Frank Dresser wrote:
None of the esoterica below should worry anyone who just wants to hook up a
wire to his radio and listen to shortwave stations.


"John Doty" wrote in message
...

For a wire antenna, the field configuration near the wire is very
similar to the field inside a coaxial cable. Unsurprisingly, it has
similar behavior: the bulk of the energy tends to propagate along the
wire and not radiate. This leads to Schelkunoff's approximation: you
calculate the current distribution along the antenna as if it was a
transmission line, and then calculate the radiation due to that current
distribution. You can get the antenna impedance by calculating the
impedance of a lossy transmission line (with loss equal to the
radiation) with the assumed current distribution. You get the reception
properties by reciprocity.


OK, but if the transmission line analogy holds, shouldn't the unterminated
antenna look like a lossy stub?

Exactly!

If a stub is open, it will look like an
open or short at certain frequencies, and some sort of reactance at others.

If it's lossy, the terminal impedance of an open stub will have a
resistive component at all frequencies. A good first approximation for
an inverted L is a 500 ohm line terminated with a 5000 ohm load. Works
pretty well for wires with lengths in the range from 1/4 wavelength to
several wavelengths.

Of course, if the transmission line/antenna is terminated with it's
charactistic resistance, it will look flat.

When you match to the characteristic impedance, you get a nearly flat
frequency response, but it's down by a few dB from what you'd get by
matching to the terminal impedance.


Since the formula isn't frequency sensitive, I was wondering if it was for
terminated wires.

The characteristic impedance isn't frequency sensitive. The terminal
impedance is. Just like a stub.



Good thing. Even when the rocket fails and destroys your payload, you
get to go home and hug your wife and kids. It's happened to me twice so
far (HETE-1 on a Pegasus in 1996, and ASTRO-E1 on a M-V in 2000).



Have you seen the movie "Cape Canaveral Monsters"?

Nope.

-jpd