From a Trompeter tech paper - something I never heard before

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The most efficient Impedance to use
when transmitting any signal considering the voltages, cur·
rents and powers to be transmitted is 75 ohm, and would be
the only standard if these were the sole considerations. The
telephone industry, followed by the TV Industry, use 75 ohm
almost exclusively for the transmission of video, baseband
and I. F. frequencies. The military services during the period
1920 through 1940, were faced with a differing need for low
radiation angle omnidirectional antennas for broadcasting
ship to ship, airport tower to low flying aircraft and base
station to ground troops. The only antenna that would give
this performance was the vertical ground plane In Its many
forms, which turned out to be 50 ohm . The military standard·
ized on 50 ohm impedance and spent vast sums of money
developing cables and connectors for all of their coax
systems. At one time a military agency, Armed Services
Electro Standard Agency (ASESA) designed all military
coax components and assigned the connector UG and
cable RG designations. This has since been replaced with
the latest government/Industry committees cooperating
and producing the current general connector standard MIL·
C-39012 primarily for 50 ohm usage.

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