Home brew cavities and Duplexer, by Karl Shoemaker

Some thoughts to this madness of spending time on building

I want to clear up a couple of terms; a cavity is a single resonator, usually in the form of an electrical quarter wavelength. Several cavities cascaded together with specific lengths of coax, setup for two or more frequencies is called a duplexer. Notice singular, not "duplexers". This is the same basic semantics as several "cells" make up a "battery". There's no such thing as a "AA" battery, unless the device used ONE cell, like my Sony Walkman. I know this sounds picky; just need you to know where I'm coming from; that I want to get the terms right.

With that out of the way,..... cavities for VHF and UHF can be made cheaply with some common parts around town. For mine I used 3 pound coffee cans. You know where to find those. Seems like my wife and I are the only ones around that don't drink coffee so I go to other places with lots of drinkers, like work or neighbors. . For VHF (and remember we still have two upper VHF bands; 2 and 1 1/4 meters. (Never could get over the imslable of "220", since it's no longer an Amateur band) (Sorry for the soapbox) For 2 meters I have not gotten them to work with much "Q", so we'll concentrate our discussion for 1 1/4 meters and 70cm for UHF. With some research you could make it work anywhere. I'm not an engineer. I know my basics about resonance, reactance, and so on, so I look how the commercially made ones are done and more or less follow that. You could do that for most stuff in the world from a garage door opener to a TV antenna. Just use your knowledge and head. (Opps, I'm being logical now)


Save the can(s) and plastic cover(s). Clean them up and dry. For UHF, 70 cm you will use one can. For 1 1/4 meters use two and for 2 meters use three. For a notch can use one connector, for pass use two. For two cans "open" the top off, but keep the "top". For three do it again. so you have two or three cans soldered on top of each other, to get the physical wavelength needed. That "top" you cut off, now can be you new bottom that you will solder, later, on near completion of each cavity(s). Wait until near completion before soldering the stacked cans together to allow access to the coupling loop.

Drill a 1/4" hole in the top center of each can. This will be for the tuning rod, which is a brass 10-32 or 24 rod at the HW store. Reason is it's solderable. No Invar here. Later on that. Drill one or two 5/8" holes for each connector needed, whether pass or notch. I use a 5/8" greenlee hole punch, which costs about the same as a bit that size and makes a cleaner hole. Ever have something you are drilling suddenly "take off" on you while violently spinning around ?? Naw! you always clamp it down, right ?
Cut a piece of 3/4" rigid copper pipe. The length should be about 70% of the can, which depends on what frequency range you are building it for. Solder an adaptor from 3/4 to 1/2" on one end of the 3/4" pipe. Now make finger stock on the 1/2" end of the adaptor by cutting 6 ways, being careful not to got down too far, since the walls get thin. I go about 3/8" to 1/2" down with a hacksaw while holding it in a vice, being careful not to distort the roundness. Slightly push in each finger so it will make good contact to the plunger. The plunger is a piece of 1/2 pipe, with a cap on the lower end. Drill a 1/4" hole in the center of the 1/2" cap, solder it to the end of the 1/2" pipe, then run the rod just thru the cap's hole and solder it. Keep the rod and pipe straight in a line, since it will be rotating for tuning purposes. Just to repeat myself, the length of the 3/4" pipe should be around 70% of the length of the can, with the 1/2" the other 30%, figuring the lowest frequency you will be using; fully extending the cap, you should be able to touch the bottom of the can (not yet soldered one) with enough rod on top to turn from. Play around to come up with good plunger length for you. Shove the rod's end of the 1/2" "plunger" into the adaptor, up into the 3/4" and out the hole on the top of the can. Rotate the 1/2" pipe and if you have a lot of resistance the rod might not be straight. In that event just reheat the bottom of it, at the 1/2" cap and it should straighten out and cool again. Thread a nut on the top end (outside can) of the rod, running it down to the top of the can and solder it there. This will help you in the next step. Now solder the 3/4" pipe edge to the inside can top. I know this sounds difficult. I use a Weller solder station with the big tip, which has enough "ufth". Another way you could pre-heat the top of the can with a torch. If you have some common sense do this in an appropriate place with someone else to help you and don't burn down the place !!

Install the connector(s) for each can. I have been lazy and used the "UHF" connectors. The market is flooded with them. They are not a good connector, but they are cheap. Lately I'm trying to wean myself off them onto BNC or "N", which are true "UHF" rated and more.....

Take some round brass tubing, you can get a the RC hobby store, say around 3/16 - 1/4" in diameter. Flatten it out, hammer, vice or whatever. I usually make my coupling loops in the shape of a "P", with the face of it towards the 3/4" pipe. The face is around 3/4" long, then 1" across the top, then around 2" for the tail. The face/chin of the "P" is soldered to the connector center and the tail soldered to the can. The distance between the "P" face and the 3/4" is critical, and needs some trial and error. I usually end of around 1/8" spacing. The distance changes the coupling (and the "Q"=notch depth) and the loss as a compromise. Maybe you can find a commercially made can and take it apart to see; one that's banged up and no good to the 2-way shop (they don't have the time to do what us hams do). Start with a notch only can for the basics.

This should be about time to solder the additional can sections (if used) together. After the can has cooled and hopefully, you did not start a fire, take the plastic top, cut around the edges so it will fit tightly up inside the can. Cut a hole for the 3/4" pipe and slide it up inside the can for a stabilizer for the pipe. Make the hole a little smaller than the pipe so it's a tight fit. The 1/2" plunger should be not going outside the bottom edge. If it is,turn the rod "up" some. Now solder the bottom on. It's probably not necessary, but I like to get a complete seal. To keep the big horse flies out, right ? These are those big, hairy, horse flys that attack you as you open the repeater building in some hot summer day ! And some of them bite !

As an afterthought, you can mount some knobs on the ends of the rod to help you turn them. Another trick I do is solder another brass nut on the end and use a nut driver to tune them. This is a low-tech can, so expect some back-lash. You can flex the rod/can up and down with some pressure to see if it's "center" tuned. I have ran these cans from hot summer to well below freezing with no apparent change in frequency. Of course, the trick is to have enough notch to allow some headroom for detuning. Therefore, if you receiver needs 45 db of notch, go another 10 db.


Tune the cavity like any other, whether notching or passing. What? you don't have a spec-sweeper?? Don't sweat; use you friends! Seriously, you can use a signal generator (please don't kur-chunk your local repeater for a signal source!!) and a rig for the receiver, either max or nulling the appropriate pass/notch configuration. At this time on a duplexer, I can only add that you will be making up a harness, 1/4 wavelengths sections. For a high pass, low notch install a 1/4 wave section of coax between the cavity and the "tee" For the other way, no section, just the "tee" on the cavity. Keep in mind to do the 1/4 wave OPEN end test to find the exact velocity constant with the cable you're going to use. Most RG214 is 66%. Also I've seen it vary from batch to batch, while building collinear antennas. Opps! that's another subject.

I'm assuming you are a smart person and can improve something, in the event I left something out.

This article was last updated on June 1, 2005

If you have a fast connection you can click on the pictures for a view of the UHF version, with the bottom open (one can).

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