In order to avoid any confusion regarding SWR, we almost always write about the "antenna system". When SWR is measured it takes into account just about everything from the radio end of the coax outward. This includes the coax, the stud mount, the mount itself, the antenna, any in-line antenna accessory, the vehicle and any vehicle add-ons (roof racks, spare tire carriers, etc.). Even open doors, hoods, trunks or hatches alter the vehicle configuration and will affect SWR. Powerful antenna designs (wire-wound top-loads) create very strong electromagnetic fields and interruptions, especially within a 3-foot radius can have a negative effect on SWR. If you only focus on the antenna when having SWR issues, there is a good chance that you will completely miss the problem. Keep this in mind while reading this and any other article regarding SWR.
Assuming that the radio is working properly, it does not have a direct effect on SWR. That is, if you test SWR with one working radio and then replace it with another working radio, there should not be any change in the SWR. Read on to know why.
When the SWR meter is installed, or activated as might be the case of one built into the radio, the radio is little more then a power source. In simple terms, SWR is nothing more then a ratio of what the radio has the ability to deliver (the 1 in the ratio) and what the antenna system will not absorb (reflected back to the radio). When in the "calibrate" or "forward" mode, the antenna system is isolated from the radio. The radio is then "keyed up" (microphone transmit button is depressed) and a "calibration knob" is manually adjusted to move the meter needle to a "set" or "calibrate" line on the meter face. This process establishes the basis from which the comparison is made. Once established, the meter is switched to the "SWR" or "Reverse" mode. This opens a path for the power to feed into the antenna system. So, as long as any radio has the power to drive the meter needle to the calibration line, that which might reflect back will generally expose the same inefficiencies (percentage wise) regardless as to whether the base output is 3-watts or 8-watts.
If the antenna system is perfectly tuned to the frequency (channel) that the radio was on in the calibration mode, if the quality of the coax isn't compromised, if the mount was correctly installed, and if the antenna and counterpoise are in sync, the antenna system will absorb all the power that the radio is putting out and the SWR (ratio of available to reflected) will be 1:1. However, if there are system flaws (as found in almost every mobile installation), or if the antenna is out of tune, the system will not be able to absorb everything that the radio has the capacity to deliver and the unabsorbed power will remain within the system. For instance, if the radio has the capacity to deliver 4 watts of power but the antenna system only has the capability to absorb 96% of the power, the SWR will be 1.5:1. This 4% loss will still allow 3.84 of the 4 watts to be radiated from the antenna. SWR that is 2.0:1 or lower is desirable and SWR below 3.0:1 is mandatory.
As in most cases, there are exceptions to the general rules. Most of these that apply to this article involve radio equipment capable of delivering in excess of 50-watts of power into the antenna system. At some unknown point, wattage can affect coax cable performance. This is generally associated with the quality, or lack thereof, of the coax. Everything could be okay up to a certain point but once that threshold is breached, problems that did not exist at lower power levels could raise their ugly heads. Tbe same holds true with the antenna. The electromagnetic fields generated from the antenna with a 4-watt transmitter may not be dramatically affected by an object that is a few feet away from the antenna, however, when you ratchet up the applied power the electromagnetic field strength will increase and the same object may now become somewhat of an issue.
Bottom line ... the make, model, bells and whistles of the legal CB radio is a non-issue with respect to SWR.