Ford Peterson wrote:
Is there any intrinsic loss attributable to coupling a relatively low impedance
system to the high impedance of free space? E.g. is there any benefit to using
a higher impedance system?
=========
Hi, Ford.
A wave traveling through space has both an electric field and a magnetic
field. The orientations of the two fields are perpendicular to each
other and both look like waves that vary with time and distance, just
like voltage and current do on a transmission line. The physical
orientation of the electric field is determined by polarization of the
antenna. The electric field of a wave traveling through space is the
analogy of the voltage on a transmission line and the magnetic field is
the analogy of current, and the ratio of the electric field to the
magentic field is 377 ohms in free space.
Since the antenna launches the signal into space, by definition it
essentially provides the transformation to the impedance of free space.
At least that's how my college field theory professor described it
almost 40 years ago. Maybe he was only making an analogy. Anyway, the
junction of the feedline to the antenna is a different node in the total
system and its impedance has no direct relationship to the impedance of
free space.
On the other hand, it would seem that certain antennas must have an
inherently better ability to couple a signal into free space. I don't
know this for sure, but I suspect that a reactive feedpoint represents
an antenna that couples improperly to free space. We are able to adjust
that condition by the use of matching networks of various sorts to
override the antenna and put voltage and current (and therefore electric
field and magnetic field) back into proper phase. Please consider this
last paragraph as speculation on my part, and I'd appreciate someone
correcting me if it is erroneous.
Dave
AB7E
>
>
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|