I did think it's maybe something of value to everyone.
First thing to understand is RF isn't like DC power most of you are familiar with and it's not even exactly like AC in your house.
There is a baseline power handling capacity for a coax based on those concepts and when you look at it that way it's terrible. Your standard RG-58 has a center conductor that's around 20 AWG so if you use it for DC power you're looking at a maximum of maybe 1 or 2 amps. That means in 12V circuits the most it can handle is 15 watts or so.
So why then does coax handle 100 watts RF? Magic!
It's actually because RF power isn't really real power, it's really imaginary, too. Seriously. The only time RF power is really real is in an actual resistor. Otherwise it's complicated. Seriously. Meaning it's complex numbers where you have a real and imaginary part.
Why that is really isn't important to know about coax power handling at a basic level. You just need to have heard that because the SWR is important and SWR is one measure of how well your antenna matches your radio.
The radio develops some amount of electromagnetic energy at some frequency. The antenna is the translator of that energy from one place to the world at large. But to get to the antenna you have a feedline to carry it, which is your coax. Ideally you have a perfect match and all the energy the radio makes leaves the antenna. SWR is perfect 1:1 and there's no loss. But that's essentially impossible in the real world, so you have losses.
Which is where the power handling capacity of coax comes in. There is no one single value for what a coax can carry. First of all it's a function of frequency. Back to RG-58 for the sake of argument. At 100 MHz most manufacturers say it can handle about 300 watts. But at 400 MHz it's only about 120 watts. Nothing is changed other than frequency.
Why on Earth?
Well, the reason a cable has a power handling at DC is just a function of resistance and ultimately current through it creating heat. Like an electric heater or light bulb. That's all that contributes. With household AC the frequency is so low that practically speaking it's also just resistance.
Technically speaking anything trying to stop the flow of electrons is an impedance, which for DC is resistance is a subset (e.g. no frequency component). At very low frequency the impedance is very close to the equivalent resistance so we can model it the same at 60 Hz.
In coax it's impedance is a function of two major things, the diameter and the dielectric.
Remember that a coax looks like this:
So what dictates coax power handling are a couple of things.
First is the absolute power. There's of course just a strict limit due to the equivalent DC energy. Even on a theoretical perfect system a tiny coax will never handle 1,000 watts.
Next is SWR match. You want a good match so not much of the power stays in the coax. Let's call that EM energy that's just, oh I dunno,
standing around. You know, we could even give that it's own term, like
Standing Wave Ratio or like SWR for short.
Most important then is it's ability to resist the voltage that is created between the center and outer conductor. This is where the AC and EM stuff gets heavy but just know that Ohm's Law applies to RF, too. So putting a current though an impedance creates a voltage or vice versa a voltage on an impedance is necessary to force a current.
The fact is your 80 watts from the radio not much is simple power like that. Most of the power is due to the fairly high voltages developed to push energy off the antenna. You might remember the term "RF burns" from licensing tests. That's substantially from the high voltage.
In the case of coax this voltage can cause a break down in the dielectric. That's a fancy way of saying you create an arc. Same was you would maybe welding, shorting your battery or nature's ultimate dielectric break down - lightning. You don't want that. That's bad in RF.
So that's mainly what drives the power limit. How well that coax can resist dielectric break down. This isn't a simple answer. The material itself, thickness, humidity, temperature, elevation (coax is rated for sea level), all factor in. Duty cycle doesn't really factor in for the EM part like it does for the heating part. That's because all it takes is one cycle of RF to create a short for the damage to be done, It's like lightning. It's not a cumulative build up per se but a potential grows and grow until it just lets go.
tl:dr
Long way of saying that putting 80 watts on a coax rated for 75 is generally not a problem but don't push it. You are at the limit and if the coax manufacturer thought it could handle 90 or 100 or more they say so. But they also do have a little elbow room in that if they say 80 watts it's not like 81 watts is definitely going to do it.
In this case what's driving the limit are two things. The short length of small coax, which is RG-174 or RG-316, roughly the same thing. That's usually rated for about 80 watts at VHF. At HF it can handle I think about twice that but the flip side is at UHF and especially higher it's limited. At 500 MHz it's down to about 50 watts and by 1 GHz RG-174 can only do about 30 watts.
Also the connectors and connection between the larger and smaller coax will have an impedance and subject to the same issues of break down.
But at the same time even being at 65W or 50W on an 80W coax doesn't guarantee
absolute safety. Under the right conditions any feedline can short. It's usually due to a problem like cut shield or insulation or kink in the dielectric or something like that but could be at the end where you necessarily have to expose the coax to make connections. That's why you ideally seal the ends when you make them up.
