When measuring the SWR of a transmission line we can find a length
that is resonant. Or in other words the current is of an exact
sinusoidal form and thus the basis of SWR. When we apply the same
principles to a radiator
we cannot assume a perfect sinusoidal wave because of the field
generated outside the radiator where a degradation of amplitude
depends not only on the generated fields but also the length of the
radiator i.e a thin radiator. Thus one can see that a fractional wave
antenna if reflected as generally assumed cannot possibly be 1:1 for a
wavelength as per a perfect reflection as could or would be seen from
a transmississsion line.
The above not only shows that a transmission line is a closed circuit
as is a fractional wavelength antenna where the current travels on the
inside of the radiator and where the transmission line has current
flow on the inside of the
outer skin (brading) The antenna compendium states that an assumption
is made with MoM programs that an assumption is made that current in a
radiator is sinusoidal where as we all know that the current degrades
in aplitude dependent on radiator length. It should be seen that when
we use the term SWR we are looking at two different things, a
transmission line which is not radiating( ignoring leakage) but still
a closed circuit and a radiator that is radiating but with a non
sinusoidal current which is contrary to the assumptions made for MoM
style radiator programs. Thus it should be seen that any radiator is a
closed circuit with continuous leakage in current levels dependent on
its length via accountability for all four fouces present..
Regards
Art