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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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<-- Hey Dave... long time!

Interesting info.. I agree getting the IL at exactly 1dB is a bit of a game, trial and error; you think you've got it, but by the time you tighten the screws up (and the loops move), the IL changes.... here's a more recent example:

An IL of -1.03dB and a reject notch of -34.91dB.? Seems to be close to your table above.

and the TX side, an IL of -0.89dB and a reject notch of -34.5dB (again pretty close to your table).

So a question for you... with an IL of 1.8dB and reject notches of -39dB per cavity, are you seeing an overall (combined) reject notch of -88dB?

My combined / final plots:

(before anyone mentions the reject notch looking slightly off, the capacitor on one of the cavities seems to be a little touchy... probably could do with being replaced).

Cheers,

Ramesh, VA3UV, WA3UV