On Jan 29, 4:26*pm, K7ITM wrote:
...
To a pretty close approximation, the coupling will go as the sine of
the electrical length, so for example if you use the design for a 20dB
coupler but make it only 1/8 wave long, the response will be about
sin(45 degrees) or .707 times as much, or 3dB lower: *it will be about
a 23dB coupler at that length. *

Actually, the sine approximation isn't all that good. Since it's not
been easy for me to find on the web and others may have similar
trouble finding it, I'll reproduce a better equation he

coupling = (Zeven-Zodd)*sin(theta) / [2*Z0*cos(theta) + j*(Zeven
+Zodd)*sin(theta)]

whe
Zeven and Zodd are the even-mode and odd-mode impedances of the
coupled line pair:
Zeven is the impedance seen when testing one line of the pair,
when both lines are driven by the same voltage.
Zodd is the impedance seen when testing one line of the pair,
when the two lines are driven by equal-amplitude but 180
degree out of phase voltages.
Z0 = sqrt(Zeven*Zodd)
theta = electrical length of the coupled line section

To find the coupling in dB, just take 20*log10(coupling).

You can get the even and odd mode impedances from a program like altc,
available at http://altc.sourceforge.net/. You may also be able to
find formulas that give them to you for common structures like
stripline and microstrip just from the dimensions and relative
dielectric constant of the substrate.

In the hopes this may be valuable info for one or more of the
lurkers...

Cheers,
Tom