 1, 2003