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LOW TECH RULES OK!
GOT QRM ON 2 METRES?? TRY THIS:≠

Hereís a low tech way to fight the pagers. It assumes you can lay your hands on a large soldering iron or a gas torch. Itís a resonant cavity which peaks the 2m signal and cuts the rubbish down to size.

First you need a large can which must be tinplate so it can be soldered and it should NOT be plastic lined. I have used successfully a home brew beer kit can measuring 100mm diameter and 175mm long and one of similar size which originally contained coffee. Other large sized cans may be OK. Now you need a length of copper water pipe of 15mm diameter, two co-ax sockets, two short pieces of stiff wire and a small variable capacitor of about 30pF. If you can find one of the Jackson 804 series with the rods supporting the fixed vanes sticking out at the back this makes for easy assembly.

Thoroughly clean the can with boiling water to get rid of any residue in the seams and dry it. Using emery paper remove any plastic film from the inside and outside of the bottom of the can and from the top rim. Mark a line across the bottom of the can passing through the centre and continue this line a few centimeters up the sides. At the centre drill and file a hole so that the water pipe is a good push fit. On the line and about 5mm either side of the edge of this hole make a small hole to fit the stiff wire, and about 30mm up each side drill a hole to fit the co-ax sockets. I prefer to use single hole fixing BNC sockets but any type will do. Now thoroughly tin the bottom of the can both inside and outside around the large hole and also tin the top rim. Clean the copper water pipe, tin and wipe both ends of it and solder the fixed vanes of the variable capacitor to one end so that the shaft of the capacitor continues the line of the pipe. Make a good job of these joints as they are critical to the robustness of the finished job.

For the top you can use a piece of tinplate but I prefer fibreglass copper clad board which is easy to work and doesn't have sharp edges. A piece of board rather larger than the can end has a hole drilled at the centre to fit the bush of the capacitor. The copper side is polished ready for soldering later. Fit the capacitor to the board and finger-tighten the nut. Now push the end of the copper pipe down the can and through the hole in the bottom. With the top held in position you can mark off the end of the pipe so that between 5 and 10mm protrude. Cut off the surplus and ensure the pipe is adequately tinned. Now solder the pipe to the outside of the can taking care to keep it running co-axially up the can. Once the outside is well soldered remove the top and reach down with the iron to solder the inside to the pipe. When you are satisfied that there is a good collar both inside and out, you can bend the pieces of stiff wire into ĎLí shapes and solder them between the inner of the co-ax sockets and the small holes in the bottom of the can. Ensure that these pick-up wires are sitting symmetrically and parallel to either side of the pipe. Now it is just a question of replacing the lid and spot soldering it to the top rim in two or three places - you can make a proper job of it later when all your tests are complete. Fit a knob to the capacitor and connect the unit between your SWR meter and a 144MHz antenna.

Adjusting the capacitor should peak an incoming signal very sharply and on transmit there should be an equally sharp point of minimum SWR. Do these tests on low power first. If the filter does not resonate, it probably means your variable capacitor is too small, so add a few pF in parallel. Tests on two of these units have shown SWR better than 1.5:1 and negligible insertion loss.

A Construction Hint: if your can is much bigger than 100mm in diameter use a larger copper pipe.

G3EJF 10/11/1997
QRMcan.gif - 6kb, a very basic sketch of what you are attempting to achieve