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Before discovering that connectors can have characteristic impedance, I ordered a bunch of adapters. I've noticed that I'm getting really sub-par performance out of the antennas I'm using and think I may have ended up with some 75-ohm connectors, but I don't have a VHF/UHF SWR meter and it won't be in the budget for a while yet.

How can I determine the characteristic impedance of a connector or adapter?

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You can look at them. The characteristic impedance of a coaxial transmission line or connector is a function of the geometry of the conductors and the dielectric between them. Connectors are standardized, so for a given impedance they have a standard geometry. The pins are usually different sizes, and the amount of plastic dielectric in the connector might be different.

The specifics will vary based on the connector of course. A quick image search should turn up plenty of examples, for whatever kind of connector you have. Common ham radio connectors are BNC and N which come in 50Ω and 75Ω flavors. You might also run into a TNC connector, which is really just a BNC with a screw instead of bayonet shield.

"UHF" or SO-239/PL-259 connectors are also common. They were made in a time where "ultra high frequency" was much lower. They are fine on HF but less than optimal on VHF. They don't come in 50Ω or 75Ω flavors: there's just the one kind. They are basically shielded banana plugs and have a non-constant characteristic impedance throughout their length. Network analyzers didn't exist when they were designed, and that's why they don't work well on the upper parts of the VHF spectrum and above.

You could also measure the impedance with a network analyzer or time-domain reflectometer. But I'm guessing if you don't have an SWR meter, you won't have those things either.

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You mentioned "adapters" in your post. If you truly mean adapters, in other words, changing a BNC to an SO-239 or something similar, bear in mind that those sorts of adapters, proper impedance or not, present an "insertion loss" to your signal on both transmit and receive. Avoid using more than one such adapter if at all possible, because these insertion losses add up.

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  • $\begingroup$ I'm aware of the insertion loss, but there's no getting around the SO-239 on the back of basically every radio. Edit: But thanks! $\endgroup$
    – William
    Mar 10, 2016 at 15:51
  • $\begingroup$ I upvoted your answer, but my reputation isn't high enough for it to show up. $\endgroup$
    – William
    Mar 10, 2016 at 19:19
  • $\begingroup$ If you can add details about which type of adapters you bought, it might be easier to determine their impedance. For example, the BNC in 75 ohm uses a teflon dielectric where the 50 ohm uses a delrin dielectric, and the crimp areas of the pins are different. $\endgroup$
    – guitarpicva
    Mar 10, 2016 at 19:42
  • $\begingroup$ My experience with both Teflon and Delrin is that they're smooth, white plastics with a slightly oily feeling. How would you possibly identify that visually? A user on Reddit directed me to this picture, which was really informative. I was tempted to ask him to respond over here. Edit: accidentally submitted my comment with the Enter key. Edit 2,3,4: I can't even line break :( $\endgroup$
    – William
    Mar 12, 2016 at 0:36
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    $\begingroup$ @William-remote you are correct. The crimp area is probably a better indicator since there are plenty of engineering diagrams for them. It was just an example. $\endgroup$
    – guitarpicva
    Mar 12, 2016 at 11:03

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