6 ms·
The choice of 75 Ohm is documented in ultimate detail in Bell Systems Technical Journal. Here is a 1934 article as an example, long before anybody had even nam
by phkamp 7y ago
The choice of 75 Ohm is documented in ultimate detail in Bell Systems Technical Journal.
Here is a 1934 article as an example, long before anybody had even named microwaves:
https://archive.org/details/bstj13-4-532 https://archive.org/details/bstj13-4-532
75Ohm gave the lowest loss and thus the longest distance between repeater-amplifiers on the trans-continental carrier-telephony coax-cables.
These same cables (and their brethern: microwave links) also carried television, thus enabling the first "television networks" as opposed to "television stations".
In many cases even the stand-alone television stations paid AT&T to connect their down-town studios to the hill-top transmitter.
So television uses 75Ohm because AT&T did.
And AT&T did to minimize the number of repeater amplifiers across USA, because 75Ohm has the lowest loss.
- jacquesm 7y agoI'll tell you a neat story about coaxial cables. I worked with quite a few high power transmitters. If you go topside and check the cables to figure out why transmitter power is lost it isn't rare at all to see the cables brittle every half wave. The high frequency does mechanical damage to the cables to the point where the outer mantle will become porous enough to allow water to enter. After that it's only a matter of time, presumably a bit of water will not help with reflected power and speed up the process of desintegration. You can easily tell roughly what band the transmitter is running at just by measuring the distance between two such porous points. That's exactly a half wave length.
- aidos 7y agoThat half wavelength damage is deviously pleasing somehow.
- taneq 7y ago> The high frequency does mechanical damage to the cables How does it do this? Like, is it physical vibration driven by the EMF or what?
- willis936 7y agoMy guess would be ohmic heating.
- londons_explore 7y agoA few tens of degrees hotter makes PVC plastic insulation go brittle much much faster.
- jacquesm 7y agohttps://www.allaboutcircuits.com/textbook/alternating-current/chpt-14/standing-waves-and-resonance/ https://www.allaboutcircuits.com/textbook/alternating-curren... Has this bit in it: "Transmission lines may be damaged by the high maximum amplitudes of standing waves. Voltage antinodes may break down insulation between conductors, and current antinodes may overheat conductors." I've never seen any proof of the first but the damage I've seen is consistent with overheating.
- someguydave 7y agoThe current and voltage in coax feedlines varies with length - he is probably looking at spots where current peaks and thus ohmic heating causes physical damage.
- taneq 7y agoHot spots was my first guess too, the emphasis on the damage being mechanical threw me off though.
- jacquesm 7y agoI should have used 'physical', my bad.
- CamperBob2 7y agoThe only way there should be significant nodes and antinodes along a transmission line is if the load match is (very) bad. Not sure why this phenomenon would ever happen in the real world, unless there's a mismatch problem. This is basically the definition of VSWR. No mismatch = no standing waves. If you're seeing a pattern of degradation due to uneven heating in a piece of coax, check the antenna.
- guenthert 7y ago> 75Ohm gave the lowest loss The lowest loss of a selection of cable diameters I presume. I'd think lower loss can be achieved with greater diameters (lower specific capacitance), but costs will be prohibitive.