The last couple of posts dealt with HARDWARE to measure SWR. But why is a high SWR a BAD thing. Often quoted, a high SWR is on par with what happens when the gas gauge is almost empty. Or as we well know, if you had a Red Ryder BB gun you will shoot your eye out! Or the cute Red Head has only a slight STD.
Basically, two impacts are: 1) max RF from your rig is not getting to the ionosphere and 2) you risk serious damage to your rig. For many hams the idea of a puny signal is a far greater impact than a smoked rig. Hey, it's just more added on the plastic. But a high SWR also can cause unwanted feedback and then your signal is not only puny, but it sounds terrible.
Manufacturers will caution their radio can safely operate with a 2:1 SWR or never exceed a 3:1 SWR. Some radios will sense when your SWR is approaching a danger point and even have circuitry that will idle back the power to values that won't harm the radio. (Fold Back Current Limiting).
Disclaimer -- I actually never looked at the sausage making aspects of SWR calculations but always had an SWR bridge in line to tell me the values. But a hand calculation is an eye opener. I listed a link for K6JCA (two posts ago) and he provided some detailed math on the calculations. (Now I know why I used the meters.)
If I haven't screwed this up if you take the Reverse Voltage and divide it by the Forward Voltage that is a Factor that is plugged into an equation where the numerator is 1 + the Factor and the Denominator is 1 - the Factor. Pure logic shows the lowest SWR is 1 to 1 (0 Reflected power) and infinite when the Forward is the same as the Reflected as that puts a Zero in the denominator.
So (again if my calculations are correct) what is a 3:1 SWR. Let's assume the FWD = 5 Volts and the REF = 2.5 so what is the VSWR. Our Factor is 5/2.5 or 0.5. Thusly (1 + 0.5) / (1 - 0.5) = 1.5/0.5 = 3. That Buck Rogers is Rocket Science meaning 50% of your available power is being fed back into your rig and not the antenna! Are you sure you want to subject your ICOM IC7300 to a 3:1 SWR.
Now how about 5 V FWD and 0.5 V REF. Our factor is 0.5/5 or 0.1. Thus, the numerator is 1.05 and the denominator is 0.95 and the SWR is 1.10:1
So, you get the drift -- it is hard to get a 1:1 but that is reflective that with 5V FWD and 1.667V REF that gives a 2:1 SWR. About 1/3 of the power is being back fed into the rig and not fed into the antenna.
Notionally if your SWR is greater than 1.3 to 1 you need to fix something with your antenna! This means about 0.65 V REF with 5V FWD or about 13% not going to your antenna!
Again, a bit hurried this morning so the math may be a tad off. BUT a High SWR does suggest an eventual smoked rig and not all the juice into the wire! I am surprised at how many hams do not have in line monitoring --so get or build a meter 1st thing!
TYGNYBNAINNVNA.
73's
Pete N6QW