This is an area of discussion COMPLETELY MISSED.
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The Antenna T
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There are FIVE main types of BP-BR Cavity filters.

1) The single connector series loop and trimmer to GND type.

2) The two connector, couple a bit of energy between the coupling loops
   with either L or C.

3) The two connector, couple some energy magnetically between loops.

4) The two connector, parallel resonant circuit between connectors.

5) The single connector capacitor divider, with either L or C in the GND side.
   As in the 2" heliax duplexer for 6m.

AND, the aperture coupled Band Pass cavities as in the image Ant_T_IMG_3734.JPG.

At the notch frequency, types 1 and 5 present a Very Low impedance thus back at the antenna "T" where we need a High impedance as the energy at this frequency is to go straight past, we can use a quarter wavelength line to achieve this.

Type 4 presents a High impedance at the notch frequency so half wave lines are
used to the Antenna "T".

Types 2 and 3 and the Band Pass cavities present a High VSWR but neither High or
Low impedance exactly, this is where it gets hard. A Vector Network Analyser is a great tool here but I doubt they had one back in the 1960s. Instead, General Radio Inc. made a constant impedance adjustable line. Adjusts 20cm and they would
have used other known lengths of coax to calibrate it.

The LHS of my picture Ant_T_IMG_3734.JPG has an extra half wave of hard-line to
get the lengths right (as best I could).

So, now you know why some of the coax lengths are not quite quarter wave or half
wave. Particularly where the Rx and Tx chains join, the Antenna T.

Some example (type 4) coupling loops for 6" RFS or AEA cavities.

Note the coil. I "filsched" this idea from Repeater Builder, "Coupling Loop Research", where they increased the loop inductance with a coil not in the "main" cavity magnetic field. This made the pass-to-notch spacing, much easier to adjust. Adjustment by rotating the assy.


Alan VK2ZIW