Silver Brazing for Better Microwave Antennas
For several years, I've been making microwave horn antennas using templates generated by my HDLANT computer program. I hope some of you have found it useful as well. Now I'd like to share a better way of soldering them together.
Recently, I was showing a horn template to Bob, WA1ZJG, a retired machinist. He took his torch and a shiny stick of metal and soldered a seam together with no flux and fuss, then handed me a stick and suggested I try it.
If you've tried soldering horns or other sheets of copper, you've probably found what I have: that lots of flux is needed, the copper oxidizes easily so solder won't stick, the joint should be a perfect fit, and the solder wants to run everywhere except in the joint.
The shiny stick of metal was marked ENGELHARD SILVALOY 15. It solders copper together with no flux, fills gaps, and makes joints stronger than the copper. I've made and tested half-dozen horns for 10 GHz and 5760 with, and also repaired my 10 MHz WWV dipole after a branch fell on it.
I called Engelhard for more information, and got a catalog. The Silvaloy 15 is 15% Silver, 80% Copper, and the rest Phosphorous; no Cadmium or Lead. Silver and copper are good materials for microwaves. The phosphorous probably reduces conductivity, but not as much as alternatives, and horns are low-Q devices so conductivity isn't critical -- plated plastic horns work fine.
The downside of Silvaloy 15 is a higher soldering temperature (Engelhard insists that you call it brazing at the higher temperature). The stuff starts to flow at 1300 degrees F, and is liquid at 1475 F. I've done this with a propane torch but it took a while to get hot. Sears has a nice MAPP gas torch which is much hotter, with a fine tip for the copper seams and a big tip for the waveguide joint. The hotter torch gets the joint up to temperature quickly.
Technique is straightforward: as you heat the joint, the copper changes colors and finally looks very clean -- now the temperature is hot enough. Touch the tip of the Silvaloy stick to the joint and it should flow along the joint and fill it up. Like solder, it only takes a little. When the joint is complete, let it cool until the joint hardens. As it cools, the copper will turn black and ugly. Before it cools completely, I immerse it in water and attack the black stuff with a wire brush - it comes right off at this point. After it cools, the joint is hard and can be touched up with a file.
For materials other than copper, Engelhard says you need flux, but I haven't found it necessary with brass waveguide.
I bought a pound of Silvaloy 15 at a welding supply shop, so I have a supply for several lifetimes. Like all silver materials, it is sold by weight.
As an example, here's a template for a 5760 MHz horn designed to feed an RCA DSS offset dish: