On Tue, 09 Mar 2004 10:40:10 +0100, Jeppe wrote:
0.5 wavelenght seems like a intuitively reasonable choice for the
smaller dimension of cross section of the throat / waveguide part. If
the tip of the probe (=1/4 wavelenght antenne rod?) is very close to
the opposite wall it is hard to imagine that it would not affect the
function and the SWR (which, unfortunately, I lack equipment to
measure)

But are you sure that 1 wavelenght is the optimum value for the other
dimension of the cross section? That is more than in the examples I
have seen on the net.

Is there some reason for choosing a 2 : 1 ratio of the two dimensions?

I may add that in the first horn I made, the dimensions were 0.36 x
0.72 wave. It worked, but probably not quite optimallly (I lack real
measuring equipment.) I then tried to put the antenne rod
perpendicular to the SMALLER side instead, but that did not work at
all (very weak signal).

73

Jeppe (former OZ3FV, hav'n't renewed my licence for about 30 years :-(

Hi Jeppe,

Sorry for the delay in response, this message of yours has arrived
only today - very late.

The 2:1 ratio follows from the basic ratio metrics of microwaves, but
are not absolute. Rather than go into a difficult description that
begs graphics, visit:
http://www.fnrf.science.cmu.ac.th/th...heory%202.html

As to your question about the dimensions of 0.36 x 0.72 wave (you
will no doubt note the aspect ratio again). The answer to this is
that such components tend to work over an octave of frequency (another
2:1 ratio, pretty common that).

73's
Richard Clark, KB7QHC