I use a similar method that I find a little more accurate for setting the loops. It's very hard to set the pass for 1db IL accurately. I use this method to build VHF duplexer a from anywhere from 4 to 8 cavities.
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Using the capacitor, center the two notches around the pass. You will have to increase the span to see it. I start at 20 Mhz. Adjust the rod so the pass frequency is at the lowest insertion loss between the notches.
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Now loosen the three screws holding the loop in place. Rotate the loop until the two notches are the following frequency span across:
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Notch span.? Ins Loss.? Notch @600kHz
9 MHz.? ? ? ? ? ? ? 0.3 dB.? ? ? ? ? ? 25 dB
8 MHz.? ? ? ? ? ? ? 0.4 dB.? ? ? ? ? ? 27.5 dB
7 MHz.? ? ? ? ? ? ? 0.5 dB.? ? ? ? ? ? 30 dB
6 MHz.? ? ? ? ? ? ? 0.7 dB.? ? ? ? ? ? 32 dB
5 MHz.? ? ? ? ? ? ? 0.85 dB.? ? ? ? ? 35 dB
4 MHz.? ? ? ? ? ? ? 1.2 dB.? ? ? ? ? ? 37 dB
3 MHz.? ? ? ? ? ? ? 1.8 dB.? ? ? ? ? ? 39 dB
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The insertion loss and notch depth are additive, so you can design your duplexer in this fashion.
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One thing I also do is rebuild the harness. I believe Sinclair had two harness lengths for these at VHF, one from 137-150 and another from 148-174. If you measure these harnesses, they are actually centered around 138 and 160 MHz respectively. I tear apart the harnesses and rebuild them with approx 14 inch lengths of RG400. I'll actually build the harness with lengths very specific to the frequency used. The intercavity lengths need to be an electrical 1/4 wavelength. This needs to take into account the velocity factor of the coax in use.
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Dave Cameron?
VE7LTD?
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