"Cecil Moore" wrote in message
ups.com...
A stub is an open-circuited or short-circuited section of
transmission
line. It can be a series stub or a parallel stub. It can be
inductive,
capacitive, or resonant depending upon length. At a single
frequency, a
stub can replace an inductor or capacitor and/or cause a change in
impedance, i.e. a change in the voltage to current ratio. (Not to be
confused with software stubs.)
--
73, Cecil http://www.qsl.net/w5dxp


In their most common form stubs are multiples of a quarter wavelength
and, at least at VHF and higher, consist of a piece of coax cable.

A stub that is an odd multiple of a quarter wavelength will exhibit at
its connection end the opposite of the other end of the cable, i.e. if
the remote end of the cable is open circuit then the connection end
will exhibit a short circuit. This can be useful in making a parallel
connected notch to tune out an interfering signal, such as paging or
whatever, and can be found empirically by snipping little bits off the
cable until it has most - or at least enough - effect.

By the same token if the remote end is short circuit then the
connection end will be seen as open circuit. This is often used
professionally in aerial combiner systems where the tuned feed
resonator - usually something like a metal beer barrel in
construction - has a short on its output pickup coil which, when
connected onto the cable that interconnects the barrels, looks like an
open circuit. Thus each barrel 'sees' none of the others on the system
just the aerial.

A piece of coax that is an even multiple of a quarter wavelength
reflects the same at the connection point as at the remote end. In the
example above of a combiner, the interconnect between the barrel
connection points is a half wavelength.

Hope that helps.


--
Woody

harrogate2 at ntlworld dot com