How do I check swr on a ring terminal

Just hook a short coax jumper to it. I have some 3- and 5-foot jumpers I use with my antenna analyzer and Bird. Just make sure it's long enough to get yourself inside the cab & close the door, so you don't detune the antenna by being in its near field.

 

Yessir, it's fine. Just tune for minimum reflected, rather than worrying about the actual value of the SWR. Or, if you don't want to use up a lot of extra connectors and/or coax, go ahead and install the full feedline and meter from there. Ring terminals already show on a machine called a TDR, or Time Domain Reflectometer, as an "impedance lump", because they represent a significant departure from 50 ohms impedance. That's mostly because the spacing between the braid and center of the coax increases in order to separate the leads to attach, and then there's really not much control of the spacing between the terminals or the direction of the conductors (which way they point).
Your point about metering at the feedpoint is well taken. Ideally, in order to eliminate extra losses (minimal, actually) or having enough coax to have an effect on the match, the short jumper with rings on one end and a PL-259 on the other, will approximate the match at the feedpoint. Two or three feet of coax is such a small fraction of a wavelength at 27 MHz that it should have only minimal effect on your reading.
At the risk of calling into use "the H-word", you can comfortably do what hams have done for years: hook it up and get on the air. My Bird 4304A wattmeter isn't calibrated in SWR; it just shows watts forward and watts back (well, not really; it reads voltages). In commercial radio installations, we typically just tune the antenna for minimum reflected power (OK, voltage). Once it's as low as we can get it, we meter forward power to assure that the radio is putting out rated (or desired) power. Actual numerical SWR rarely enters into commercial installations, at least in my modest experience. All most techs do is meter the forward and subtract the reflected to get a net power output.
While I don't recommend buying a $550 meter for the occasional CB install, the idea is the same: just tune for minimum reflected, regardless of the length of the coax.

And you might very well get differing SWR values depending where along the coax sections you put the meter, but the goal is still to just minimize reflected energy. I believe most folks would be better off by just removing the SWR scale from their meters, and just making the needle stay as far to the left as possible.

Hope that helps.
73

 

Luwi, for way less than the cost of an analyzer, you ought to be able to find a shop who'll use one to tune it for you. After it's set, then you can meter the SWR with your own stuff (or even the built-in meter on the radio, if it has one) and write it down someplace -- vehicle maintenance log, Sharpie on the windshield, something -- for reference. That way you'll know what the meter should read in case something changes.

Sorry the previous post was so long, but the gist of it was, "Just tune for minimum reflected power" wherever you meter it.

I've oft times been told that when asked what time it is, I tell them how to build a clock.....

 

I bought one of those analyzers shortly after they came out, and it's been one of the best investments I've made. Bring one to Field Day, and you are the most popular person there during setup... at least until lunch!

 

A nice cross-needle meter from, f'r instance, MFJ can give you forward & reflected watts simultaneously. The ones on two of my antenna tuners here at the shack also have an SWR scale behind them. SWR is read at the point where the two needles cross. http://www.mfjenterprises.com/Product.php?productid=MFJ-815C is a good example, but the power scales are pretty high for typical CB power levels -- 300 and 3,000 watts.
A lower power (30 and 300 watts) would be a better choice for CB stations: http://www.mfjenterprises.com/Product.php?productid=MFJ-860
Remember, too, that antenna coils don't radiate. So a two-foot CB antenna can reliably be predicted to be a less efficient radiator than a six-foot CB antenna, even if both are tuned to be resonant at the right frequency. The two-foot antenna has to make up for the missing six-and-one-half feet (more or less) that would be present in a 108" quarter wave antenna. The quarter wave antenna and the two shortened ones may all show as 50 ohms (the quarter wave will be more like 36 ohms, I think) but the shorter the whip, the smaller fraction of the antenna is available to send RF on its way -- or to catch it on receive. If you had a big enough coil, you could make a 10 penny nail resonant at 27 MHz, but it would be horribly inefficient.

And your research is correct. SWR is just the simplest parameter to test, and the cheapest. But a dummy load on the end of the feedline will show about 1.0:1 and still not radiate. Ideally we hope to see 50 ohms of pure resistance, but that just means that all of the transmitter's power will be transferred to the load. Reactance can be added to a radiating element which has a radiation resistance other than 50 ohms to keep the transmitter putting out all the power it's made to.

For years hams have put up wire antennas that were cut to what the formulas say will be resonant at the frequency, and put up with the losses incurred by mismatches between the feedline and the antenna. Using an "antenna tuner" at the transmitter end of the feedline dials in enough reactance (capacitive, inductive, or a combination) to transform the transmitter's output impedance to whatever impedance shows up at that end of the feedline. Since it doesn't actually alter the tuning of the antenna, however, some of the power is sacrificed, dissipating in either the antenna tuner or the feedline. But whatever *does* reach the antenna, however, will be radiated because it has been cut to be resonant on the desired frequency.
I'll defer to MsJamie at this point, however. She's a jenn-you-wine engineer who specializes in antenna design & stuff.
Glad to see you've been studying up on this stuff. Having an understanding of what's going on in the system and why will help you be able to reason out solutions for problems if they arise, rather than just throwing money at someone else's ideas with pretty packaging at the TS.