Simply Half-Wave Trapped Antennas Part 3 - Constructing Traps

Constructing Traps
It goes without saying that almost anything can be used as a coil form for the inductor in a trap from air to iron. Classic traps in the days of yore was one made out of the Air Dux™ coil stock (shown below) where one sawed off a hunk for the correct inductance.

Of course, wood dowels, PVC, pill or other plastic bottles -- even card board tubes -- can be used. Take, for instance, this trap made out of a toilet paper tube, a hunk of wire, and a couple of 27 pF silver mica capacitors in series. Obviously, it will not stand up to a cat 5 hurricane -- or even a moderate drizzle -- but it can be "hardened" and weather-proofed and/or hidden in an attic where it will serve duty. (I Would not run more than 100 watts through it unless you have your track shoes on and your homeowner's insurance paid up.)

Also, there's no law that says you have to have a round coil either. A piece of wood with a square profile would work just fine. The take-away here is to use your imagination and see what you have on hand. My preferred material is PVC and generally in 1-inch to 2-inch diameters. The material is rugged, weather-proof, and easy to work with.

Here are a pair of traps I fashioned with some PVC, a 27 pF silver mica cap, some screws, and solder lugs:

The bottom line on constructing traps is the more perusal and experimentation you do, the better your antenna building kills will become. I did a post before this series with a bunch of pictures of traps I built for a vertical antenna. You are invited to peruse those -- as the rest of the blog if you like.

Testing Traps
Once you build your traps you will need to test them; measure their resonant frequencies. I like to measure each component before I assemble the trap and then measure the resonant frequency of the resulting trap. As you will find out, components are not always dead on with respect to their marked value nor are the coils you wind going to exhibit the calculated inductance. Consequently, the resulting resonant frequency will not be what you calculate either. But not to worry. All you need to do is get things "close enough for government work".

First, let me digress and recommend you take a quick gander at this as a hint on tools you should be considering if you would like to progress further away from being an appliance operator.

All of that said about the "nice-to-own" tools, here's some notes and comments on ways to measure the resonant frequencies of your traps. First up is a video by yet another ham who does a great series on how-to projects, Peter Parker, VK3YE. It pretty much describes a way to use your radio and a broadband antenna (i.e., a long length of wire) to roughly determine the traps. It is fairly self-explanatory, illustrates a "cheap method" of doing this since you already have your HF radio, and it follows through with how Peter calculated the values necessary to achieve resonance at the desired frequency. Finally, he shows the resulting traps which -- along with my pictures and pictures of other traps on the internet -- should give you soem good ideas how to fabricate yours.

Unfortunately, he flashes by a piece of information from W8KI's website on traps that speaks to the desirability of component traps versus coaxial traps:

- Trap loss greatly exaggerated by advertising hype
- Traps should not be resonant at the actual planned operating frequency
- Coaxial traps are more lossy than articles conclude
- Coaxial stubs used as capacitors cannot be calculated using pF/foot unless the stub is a very small - fraction of a wavelength long
- Coaxial stubs have a low Q (are relatively lossy) compared to normal lumped components

The actual website is here and it is definitely worth a glance as he develops this theme with calculations. The bottom line is that you don't sacrifice anything by building traps with discreet components versus pieces of coax, the busy-body naggers notwithstanding.

In the link above where I made the recommendations as to the inexpensive test equipment you should avail yourselves of, I mentioned an RX noise bridge. These can be used in conjunction with the receiver on your HF rig to measure the resonant frequency and a good explanation on how to build one as well as use it can be found here. Note that the noise bridge will substitute for Peter's broadband antenna and provide you the antenna radiation resistance as well as the inductive or capacitive reactance.

There is a great article on an antenna noise bridge detector here.

..and another one here.

By this time you should have a couple of traps resonant just below the highest band of your dual-band antenna. (That is, around 13.5 MHz for a 20m/40m antenna.) Next we'll take up building the antenna itself. But rest assured, the toughest part of the job is done.