I keep forgetting some of these principles.

Thanks very much for the detailed and helpful explanations, Roy.

73,

Chuck

Roy Lewallen wrote:
chuck wrote:

As a follow-up, is there a practical way to determine how much current
unbalance will cause a one dB reduction in power delivered to the
antenna, the "lost power" being that power radiated by the
transmission line?

It seems like a rather complex modeling problem.

Thanks!

Chuck, NT3G


Nope. You can't generally say that one part of an antenna is radiating a
particular amount of the total power. Each part of the antenna creates a
field, and it interacts with the fields from all other parts of the
antenna. The total power radiated has to equal the total power input
less loss, but that's all you can say for sure. An example will help
illustrate the problem. Consider a parasitic element in a Yagi. It has
considerable current and contributes a great deal to the overall
pattern. Yet the total power input to the Yagi element is zero. With
zero power input, it can't, by itself, be radiating any power. What it
does is intercept some of the power radiated by the driven element and
re-radiates it with a different phase and amplitude. So how would you
apportion the power radiated by the driven element and the parasitic
element?

You might take a look at the current in the driven element and note that
it increases or decreases as you put the parasitic element in place and
remove it. But the current can either increase or decrease, depending on
the length and spacing of the parasitic element. So has the parasitic
element increased or decreased the power radiated by the driven element?
There's no answer.

You can look at the change in pattern in some idealized cases by
modeling. This can tell you what range of effects you might expect in a
real situation.

Roy Lewallen, W7EL