Coax impedence, 50 Ohm and 75 Ohm, why is it so?

Foundations of Amateur Radio

There is a recurring question that never seems to get a straight answer. Why are we using 50 Ohm impedance and not 75 Ohm? The more people you ask, the more answers you get. There'll be commentary about standing waves, SWR, loss, incompatibility, soldering, cost, velocity factor, diameter, susceptibility to noise and the list goes on and grows, the more people you ask, the longer the list.

Of course as time goes by, people remember stories told to them, guess, or even, how to say this, make stuff up.

To steal a phrase: "Why is it so?"

In the 1930's, when most of us were not even the apple in the eye of their parents - let alone their grand parents - coaxial cable was being developed for kilowatt radio transmitters.

There are two aspects to consider, the amount of loss against length and the ability of the coax to handle power.

Without going into the maths, there's plenty of that online, the lowest loss for air-dielectric cable is 77 Ohms. If we look at the peak power handling, that occurs at 30 Ohms, that is, at 77 Ohms, coax is best at getting signal across the cable with the lowest amount of loss and at 30 Ohms, coax is best at dealing with high power. Clearly a compromise is needed.

So, the mean between 77 Ohm and 30 Ohm is 53.5 Ohm and the geometric mean is 48 Ohm, so, 50 Ohm is a compromise between power handling and signal loss, for air dielectric.

So, obviously, 75 Ohm is used for TV reception and not for transmission. Except it ain't so.

In 1938, Roy Plunkett invented PTFE or Teflon. This material wasn't around when 50 Ohm was decided on.

If you remember, coax consists of a few parts, the centre and the shield, each conductors that we use to move our signal around and something in between, the dielectric, which stops the two conductors touching, with a cover over the top of that for good measure to protect against shorting and damage. The dielectric can be an air gap, or some form of plastic like PTFE.

Electrically, the dielectric constant for Air is 1, for foam PTFE it's 1.43 and for solid PTFE it's 2.2. Turns out that this makes quite the difference. Our lowest loss coax, is 77 Ohm for coax with an air dielectric, but drops to 64 Ohms with foam and 52 Ohm with solid PTFE. So, rather serendipitously, 50 Ohm was a grand choice, good power handling capability and low loss with a solid PTFE core.

Now, why are we using 75 Ohm for TV? One suggestion is that it's another compromise between low loss and cable flexibility.

What does all this mean for you?

In a nut-shell, 75 Ohm coax is one type of compromise, 50 Ohm coax is another. You can use either, but they won't be the same and won't react the same. Calculations made for one, will not apply to the other and loss and power handling will be different. This means that your roll of cheap Quad Shield RG6 is perfectly fine for some aspects of our hobby and not for others.

Here's an interesting tid-bit to tide you over until next we meet. If we compare RG58, common in Amateur Radio to RG6, common in TV, the losses are quite different. For 100m of coax, at different frequencies, these start to add up. At 1 MHz, the difference in loss is .6 of a dB, at 10 MHz, it's 2.2 dB and at 145 MHz, it's 10.7 dB. To be clear, the loss for RG6 is lower across the board.

This really means that you shouldn't be afraid to experiment. There is nothing particularly special about the different types of coax and each choice has it's advantages and dis-advantages.

I'm Onno VK6FLAB