The above chart shows common SWR curves. The actual SWR for any installation is likely to be different. In some cases, the curves in the chart are over-emphasized in order to illustrate the general appearance of a particular SWR curve. This article deals specifically with ground plane dependent antennas. When it comes to no-ground plane antennas, most of the tuning procedures are the same but issues dealing with insufficient ground plane and/or chassis grounded mounts does not apply. In any case, you will not be any dumber for knowing the information in this article whether using a ground plane or no-ground plane antenna system.
Standing wave ratio (SWR) is simply a ratio of what voltage or current your transmitter has the ability to deliver compared to how much of that is absorbed into the antenna system. Un-tuned antennas or problems with the installation or components will prevent the antenna system from absorbing the transmitters potential thus reflecting it back to the transmitter. High SWR means high reflected power and that can be damaging to the transmitter.
For the most part, people tend to place every problem on the antenna when a problem occurs. The fact of the matter is, the antenna is just a part of the installation and there are many other things that can cause SWR problems. An SWR meter cannot isolate one item from another … it sees everything from the radio out to the antenna. That includes the coax, the mount, the mount location, and everything in and around the antennas near field of radiation (the vehicle, everything on it and everything around it). If you plan to drive with the hood and doors open then go ahead and tune the antenna with the hood and doors open. The antenna must be tuned under conditions that it will most likely be used … all openings closed and away from structures. Antennas are sensitive to their surrounding, especially metallic surfaces that are running parallel to the antenna and within a couple of feet distance. And … DO NOT test and tune the antenna with the rubber tip off because as soon as you put it back on, the SWR changes. Take the SWR readings, remove the tip, make the adjustment, replace the tip and retest.
Keep in mind that transmitting antennas need to release electromagnetic energy into free space and therefore they need free space around them to release that energy. If that is not given due to the way the antenna is installed, the antenna will not perform to its capacity. It's like trying to serve pizza through a screen door. Even if the SWR is in a safe range (below 2.0:1), objects in the near-field of radiation can absorb energy and distort the radiation pattern. Antennas need room to operate, so it is up to the installer to install them in a way that allows them as much unimpeded free space as possible.
Even though SWR has an effect on both the transmission of radio waves as well as the reception of radio waves, you can listen (receive) all you want with a fouled up antenna system with high SWR. You cannot, however, transmit into a high SWR antenna system without the possibility of damaging the radio transmitter. The radio has separate circuits for transmitting and receiving. You could damage the transmit circuits but still be able to receive radio traffic. If, during SWR testing, the radio lacks the ability to drive the SWR meters needle to the "calibrate" line, either the transmitter is damaged or the power going to the radio is insufficient to fulfill the needs of the transmitter. You cannot depend solely on a voltage reading because you can get a 12v reading from a single strand of wire. Wires that are too thin to carry enough current will also starve the transmitter and could be the blame for SWR meter needles not having the ability to swing to the calibrate line..
When tuning an antenna, it is necessary to have two SWR data points. These would come from the lowest channel and the highest channel (i.e. CH1 & CH40 on a 40-channel CB radio). However, if you never use channels above CH20, you could use the SWR readings from CH1 and CH20 to maximize efficiency within the channel range that you use. If you should happen to use less than the entire band for tuning, be aware that operating the transmitter too many channels above or below your reference channels could potentially leave you operating at an SWR too high for safe use. In order to understand and/or troubleshoot a complete antenna installation, a minimum of three SWR data points should be known … on the lowest channel or frequency, on the highest channel and frequency and on the middle channel or frequency. We provide a PDF plotting chart on this website for your use. View and print using this link.
In most instances, your goal when tuning an antenna is to get the SWR on the lowest channel to be the same as the SWR on the highest channel on the transmitter. That would not apply if you want the antenna tuned for a specific set of channels (i.e. 1-20, 10-30, etc). However, you still need readings on the hi-lo channels within the set you intend to use.
Common SWR curve discussion, top to bottom on the charts right-hand list of conditions.
Long Indication: This curve indicates an electrically [not physically] long antenna. Whenever the SWR on the highest channel is a bigger number than it is on the lowest channel, the antenna system appears electrically long to the transmitter. Depending on the type of antenna, the typical solution is to turn down a tuning screw, remove wire, cut part of the antenna off (if the wire is impregnated in fiberglass or is a solid metal rod), or lower the metal rod into the base loading coil.
Narrow Bandwidth: After the antenna has been tuned and the SWR balanced on the lowest and highest frequencies [channels] and the SWR at the two reference points is above 2.5:1, a mid band SWR test should be performed. If it is below 1.5:1, then it is most likely that the antenna just doesn't have enough bandwidth to cover the entire frequency range of the radio. Usually, a physically longer antenna is needed. However, if the SWR is above 1.5:1 mid-band the most likely causes are insufficient ground plane supplied by the vehicle, insufficient or no chassis ground at the mount, too much of the antenna running parallel to parts of the vehicle or low quality and/or reactive issues with the coaxial cable that are often corrected by using 18-feet of good quality coax.
High & Flat: If the SWR is above 2.0:1 on the lowest and highest channels and is only a few tenths of a point lower at mid-band, the most common causes are insufficient ground plane, mounts that are not chassis grounded and/or really cruddy coaxial cable assemblies. There isn't much a person can do when it comes to insufficient ground plane. There is either enough, or not enough supplied by the vehicle. You can do a quick check of the chassis ground by simply clamping one end of a jumper cable to the antenna mount (not the antenna base) and the other to a known good chassis ground point on the vehicle then retesting the SWR. If it drops into a reasonable range and the mid-band SWR is now below 1.5:1, you know you have a chassis ground issue to deal with. A lot of mounts and/or mounting locations are either powder coated or painted, both which insulate the antenna stud mount and/or mount from chassis ground. If you are going to check chassis ground at the mount with a meter, make sure that the coax is disconnected from the radio to avoid a false reading via the radios circuitry. The mount and mounting surface must have chassis ground separate from that at the radio.
Acceptable Bandwidth: If SWR is below 2.0:1 at the lowest and highest channels and has a nice dip in the mid-band position, go have a sandwich and a cold drink. You got it right and should reward yourself.
Broad Bandwidth: When SWR across the band is at or below 1.5:1 on all channels, you have one of those premium installations. Typically this will occur with antennas that are over 45-inches in length and ground plane, chassis ground, mounting location and your coaxial cable are better than average. In most cases, the taller the antenna, the more bandwidth you will realize and the lower the SWR will be across the entire spectrum. But, don't let that draw you into dangerous territory. If you are using your vehicle off-road or where there is a lot of low hanging foliage, a shorter antenna with somewhat less bandwidth is always better than a broken antenna. I [the writer of this article) am a two-antenna user. Day to day I run a short antenna. It has less bandwidth but I never use channels above 20 and have balanced the SWR on CH1 and CH20 to optimize performance in the lower 20 channels. I also pack a previously tuned 4-foot antenna behind my seat "just in case" I find myself out in the boonies and want/need more efficiency.
Short Indication: Hate it when this happens, as rare as it may be. But, anytime the SWR is higher on CH1 than it is on CH40, the antenna is electrically short. The most common cause of this is insufficient ground plane ... just not enough horizontal metal surface for the antenna to use as counterpoise. If the antenna being used has some means to be lengthened, i.e. a tuning screw, adjustable top piece, or a stinger that can be moved higher in the loading coil, you at least have some ability to compensate for the condition. A few times a year we run into this issue. In the case of our tunable tip antennas, going to the hardware store and buying a longer screw can solve the problem. Also, our "KW" series and "FS" series antennas are technically similar but we do have additional wire on the "KW" series antennas (because they are so popular with over-the-road truckers and their mostly fiberglass bodied trucks). Extra wire and longer tuning screws can make up for some installations that are lacking ground plane, but not those that have no ground plane at all. In those cases, there are few options other than the use of a no-ground plane antenna system.