RadioBanter - A "silly" question

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May 12th 05 05:35 PM

A "silly" question

Most radio and RF distribution systems for transmitters
are 50 Ohms. A lot of test equipment is 50 Ohms.
But TV uses 75 Ohms in both video and RF distribution
systems. I have dug through all of my refference books,
inclding one from 1947 and even that far back, when color
TV was still a dream, they used 75 Ohms.

I must have slept through this in class 30 years ago
becvause I am sure there is a logical explanation.
Does anyone know why TV video, antenna systems?("F" connectors),
and cable all use 75 while real radio uses 50 OHms

Bonus quesiton: How can you tell a 50 Ohm and a 75 Ohm
BNC connector apart just by visual inspection?

And guess which BNC was most commonly used in TV facilities
up until HDTV came along?

Terry

Michael A. Terrell

May 12th 05 10:26 PM

wrote:

Most radio and RF distribution systems for transmitters
are 50 Ohms. A lot of test equipment is 50 Ohms.
But TV uses 75 Ohms in both video and RF distribution
systems. I have dug through all of my refference books,
inclding one from 1947 and even that far back, when color
TV was still a dream, they used 75 Ohms.

75 ohm cable was used because it has lower loss than 50 ohm cable of
the same size.

I must have slept through this in class 30 years ago
becvause I am sure there is a logical explanation.
Does anyone know why TV video, antenna systems?("F" connectors),
and cable all use 75 while real radio uses 50 OHms

75 ohm cable was used because it has lower loss than 50 ohm cable of
the same size.

Bonus quesiton: How can you tell a 50 Ohm and a 75 Ohm
BNC connector apart just by visual inspection?

The diameter of the center pin and collet are different for the two
different impedances.

And guess which BNC was most commonly used in TV facilities
up until HDTV came along?

75 ohm, in equal amounts of both genders for video signals, and 50
ohm for remote control is all I saw in pre HDTV studios. The 50 ohm was
used on a audio/video router for the remote control panels.

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida

Michael A. Terrell

May 13th 05 07:49 AM

wrote:

Folded dipoles were pretty common for TV. They are around 300 ohms. 4:1
baluns are common, so I figured 300/4 was how they got 75 ohms.

75 Ohm coax was used in early RADAR equipment for the video from the
receiver to the displays, as well as the internal video wiring.

It's hard to think of anything cheaper than the F connector, and for
mass market, you want cheap.

The "F": connector wasn't the first connector designed for 75 Ohm TV
coax. A lot of early MATV devices had the coax crimped directly to them
with a crimp ring used on the cheap "F" fittings. You had to cut the
ring off to remove an item for service. This was still common into the
early '70s. Early CATV systems were connected with "N" connectors that
had to be hand assembled and soldered to braided shield coax cable.
Drops to homes didn't use directional coupler type taps. They used a
"Stinger". You stripped the jacket off the cable and bolted a split
block over the .412 inch coax, then used a tool that cut a clean hole in
the shield. After the tool was backed off, you threaded the proper
"Stinger" into the hole, and connected the drop to the house. This
system was barely usable on 12 channel systems, and had so much RF
radiation that it had to be replaced when more channels were added to
prevent interference with other RF services.

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida

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