> As received, each cavity was tested separately using a DSA815 spectrum analyzer with tracking generator and a high quality 50 ohm load.
> The insertion loss of each was 0.7 dB and the notch depth was at least -35 dB.
35 dB notch depth is a bit low with loops set for 0.7 dB IL.
> Touching the trimmer with the insulated tuning tool caused the notch to flatten out at about the 15 dB level.
That indicates the capacitor is shorting out.? Piston trimmer caps are rather fragile beasts.? There are several concentric plates in the piston and stator.? The slightest eccentricity will cause them to short.? It is almost guaranteed to need to be replaced.? You can unscrew the rotor all the way, and, using a magnifying glass, look closely at the plates on both sides.? I bet you'll see either a slight distortion, pit, or something that will be the reason for it shorting out.
> I'm also curious about the cable lengths. The inter cavity cables are the same length for both the transmit
> and receive branches. They are RG-214 precisely 12.125 inches long measured tip to tip.
> I would have expected the lengths to have been slightly different because of the different frequencies,
> but maybe at a 600 kHz split the difference is negligible since it's only about one tenth of an inch or less. Comments?
Between adjacent cavities - cable lengths would not vary because the frequencies are so close.? Between cavities and the antenna tee, the correct length is a function of Z of the opposing cavity at its notch frequency (i.e. at the opposing side's pass frequency), with the goal of rotating that Z to be back at the T to make it appear as an open circuit.? Many times, but not always, the lengths to the tee end up being equal.
>? The other length I'm curious about is what Telewave refers to as the phasing cables. These are the cables
> between the cavities and the antenna TEE. Their documentation states these cables should be
> 1/2 wavelength at the reject frequency.
Telewave is decidely wrong.
> The lengths of the RG-214 cables on this duplexer are both the
> same and only 13.5 inches long tip to tip.
> These seem way short since 1/2 wave cables at these frequencies is 26 inches and change. Comments?
Optimized cable lengths for the ham band:
Between cavities:? 12.125" RG214 tip to tip
Tee to first cavity on each side: 12.625" RG214 tip to tip
The latter works out to about 84 degrees...nowhere near 180 degrees.
??? ??? ??? ??? --- Jeff WN3A
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