Recently, I have posted back to back about maximizing power at the antenna, and about characteristic impedance. These conversations have touched on reflected waves and associated loss. I thought it might be good to drill down on this just a bit more, as these have been topics on my mind as I have been making changes to my humble ham shack.
I like a good analogy. When imagining and describing RF radiating from an antenna, or traveling along a coax, we often describe it as waves. These waves can’t be seen, but we can imagine some of their properties by substituting the behavior of visible, tangible waves we are familiar with – sea waves.
A big beach with a slow, gradual rise in elevation will allow waves to come in from the sea and slowly dissipate, using the majority of their energy to dampen the farthest inland reaches of the beach.
By contrast, an abrupt sea wall creates a violent place where an incoming wave crashes into the wall, sending spray into the air, and causing a new wave to instantly form and bounce back in the opposite direction. The new wave collides with the next incoming wave where their opposing forces interact. In the subsequent interactions, much energy which could have been used to dampen the beach beyond the sea wall, gets wasted.
We might say that our gradual beach example has a desirable characteristic impedance – well suited for the arriving sea waves to reach as much sand as possible. We could also say that the beach with the sea wall is poorly matched, and because of this the energy in the waves is reflected back out to sea where it interacts in ways that dissipate wave energy before the waves can reach the sandy beach.
Using a VSWR meter we can measure the magnitude of wave reflections happening in our system.
Return Loss is the measure of how much power is being reflected from our load, which in our case is our antenna system. The better we do at matching the load to its “ideal” characteristic impedance, the lower the reflected power will be. We will always have some amount of reflected power. (Even in our gradual beach analogy, after each way reached its apex, there was going to be some water changing direction and making its way back toward the sea.)
This kind of loss is expressed in dB relative to the power we used to transmit. Since the value of the reflected power will always be less than the transmitted power, we express this value as a negative number. It is correct to think of RETURN LOSS as the amount of power reflected that didn’t get absorbed by our load.
There is another sort of loss that often gets lost in the return loss discussion, but it should be understood as something separate. This is mismatch loss. We introduce mismatch loss by including imperfect components into our antenna system. At every connector, coupler, adapter, and so forth we add an opportunity for a distinct change in characteristic impedance. These kinds of changes are hardly noticeable compared to typical return loss problems, yet they do contribute to overall loss. For this reason, we can try to minimize the possibilities for mismatch by using the fewest possible components in the design.