Before we get too far into a discussion of what metals *can* be used to fabricate a duplexer, on 6m or on any other band, we should perhaps consider what metals *should* be used for that purpose.
If we examine the basic differences between cheap duplexers that drift all over the place and become noisy in a short time, and expensive duplexers that are rock-stable and noise-free for years, we find that they boil down to just a few: Materials used in construction and workmanship. An ideal duplexer is made of materials with complementary coefficients of expansion, meaning that as the outside body of the cavity expands and contracts with temperature changes, the inner elements move in such a way that the tuning is not affected to any significant degree. Before you conclude that your duplexer doesn't need any fancy temperature compensation because it is in a temperature-controlled radio room, think again- ambient temperature is only part of the picture. Let's suppose that you have a 125 watt transmitter feeding a six-cavity duplexer that has a typical insertion loss of 2.2 dB. That power loss of 2.2 dB equates to about 50 watts that is consumed as heat in your duplexer, and you can bet that the duplexer components will start moving on long transmissions. If a poorly-made duplexer is subjected to constant cycles of heating and cooling, the contact surfaces will wear and sparking will occur since the movement is happening while transmit power is applied. High-quality duplexers have beryllium contact fingers working on a silver plating. The interior of the cavity may also be silver-plated to increase the efficiency due to skin effect. Cheap duplexers may have simple friction contacts made of copper or brass. So, yes, it is possible to make duplexers from almost any metal that can be brazed, welded, or soldered. However, combining diverse materials to make a duplexer that is effective, efficient, and stable can be a challenge. YMMV... 73, Eric Lemmon WB6FLY
