CHAPTER 7

Lines between cavities and the antenna "T"

For some reason this chapter is missing. This is my input. Alan VK2ZIW 2m and 6m cavities

Lines between cavities

Obviously good coax cables. Loss is not a problem for the short lengths used here, but leakage is important. Use coax, either double shielded or with good braid.

Cable lengths. Are they always y/4 ?

No, this is where cavity types matter.

With BpBr cavities of the single loop and series capacitor type, we are lucky because, at the notch frequency, the input impedance is very low. So, just use a y/4 cable as it presents a high Z at the other end.

However, BpBr cavities of the two loop and small C or L across the connectors type, the input impedance at the notch frequency may be anything. All that is known is, they present a high VSWR at the notch frequency.
So, to present a high impedance, a different length coax must be used. How do we find this length? Well, General Radio made an adjustable in length 50 ohm coax line, back in the '70s. (I have one)

Measurement: Put 50 ohm terminator on a "T" piece and the adjustable line to your cavity. Measure the SWR into this combination. Adjust the line length for best SWR. If your adjustable line is longer than y/2, (mine is) subtract y/2 from your length. Remember the velocity factor of your adjustable line will not be the same as your coax. Now make a lead and test it. Accuracy within 1cm for 2m will do. You could use a zillion coax joiners, male to male, female to female, end to end, say 10 of them for 2m. If you have a network analyser go for it.

This is the stuff the manufacturer's literature sais nothing about other than, it's critical.

The antenna "T"

A circulator would be nice. A rare device. Not needed here.

We want all the Rx signal to go down the Rx cavity path and not experience a bad SWR caused by the lead to the Tx cavities. As mentioned above, with BpBr cavities of the single loop and series capacitor type, we just use y/4 lines. Likewise, the Tx signal needs to not see a bad SWR at the "T" caused by the Rx chain.

BpBr cavities of the single loop and series capacitor type make life easy!!!

Bandpass cavities also have this problem, so are not used as the 1st cavity after the antenna "T".

Unless somebody has done the hard work and worked out the correct coax lengths.

============ Testing ================

If you've never seen the cavity set before and suspect it may have been fiddled with, start by putting terminators on the Rx and Tx ports and measure the SWR into the antenna "T". You should see good SWR at both the Rx and Tx frequencies. If not. move the terminators to, after the 1st cavities, and test again. Cavities further down the chain may be off frequency.
If you have just a tracking sig-gen and spectrum analyser, make up an RF bridge. Two 47ohm surface mount resistors, and a torroid, 4x BNC sockets, does me fine for SWR, 30 to 600MHz. You could buy one for many $$$.
Today's programmable DDS synthesiser chips make a sweep generator, just a simple software exercise.