On Wed, 01 Mar 2006 20:37:16 GMT, David wrote:

On Wed, 01 Mar 2006 20:00:22 GMT, Bob Miller
wrote:

On Wed, 01 Mar 2006 18:20:29 +0100, Charly wrote:


Dear all,

Usualy, people use 9:1 balun to connect a wire or dipole antenna to a 50 ohm coax.

A resonant dipole, I believe, is about 72 ohms. Connecting it through
75 ohm TV coax to a 50 ohm receiver input should be a near ideal
match.

On non-resonant frequencies, the dipole will present different
matches. That shouldn't be a problem, though, just for listening on a
sensitive receiver.

A balun won't give you a decent match except on the single frequency
it is designed for. That's one problem you run into, putting a balun
on a multi-band or multi-frequency antenna.

That's not the point.

One of the big rules in impedance matching is that the load (gozinta)
have equal or higher impedance than the source (gozowta). You never
go from a HiZ generator into a LoZ cable. Major faux pas.

If the antenna's highest impedance is 450 Ohms, and it's going through
a 9:1 matcher it will never be choked by a 50 (or 75) Ohm cable.

Why do you think an antenna for multi-frequency use has an impedance
no higher than 450 ohms? As an example, check the impedances of the
all band doublet at http://www.cebik.com/wire/abd.html

bob
k5qwg



Think of impedance as hose diameter and the signal as a solid stream
of water.