Dilon Earl wrote:
On Tue, 15 Jul 2003 01:20:02 -0700, Roy Lewallen
wrote:


No, an antenna doesn't "match" the impedance of free space. The input
impedance of an antenna is the ratio of V to I. The impedance of free
space is the ratio of the E field to the H field of a plane wave. They
both happen to have units of ohms, but they're different things and
there's no "matching" going on. If you apply 100 watts to an antenna,
resonant or not, 100 watts will be radiated, less loss, regardless of
the antenna's input impedance.
Roy Lewallen, W7EL


Roy;
Hope you don't mind if I ask you a couple of questions about your
last sentence.
Given a 75 ohm dipole fed 100 watts with 75 0hm coax. Assume no
losses, now if the antenna's input impedance is changed to say 50 ohms
(non-reactive). You would have some loss with the mismatch between
the coax and antenna?

Mismatch does not cause loss (that is, conversion of electrical energy
to heat), except that the loss of a lossy transmission line will
increase (very slightly unless initial loss is great) when SWR is
elevated. In this case, the line SWR would be 1.5:1, which would not
cause a significant amount of extra loss even if the cable were very
lossy when matched. So the answer is no.

The transmitter would see 50 ohms?


The transmitter could see any of a variety of impedances, depending on
the length of the 75 ohm transmission line. Only if the line were an
exact multiple of an electrical half wavelength would the transmitter
see 50 ohms, resistive. If the line were an odd number of quarter
wavelengths, the transmitter would see 112.5 ohms, resistive. At all
other lengths, the transmitter would see a complex (partly resistive and
partly reactive) load.

There is a term called "mismatch loss", which is widely misunderstood in
the amateur community because of its name. It doesn't really represent
loss at all, but a signal reduction for other reasons. I've explained
this before in this newsgroup, so if you're interested, you should be
able to find my earlier postings via http://www.groups.google.com.

Roy Lewallen, W7EL