L match antenna tuning unit for 80 40 and 20 meters

as biult with found items
Ralph Klimek Sept 2007

The roller inductor was purchased at a hamfest about 10 tens ago and was mouldering away at the bottom of my junk box. It was originally a double roller that came from some fixed frequency Collins transmitter of  some sort sort.  I cut the double assembly apart and added some extra hardware to make it a standalone unit. There was no provision for an external drive shaft so I drilled it and glued a fibre glass rod to serve as a drive shaft. It was originally designed for screw driver adjustment only. There were no proper bearings so the mechanical arrangement exists so as to remove as much mechanical stress on the bakelite on bakelite journal bearing. The flexible coupler was required because lacking a lathe, my centering on the drive shaft is less than concentric.

An excellent source of fiberglass rod material is the inner strength element from heavy fibre optic cables. This material is also excellent for use as a contact cleaner or spot polishing tool, indeed the local electronics shop sells just such a tool which is nothing more than a short length of this material.

The variable capacitor is one I picked up from a mystery piece of WW2 vintage radio equipment from a hard rubbish collection over thirty years ago. It has been in assorted projects of mine that came and went but I suspect that it has found its final home in this equipment.

This ATU is put in a mystery die-cast box, another castoff.  The business with all the copper sheet is motivated by the cylinderical roller inductor. It wants to induce a current that would otherwise circulate in a very lossy path, through the die-cast metal (zinc+rubbish), and through a dubious lid connection. If the circulating current had a nice low impedance copper path, that would mean extra precious Watts would make it to the antenna. The copper sheet was recovered from a dead copper header tank, otherwise copper sheet is now prohibitively costly.

There is an extra refinement, I did not want heavy RF circulating currents to go through the shaft wiper and bearings of the variable capacitor, so the entire capacitor frame floats and the connection is made by a flexible copper strap directly to the shaft, so there is no sliding contact. Sliding contacts are unavoidable in a roller inductor but at least the sliding contacts are precious metal plated.

The flexible shaft couplers are made from Unobtainium and this project exhausted a lifetime collection stock of them!

A good source of copper sheet could be the long dead dead electric hot water service in your roofspace. Hobby shops can sell you small copper sheets, which at the asking price is weight for weight more expensive than pure Silver!

The bearings for the rotating elements are made from brass angle stock.  In general, I now find that when building RF projects into aluminuim and especially die-cast boxes, the ground areas under coax connectors and any elements that should be grounded to a low impedance RF ground ; should be soldered to copper flashing. You cannot trust a ground to zinc or pot metal die-cast material. Notice what is underneath the BNC connectors.  It does make a differance.

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