Home > Antennas > Old question: 75 or 50 ohm coax?

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…

  1. 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.
  2. 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.
  3. 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.)

Categories: Antennas Tags: ,
  1. 2010/09/04 at 1:42 PM

    Actually most broadcast coax has been 75ohm since the 1960’s – Belden 8281 and 8241 (and the Gepco equivalents) are 75ohm – standard practice until the 1990’s was to put 50ohm BNC (less expensive than 75ohm and for analog signals not a consideration – to tell the difference between a 50ohm and 75ohm BNC – the 50ohm has a dielectric around the center conductor the 75ohm does not) with the advent of “inexpensive” SDI interfaces (Serial Digital Interface – standard definition component video and audio SMPTE 252M or CCIR601) which at 270Mbps serial data rate became more sensitive to the impedance mismatch considering the number of connections that might be in a specific production path and overall cable length.

    In fact one of the best test methods today in characterizing SDI and HDSDI (1.39Gbps for up to 1080i and coming soon 3Gbps for 3D and 1080p) signal paths is to look at the 3rd harmonic of the primary, once that drops below a specific threshold you’ve fallen off the digital cliff!

    One note about the older 8281/41 is that they had very precise propagation dealy to physical measurement – a “Zero timed” analog facility was a lot of specifically cut cable lengths

    • 2010/09/06 at 4:54 PM

      Hi Stu. How did you find this blog?

      Anyway, good note, thanks. Yes, 75 is major in broadcast distribution. I thought about it earlier, but decided not to make the article any longer (not sure hams would be that interested.) BTW, I still have a pile of 75 BNC’d coax in the basement, if anyone local ever needs it for projects. (I wish I had more use for it! Maybe when TIA starts using SDI.)

      So, looking into the BNC I can tell the difference between 50 and 75 (M or F)? I’ve always wanted to know how to tell… I know the difference for N connectors… the 75 center pin is noticeably smaller. (And a 50 into a 75 will ruin it.)

      • 2010/09/06 at 6:52 PM

        Poking about looking for trunking information – I have s jpg that shows the difference between 50 & 75 Ohm BNC’s – where is the best place for you to grab it?

  2. 2010/09/07 at 4:15 AM

    Is it something you can just link to from here? On your website?

  3. 2011/11/13 at 10:31 PM

    certainly like your website however you need to take a look at the spelling on quite a few of your posts. A number of them are rife with spelling issues and I to find it very bothersome to inform the reality on the other hand I will definitely come back again.

    • 2011/11/20 at 10:09 AM

      There are a few contributors to this blog… and over the last year I’ve not done it justice in terms of moderation and editing for errors. That said, when I see an error, it will get fixed. Thanks for the note.

  4. 2013/09/23 at 12:06 AM

    Is there a way to tell if the coaxial cable I have is 50 ohm or 75 ohm? I am connecting external antennas to a 50 ohm TNC on an Axesstel MV440 3G Cellular router. In trying to figure out what I’m doing I have come to understand that the cable needs to be 50 ohm impedance, yet I don’t have a clue how to discover what the impedance is on the cable I have. Of course I would like to get along without buying anything else and use this coax. Any help??

  5. Jennifer
    2013/09/28 at 1:17 PM

    Reading through this, I was interested in the 50 – 72 ohm transformer. Okay, so, what is the best way to start this transformer, by that I mean, the 75 ohm turns would, obviously be 10 turns, the 50 the 12 turns, but…. what size coils? on a ferrite ring? I’m building a 70 cms folded dipole (F = 434.5). I could use a half wave stub, and I mused this would do the same thing.
    best 73 de Jennifer.

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