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Author Topic: duplexers vs notch filters  (Read 853 times)
N9DO
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Posts: 55




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« on: February 19, 2002, 12:53:56 PM »

What is the main differences between band pass-band reject duplexers and notch duplexers. Is one superior to the other and why? We've been using a notch filter duplexer on our repeater for years and now we are being told we need to change to band-pass band-reject duplexer, why?
Thanks N9DO
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WB2WIK
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Posts: 20666




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« Reply #1 on: February 20, 2002, 01:55:56 PM »

Who told you that you must change?  Unless you're interfering with another service, this doesn't make much sense.

Most common duplexers are BpBr (band pass/band reject) designs or Bp/notch designs.  The popular "Phelps Dodge" (changed to Celwave many years ago) designs are notch cavities having a bandpass characteristic and are highly effective, making a lot of use of the available volume.  The popular "Wacom" designs are BpBr cavities not having nearly as steep nor narrow a notch, but providng excellent characteristics for amateur band repeater spacing, e.g., 600 kHz at 146 MHz, etc.

One disadvantage of the "notch" arrays is that the notch is so narrow, and so deep, that it's easy for these arrays to become misaligned with vibration, age, and handling.  The notches can also be so narrow, you can almost "deviate" out of the notch -- no kidding.  They are truly tight.  The BpBr designs are a bit more forgiving.  In all cases, commercial cavity filters are built using temperature compensated components so they should not drift much with temperature changes.  (Actually, the tuning plungers are usually made of Invar, a metal having a zero temperature coefficient; it neither expands nor contracts with temperature changes, over a fairly wide range.)

WB2WIK/6
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