On Fri, 18 Feb 2005 11:42:56 +0800, budgie wrote:

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..


Usually a bandpass filter will present a low impedance out of band.
That is where I was with the quarter wave cables. But you are right,
it may be somewhat of a problem with that wide of a span. Guess I
didn't read close enough.

I read the original post again and maybe understand better what is
trying to be accomplished.
With the back to back diplexers there may be a termination problem
with combining them. ?? Maybe try a load resistor at the junction
where they recombine.

The simplest solution seems to be two hybrid power splitters back to
back. No band pass filters needed. Like two tv cable splitters. Of
course that means 6 db of loss.

73
Gary K4FMX

Hi guys,

"budgie" wrote in message
...
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.

Yes, this is correct; the problem is that close to the 500MHz split, the
filters combine together so as to create several resonances. I'll try the T
combiner approach; this should tend to 'de-Q' the resonances.

An isolator (or circulator) would work here, but I'm not aware of any that
cover such a wide bandwidth.

The suggestions for 'quarter wavelength' anythings aren't appropriate given
the wide bandwidth.

Thanks for all the help,
---Joel Kolstad