On Wed, 01 Mar 2006 09:19:37 -0600, Bob Bob wrote:

Check the resistor.

One of the first things done. At DC of course. Dont think I ever
replaced it. Was considering playing with it last time but since moving
the frequency also bought it back to normal operating conditon I couldnt
see it as a reason.

Hi Bob,

DC is so remote from the application as to be only an approximation.
There's also the prospects of reactance to consider too. When you say
you can see a frequency dependency, you are almost guaranteeing that
"tuning" has been injected into an otherwise wideband design.

From your position in the company you have two paths:
1. Announce the design is FUBAR, or
2. Find an ad-hoc solution and forget theory because you are in no
position to re-engineer the design.

#2 is a dangerous path to take for the sake of the company's
perspective, although it may be more politic if the design department
is populated with prima-donnas.

#1 will accomplish one of two things, the design will be corrected, or
you will be educated - possibly both. Most designers appreciate
hearing what your experience has revealed. Most of my techs enjoyed
pounding my designs to find the weak seams. One fellow had a small
transistor radio that he would put on top of the microprocessor to
listen to the software running. He could always tell when one of my
patches went south.

73's
Richard Clark, KB7QHC

How thick is the board material, and what material is it? The
propagation velocity depends on the permittivity (dielectric constant)
of the material. The impedance depends on the material and the
spacings and trace widths. Presumably there's a ground plane behind
the microstrip lines (else they aren't microstrip). There are many web
sites that will let you play with microstrip designs, and some that
will give you the response of a coupler like you're describing. Do a
web search for things like "directional coupler" and "90 degree
[microstrip] hybrid". But if you plot the coupler's coupling versus
frequency, you'll find it's zero at DC, increasing to a fairly broad
maximum when the freq makes it 1/4 wave long (accounting for the
velocity factor), and falling again to zero at twice that frequency
where the line is 1/2 wave long. That pattern repeats. If you account
for the response, the coupler is useful over a broad range of
frequencies, as the directivity stays good even as the coupling
decreases (if it's accurately made). You can extend the frequency
range (make the peak even broader) by "tapering" the coupling. (Easier
to see in a picture than trying to explain in words...basically a
cascade of sections, with the center one coupled most closely.)

A point to note: if you make the coupled line say 5/4 wave long at
10GHz, it will couple nicely at 10GHz, but you only have to move by
2GHz in either direction to hit a null at 4/4 and 6/4 wave long for the
same physical line length. But if you make the coupled line 1/4 wave
long, then you don't see a null till 20GHz, and the coupler should be
quite useable between 8 and 12GHz. You can make the coupled section
short by leading the ends to the 50 ohm load and the diode detector
away from the coupled section, at right angles to it, so you don't have
to worry about the length of the resistor and the diode adding in some
difficult-to-calculate way to the overall length. Perhaps they already
are done that way, but from your description that's not clear to me.

Cheers,
Tom

Bob, VK2YQA wrote:
"The question is this. If one hangs a 1/2-wave dipole in free space I
assume it receives such that current maximums are at the centre and
voltage maximums at the ends. Is this the case whether a feedline is
connected or not?"

Yes, but you must have continuity between both halves of the dipole. If
you disconnect the feedline leaving an open circuit gap in the dipole at
its centre, you no longer have a 1/2-wave dipole of the same frequency.
You have (2) lengths of wire and each has its first resonance at about
twice the frequency of first resonance of your original dipole.

Best rergards, Richard Harrison, KB5WZI