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Comment by phkamp | Hacker News Reader

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

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.

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I'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.

That half wavelength damage is deviously pleasing somehow.

> 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?

My guess would be ohmic heating.

A few tens of degrees hotter makes PVC plastic insulation go brittle much much faster.

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.

The 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.

Hot spots was my first guess too, the emphasis on the damage being mechanical threw me off though.

I should have used 'physical', my bad.

The 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.

Even if the antenna is well matched when it's set up... is it still well matched while it's loaded with snow?

At high enough power levels that doesn't even matter. No antenna is perfect and even if it is when it is installed it won't remain so over time.

If the coax is operated at max power handling capability with a good match, then it wouldn't take much reflection (anywhere in the system) to cause internal arcs or heating. No real component has an infinitely low reflection coefficient.

> 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.

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