VSWR is an outdated term from the 50s/60s. It does not tell you much about the antenna system. It gives you two possible antenna system impedances. For example, a VSWR = 2 tells you that the antenna system impedance is 25 or 100 ohms. The only radio-related industry using the term VSWR today is antenna manufacturers to describe the bandwidth of their antenna measured at the feed point. For example, antenna bandwidth = 1.5 VSWR @ 144 to 148 MHz. If you swept the antenna, you would see the VSWR dip around 1.1 @ 146 MHz. It tells you the antenna impedance ranges from 33 to 75 ohms. VSWR is linear and ranges from 1 (perfect 50 ohms) to infinity (open or short circuit).
Return Loss (RL) describes the amount of power reflected from the load and is expressed in decibels. RL = 10log (Pf/Pr). It is a relative measurement taken from an absolute measurement. For example, if the absolute forward power = 100 watts (50 dBm), and the reflected = 10 watts (40 dBm), then 50 dBm – 40 dBm = 10 dB RL. It tells you your antenna system efficiency is 90%. RL 10 = VSWR 2. Another example is forward power = 100 watts (50 dBm), return power = 1 watt (30 dBm), RL = 20 dB, and VSWR = 1.22, efficiency = 99%.
Return Loss shines if you know how to use it. It becomes an excellent troubleshooting and modeling tool. It helps connect the dots. True story I have seen many times, especially ham radio operators. I got a call from a fellow ham telling me his UHF range suks. He cannot kerchunk repeaters, and the simplex range is limited to a few blocks. He cannot understand because he says his antenna VSWR is good at 1.8:1.
So I head over with my Anritsu to sweep his antenna. Upon arrival, I looked at the antenna system to get an idea of how long the coax run was, and that is when I spotted the first problem. Go inside, sweep the antenna, and see the antenna has an RL = 0 dB reflecting 100% of the signal. An RL = 0dB is a worse-case short or open circuit. An RL of 0 dB is equal to VSWR = Infinity. Jim is perplexed and pushes back, saying that his VSWR is good. I replied that he was measuring the VSWR of his 75 feet of LMR-195 coax. That confused him. How in the world does a coax have a VSWR, he asked?
The answer is simple: coax has Insertion Loss measured in dB at a specific frequency. If you input 100 watts (50 dBm) @ 450 MHz at one end of the coax and you get 50 watts (47 dBm) out at the other end, the coax has 50 dBm – 47 dBm = 3 dB Insertion Loss, aka attenuation. Insertion Loss is a one-way trip. Return Loss is a two-way trip, forward and reflected. RL = 2 x Insertion Loss. So, if you swept the coax with an open or short circuit at the far end, you would measure the RL = 6 dB with a SWR = 3. If you have heard Uncle Elmer say you can operate with a VSWR = 3, raise your hand.
Jim used 79 feet of LMR-195 coax measured on the Anritsu Distance to Fault mode. LMR-195 has an insertion loss of 6.9 dB/100-ft @ 445 MHz. If you understand and do the math, the problem will scream at you. The coax insertion loss =. 79 x 6.9 dB = 5.4 dB, which means the Return Loss equals 2 x 5.4 dB = 10.88 dB. Convert that to VSWR = 1.8, which is what his VSWR meter told him. Jim never considered the influence coax has on VSWR and RL measurements. You can only measure the antennas' RL or VSWR at the antenna's feed point. You cannot measure the antennas RL or VSWR in the shack unless you account for the coax. Easy to do when working with RL, you add the coax RL. Try doing that with VSWR Uncle Elmer.
Jim replaced the coax with LMR400 which gave him an acceptable Insertion Loss of 2.1 dB and rplace his damaged antenna. With a proper design, Jim increased his TX and Rx signal levels 3.3 dB more than twice power levels. His VSWR is now down 1.1:1 and he knows if he sees his VSWR rise to 1.2: 1 knows he has a serious antenna problem on his hands.
