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