On Wed, 10 Mar 2004 08:47:27 -0600 (CST),
(Richard Harrison) wrote:

Jeppe wrote:
"So my problem is really mostly that I am trying to guess how close the
tip of the 1/4 wavelength probe antenna rod can be to the opposite wall
of the wave guide - without "detuning" the probe / affecting the SWR in
a detrimental way."

I don`t know, so I`m glad you asked the question. This is a good
newsgroup and I`d wager that someone will provide the answer. It`s been
a long time since I worked with microwaves and then it was with
commercial equipment.


Hi Guys,

I thought that was pretty well evident by the photograph alone. It
should also be quite obvious by the sheer ratio metrics of wavelength
dimensions: A quarter wave element penetrating into a half wave
dimension finds the tip of the element one quarter wave from the
opposite wall. When you reduce the fractions, the answer is simply
half way across. It is like a key, if it doesn't fit, you've done
something very wrong.

You can observe that the tip resides at a spot defined by the
intersection of two lines drawn from the four corners of the interior
box dimension of the throat.

The physics of wavelength, the size of scale for microwaves, the
regularity of surfaces, and common sense combine naturally to allow
anyone to manipulate and visualize all interactions with more ease
than attempting to perform the same operations at HF.

Adding inline lumped reactances is also visually elegant. You erect
short walls on opposite faces of the throat to build inductors or
capacitors (capacitive and inductive elements are 90° from each other;
inductors are erected across the shorter face, capacitors across the
longer face - all should note the correlation to e-field and h-field
geometries) and if you want to build a resonant trap (LC) you
construct a window frame (the combination of the four short walls
built) or iris occupying the interior of the throat.

One dimension that has lacked discussion is how far from the back of
the throat (the back wall) should this probe be placed. The link
offered reveals that this, too, maintains the standard ratio metric of
a quarter wavelength. However, other sources of mine suggest this is
variable for matching issues and may be as great as 3/8ths wavelength
or as short as 1/5th wavelength. Such adjustments add inductance or
capacitance to the rod to balance out its reactance (if present).
This is of less importance to receiving and the recommended quarter
wave placement is sufficient.

73's
Richard Clark, KB7QHC