|SRF/SWR/CAL switch and SWR CAL Knob
|What it does
|Changes what your meter is telling you.
|Where to set it
|SR/F for normal operation, SWR/CAL for antenna testing
Thanks for stopping by, we're hopeful this article can prove helpful! Many CB radios have a built-in meter for testing your antenna system. It's an extremely useful function to have as it can make your system work well. This testing process revolves around something called "S.W.R." or Standing Wave Ratio. Many Cobra CB's, like the Cobra 29 LTD Classic and the Cobra 29 LX, have both a switch, or a button, and an SWR CAL knob. Some radios do not have the SWR calibration knob and instead will automatically calibrate your meter (not the antenna!). Research which radio has these features first. We'll explain in easy to understand language what all of these functions are for, and how to correctly use them.
Checking, and tuning your antennas SWR is important for two reasons. First, this is the easiest way for you to tell if your antenna system (coax + mount + antenna + etc) was properly installed. Second, if your SWR reading is too high (higher number = reflected, wasted power) then you can damage your radio. Considering how important both of these points are, you'll always want to check the SWR reading of your antenna system. Either by using the built-in meter that many radios have, or by buying an external SWR meter like our SWRKIT. What you are doing is making sure that when you transmit, your signal is passing through your antenna system like it should. If your SWR reading is too high then your transmit power isn't leaving through your antenna properly, and part of your radio's output power is feeding back to the radio. Remember, the closer your ratio is to 1:1 (standing wave ratio), the more optimised your CB system will be. Low SWR readings will provide a long lasting and better performing CB radio.
Checking SWR with radios that have an SWR CAL knob
Checking SWR with radios that have built-in automatic SWR calibration (this is your radio if you have an SWR meter, but do not have an SWR CAL knob, a very helpful feature). Unsure which you have, do your own reseach to find out!
It's important to note, contrary to popular opinion, your SWR does not affect your squelch. Instead, tuning your antenna system to your vehicle directly affects the peak performance of your CB system. When we talk about swr meter tuning, really what we mean is making your antenna work on the channels (frequencies) you're operating on. If your antenna is not tuned correctly, your radio's peak performance will suffer and you run the risk of damaging your radio. Proper SWR tuning is vital to the health and performance of your radio system!
SWR is a reflection of the impedence matching in your antenna system. The SWR Meter really measures impedence, which in simple terms, is the measurement how efficiently your signal can pass through your antenna system. If you have an impedence mismatch, some amount of your radiated signal will not be able to pass through your antenna system. Instead, part of it will get reflected back to your transceiver. When this ocurrs, an amount of your potential output isn't radiated away from your antenna and instead travels back along your system creating standing waves.
The answer depends on what your meter reading was, and what you are doing with your CB radio. If your reading was less than a 2, and you mainly use your CB for casual use, then you are done. Congrats! There is nothing else you need to do. However, if your SWR reading was above a 2, or if you want to talk on a different channel (or channels) or if you want to optimize your system for the best peak performance, keep reading.
The process can be summed up like this. 1. Check your SWR reading. 2. Adjust something on your antenna. 3. Re-check your SWR to verify your adjustments. Here's a step by step process for setting or tuning the SWR on your CB antenna:
Your CB radio, antenna, and cable are all rated at 50 ohms. This reading is reflected in your SWR meter. If you're using a co-phase, or dual antenna system, the coax cable is rated at 75 ohms per side. When two 75 OHM cables meet, the impedence becomes 50, thus allowing your two antennas to actually act as a 50 ohm system - which is what your antenna and radio are designed for.
Helpful websites such as Wikipedia, Firestik and the ARRL.org are all great tools to use when trying to learn more about SWR. Find an article you need, and they can point you in the right direction. In radio engineering and telecommunications, standing wave ratio is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the transmission line, and SWR is defined as the ratio of the partial standing wave's amplitude at an antinode to the amplitude at a node along the line. The SWR is usually thought of in terms of the maximum and minimum AC voltages along the transmission line, thus called the voltage standing wave ratio or VSWR. For example, the VSWR value 1.2:1 denotes an AC voltage due to standing waves along the transmission line reaching a peak value 1.2 times that of the minimum AC voltage along that line. The SWR can as well be defined as the ratio of the maximum amplitude to minimum amplitude of the transmission line's currents, electric field strength, or the magnetic field strength. Neglecting transmission line loss, these ratios are identical. The power standing wave ratio is defined as the square of the VSWR, however this terminology has no physical relation to actual powers involved in transmission. Wikipedia
A common misconception with SWR tuning is that you are tuning your radio, or tuning your antenna to your radio. This isn't what's happening at all, rather, you are tuning your antenna to the vehicle on which it's mounted. Once you've set your SWR, changing radios will NOT affect your readings. You do not tune your antenna to your CB! Additional research on radio and antenna tuning can be found on wikipedia.
If you need helpful assistance with tuning your SWR readings from an antenna you purchased from us, please call, email, or stop by or retail location for help.
|I thought I tuned my CB to my antenna?
|This is a common misconception. To be very helpful here: You are not tuning your CB to your antenna. Nor are you tuning your antenna to your CB. Instead, you are using your radio meter to CHECK your SWR reading, and then tuning your antenna(s) to your vehicle. This is an important distinction! Your antenna is tuned to the ground plane your vehicle provides, and once you've set the SWR (aka tuned your antenna), then it is ready for any radio you want to use. Let me make that abundantly clear. Once you have a low SWR reading, you can use any CB you want. Your antenna is NOT tuned to your radio!
|My SWR Reading is Higher than a 2
|Follow the steps listed above and check your reading on channels 1, 20, and 40. If you find an SWR reading at or above a 2 on all channels, that may indicate something is installed incorrectly, or broken. You may need to replace your coax cable, mounts, and antenna(s) to resolve.
|What do I DO to my antennas to adjust them?
|First, check your antenna's package for the specific instructions for tuning. The Firestik FS series CB Antenna, and other similar fiberglass antennas, have a screw at the top of the antenna. Adjust this up or down based on your SWR readings. Some fiberglass antennas do not have a "tuneable tip", with these "cut to tune" antennas like the Firestik KW series CB antenna you need to remove the antenna's cap, then cut off the outer insulation until you can get to the copper wire. Cut this wire down to tune (if your reading on 40 is higher than 1) * it's not uncommon to have to remove A LOT of copper wire to get these antennas tuned. For steel whip antennas, like the Wilson 2000 Trucker Antenna, you may need to use bolt cutters to trim the "whip" portion of the antenna. Remove the whip from the holder (held in with set screws) and either move up or down according to the readings. If your readings tell you to go down more, you will need to cut the whip with bolt cutters. (this is pretty common).
|My radio's SWR light or "ANT" light is coming on?
|This means your SWR reading is too high and you can damage your radio by talking. Either attempt to tune your SWR using the steps above, or start replacing your antenna system parts until it's resolved.
After doing this line of work for over 20 years, I can tell you I've heard every name / butchered expression for the term "SWR". Likely you've heard people call it SRW - getting the W and R backwards. I've also heard SWIRLS, standard wave, and S-doubleyahs, to name a few. While people are trying to be helpful, if you encounter these strange sayings and don't know what it means, you're not alone. As with any part of the English language, when people use it, it changes. Just know the correct acronym is either SWR or VSWR, and squelch out the nonsense! Perform your own research and you'll see similar results.
One last thing to remember is to be clear when talking about tuning, and what you're after. Tuning a CB radio usually means a technician peforming a modification inside the radio to optimize performance. Tuning an antenna is completely different, and as mentioned above, and is NOT tuning your radio at all. Your antenna system and radio operate as two parts of a whole system, and tuning is done independantly of each other.
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