I love building. The biggest reward in creating a circuit is watching it work when you fire it up. One of my first projects, when I was first licensed in 1975, was to build a 3 band trapped diploe. I used molded mica transmitting silver mica capacitors and B&W coil stock. Once the traps were constructed, and checked with a grid dip meter, they were ready for installation into the wire antenna. Through the years, ceramic doorknobs became more common place than the old micas, I suppose due to availability and not form, fit and function. The mica was and always will be a superior capacitor for use in and outdoor antenna trap. That 20-40-80 meter dipole worked faithfully for many years.
Molded silver micas are made with a hermetically sealed envelope, impervious to weather of all sorts where and the doorknobs are made from porous ceramic. They don’t hold up to the Midwest elements like snow, ice, water, freezing and thawing. Eventually they pop apart. The mica is great for small traps made for a 200 watt or less transmitter and will last a long time. Micas are made both with solder terminals and screw terminals. I prefer the screw type. They have 6-32 solid brass tapped screw holes and can be soldered if you insist and a tight mechanical connection is made. The solder joint is also subject to weather over time. Micas are also available in literally hundreds of values allowing latitude in design and the ability to change values sliding the resonant frequency of the trap a little if need be. It is not uncommon to parallel molded micas as well. Micas are 30%-60% less than the current cost of ceramic doorknobs. RF current handling is greater for the silver mica.
Too many new hams take the easy route of a random wire and an antenna tuner. Live a little. Build traps. If you can’t find coil stock, wind your own inductor on 3” thin wall PVC. Used 10 gauge enameled wire and then urethane the winding in place once its wound. We also have ceramic insulators you can run through the middle to take the strain off of the trap. There lots of construction stories on how to build traps. Try the Radio Amateur’s Handbook for example.
I made these traps early in my ham career. Conveniently, the same year Jim and I started Surplus Sales. So, for the next 40+ years of buying parts for the surplus business, I always kept an eye out for silver micas. Handy little buggers. They have also been a great seller through the years. We have not raised prices on these for decades. And to make matters even more tantalizing for the reader contemplating building an old fashioned trapped dipole, I acquired another 500 pounds or so of molded silver mica caps to add to stock. Not much chance we will run out anytime soon! Check out our web page for micas and keep an eye on it as it expands very soon! Fistell Electronic’s inventory has only had the surface scratched but we just found a pallet of micas during our move to the new building.