Old question: 75 or 50 ohm coax?
Last month while rummaging around the basement I found a couple old dipole antennas that I made 20 years ago… but… I was surprised to find they used 75 ohm coax for lead-in cable. These days I use 50 ohm for my shack… and it’s always in short supply around here. (After all, who can afford nice quality 50 ohm coax?)
So, why did I use 75 ohm cable 20 years ago? I don’t remember! But, if I had to guess, I’d say it was because…
- A dipole antenna has a characteristic feed point of 72 ohms. So, 75 ohm provides a good match. Power transfer from the cable to the antenna is dependent on the match of the impedances. The optimal match is when they’re identical.
- You can buy very good quality quad-shielded 75 ohm cable at a reasonable price. Why? Because it’s commonly used in home cable TV, off-air TV, and satellite TV installations. The high demand reduces the cost. By contrast, 50 ohm is used mainly by hams, commercial broadcast, and communication sites. So, not nearly as common.
- From theory, the optimal impedance for a “dielectric-filled” line is about 77 ohms. So, 75 ohm line provides the lowest loss of any coax. That’s the theory at least. (See reference link below.)
Of course, there’s always a gotcha, isn’t there? The transmitter provides a 50 ohm output. So, the mismatch is 50:75, which causes reflected power with an SWR of about 1.5:1, right off the bat.
Of course, if you decide to use 50 ohm coax to avoid this problem, then you get a similar mismatch at the antenna, 50:72. So, there’s really no winning here, is there?
However, some useful solutions could be:
- Use an antenna that’s 50 ohms, not a dipole. For example, a vertical with properly sloped radials is around 50 ohms.
- Tune the coax itself to a multiple half-wave (be sure to account for velocity factor), thereby placing the null of the wave at the mismatch reflection point. Tricky, eh?
- Use a 75-to-50 ohm transformer. But, who has that around the shack? I suppose you could build one. 10 turns on top of 12 turns might work.
- Locate the mismatch at the antenna tuner. The idea is that you can best manage the mismatch by compensating with the tuner input and output controls.
Well actually, that last point is my own theory, and I think it’s how I dealt with this issue 20 years ago, and why my lead-in coax was 75 ohms.
Of course… looking at my old dipoles, there was one little problem: the coax was directly connected to the antennas. There was no balun or choke. Not a big deal, but I could have improved the setup even more by providing one of those. But, that’s the subject for another article.
Interested in reading more? Check out: Using 75 Ohm Cable TV Hardline (related to 2 GHz systems, but still a good read.)