Duplexer Basics

Cavity filters can be used as narrow BandPass filters, a single tuned circuit with a very high Q (2000+) coupled into and out of by various methods, usually a loop coupling to the, in cavity, magnetic field.
They can also be a narrow BandReject filter, usually done with one coupling loop and where the cavity, at resonance, presents a very low impedance. Thus is used to "short out" the signal on the coax line at a Tee junction.
Because our Ham Radio repeater split frequencies are so narrow, particularly on 6m and 2m, we need a different cavity type, the BandPass AND BandRegect type (BP-BR).

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 UHF cavities as in the image Ant_T_IMG_3734b.jpg.

Examples:

Type 1

Lepzig UHF duplexer.....
Note: No connectors, hardwired and Gold trimmer.

Type 2

Alan VK2ZIW's 5Note phase swap of LHS loop, makes for larger L.

Type 3

Alan VK2ZIW's UHF test cavityMagnetic coupling.

Type 4

image from Coupling Loop Research, RepeaterBuilderTwo connectors parallel resonant circuit between.

Type 5

A 1972 article on 2" heliax as resonators for 6m

This below is an area of discussion NEEDS TO BE ADDED on Repeater Builder.

The Antenna Tee Where the two cavity chains join to feed one antenna.

At the notch frequency, types 1 and 5 (above) 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_3734b.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 examples of home-made (type 4) coupling loops for 6" RFS or AEA cavities.

IMG_3733b.jpg

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. See also reference (e).

Tuning Basics

I've made up the loops, set them up for 146.9MHz pass and 146.9 notch, make up all the 1/4 and 1/2 wave patch leads, connect it all up, this chain works, a treat. But, the other set, individually look good but together, the pass is a double hump and 6db down. What do I do?? I tried putting a bandpass cavity between the two BP-BR cavitiies, no effect. I tried a 5db pad from the VNA, no effect.

OK then, how identical are my two loops? PocketVNA 10db loss double hump, bad

At the end of the day, we want to know what works and is easy.
And, to know WHAT DOESN'T WORK. This is MOST important.
The secrets Vari Notch tuning

References:

a) A 1972 article on 2" heliax as resonators for 6m
b) A 1965 IEEE article by Phelps Dodge
c) A general article by EMR Corp
d) A 2010 article by Sinclair
e) Where the loop cannot be rotated...VE2AZX
f) Motorola T1500 UHF series, with internal pictures.
g) TX-RX Coupling Loop Project on the Repeater Builder website but no followup, a full duplexer build.
h) andrew-about-rf-communications.pdf Section 4, a general article.

Alan VK2ZIW