On Thu, 17 Feb 2005 21:36:05 -0500, Gary Schafer wrote:

On Thu, 17 Feb 2005 17:35:11 +0800, budgie wrote:

On Wednesday, 16 Feb 2005 09:42:58 -500, "Asimov"
wrote:

"budgie" bravely wrote to "All" (16 Feb 05 12:52:14)
--- on the heady topic of " Something like a diplexer"

bu From: budgie
bu Xref: aeinews rec.radio.amateur.homebrew:2181

You missed a step called a directional coupler. It's a sort of
transformer with 2 input ports and 1 output port. The 2 inputs don't
see one another but their power is combined at the output.

bu You don't actually *need* a directional coupler. I have seen window
bu preselectors with five separate sections for sub-bands between 403 and
bu 520 MHz. The configuration is symmetrical (in/out) with simple bandpass
bu filter segments and coaxial split/combine harnesses. Not a DC or
bu hybrid in sight. I am presuming - not having swept one - that at "off"
bu frequencies each parallelled leg presents a high enough impedance to
bu the split junction that the effect is negligible.

Yes but it requires some effort and cost to build the 2nd BP filters so
I wonder if it's any more difficult to do either? What do you figure?

I'm not sure we are on the same wavelength here (no pun intended). For
*non-overlapping* filter sections, the bandsplit through two bandpass sections
25-500 and say 550-3000 should be able to be achieved with T-pieces at the input
and output ends (although it appears the O/P didn't achieve this).

I am not sure exactly what is trying to be accomplished here but if it
is to combine two different bandpass filters then yes a T. The cable
from the low pass filter to the T should be a quarter wave length at
the frequency of the high pass filter.

The cable from the high pass filter to the T should then be a quarter
wave length of the low pass filter.

The low pass filter will look like a short circuit at the high pass
frequency. With the quarter wave length cable that short circuit will
then be transformed to a very high impedance to the hi pass circuit.
That effectively isolates one from the other.

Do the another T and cable setup at the input and output.

If I understood the O/P he was trying to split and then recombine the 25-3000
band with a split ("crossover") at 500 MHz. Assuming a 50R in/out impedance
across the passband, the fiter sections presumably present a substantially
different (usually much higher) port impedance out of band. A simple T combiner
therefore presents a minimal impedance mismatch except near the split frequency.

Also I'm still thinking about how to make a quarter-wavelength line at 25-500
MHz or at 500-3000 MHz..