Henry Kolesnik wrote:

I know that a shorted 1/4 wave stub exhibits a very high impedance. But
for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance
or a short. There are claims that this can be used to filter the even
harmonics. Shorts can't diisipate power and must reflect, so how does a
stub work?

Consider the following configuration:

Source----ideal 1 WL feedline-----------+-----matched load
|
|Stub
|
|
open

If the stub is 1/4WL, the forward voltage and reflected voltage in the
stub is the same as on the feedline. At the mouth of the stub, they
are 180 degrees out of phase and superpose to zero volts which obeys
Ohm's law and delivers zero power to the load.

Double the frequency. That makes the stub 1/2WL. The forward voltage and
reflected voltage are essentially the same as in the 1/4WL stub but this
time they are in phase and superpose to a maximum value which obeys Ohm's
law and delivers maximum power to the load.

Absolutely nothing except superposition and interference happens at the
mouth of a stub. All the reflected action happens at the physical open
circuit. Virtual impedances are only a V/I ratio and CANNOT cause
reflections. Absolutely no reflections are happening at the mouth of
the stub (unless a physical impedance discontinuity exists there).

Consider this. If a stub really presented an infinite impedance, you could
simply remove it and nothing would change. Something inside the stub happens
to cause that infinite virtual impedance. Hint: Stubs have a near-infinite SWR.
To understand that assertion, consider how much voltage exists at a voltage
minimum inside a stub. It is nearly zero.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----