Simple Half-Wave Trapped Antennas Part 1 - The Basics

I was asked by a fellow ham to make a "how to" tutorial available on trap dipoles. A Power Point summary was suggested but I prefer to do a fuller, more complete explanation and, to that end, I am creating some posts on my blog. It is loosely aimed at the following objectives:

(1) Understanding the principles behind a trap dipole antenna without the concomitant EE degree math.

(2) Fabricating a trap antenna for the constructionally impaired from [mostly] ordinary "household" items.

I will probably break this up into four posts on the blog -- this one on the basics, a subsequent one on trap theory and calculations, a third on building traps, and a final one on putting the whole antenna together.

So let's charge right into it, shall we?

A trap [half-wave] dipole antenna is a device that will be resonant on two or more bands by means of one or more pairs of parallel resonant LC "tank circuits". Such a circuit is an inductance and a capacitance hooked together in parallel that exhibits the property of presenting an infinite -- or almost infinite impedance -- at its resonant frequencies.

But let's back up a minute and refresh our memories of dipole antennas. Sparing you the 1,000 words, here's a picture:

If you need a refresher on the derivation of the half-wave antenna formula, you can go here which I heartily recommend doing as it even has a video explanation of dipole antennas. Also, you will see that these antennas are fed with 72-ohm coaxial cable in the literature but, fret not, your 50-ohm coax will be fine for the moment. (But, down the road, you will come to realize that the RF loss of even the best coax is a mega-bummer if you like QRP.)

So, for the sake of argument, we would like to have a half-wave dipole for 40 meters. Hence:

Also, we wish to have a half-wave dipole for 20 meters. similarly:

And, while half-wave dipole antenna calculators abound on the internet, the above was derived from a site put up by West Mountain Radio who also went ahead and published the computations for all of the amateur HF bands, below.

So, armed with these, see that a 40 meter antenna is roughly twice as long as a 20 meter antenna and that we could probably use one antenna on both bands or actually terminate our coax into an antenna with BOTH 40m and 20m elements. And, while there is nothing wrong with either solution, there are drawbacks to each approach. (There are, in fact, problems with trap dipole antennas as well. Hey, nobody's perfect! )

Using the 20m band on both 20m and 40m will cause mismatch problems and compromise the efficiency of the 40m operation a great deal. Conversely, using a 40m antenna on 20m does not work out so well on 20m as it isn't a resonant match on that band. Still in all, it works far better than using the 20m antenna for 40m.

But I prattle.

The other alternative -- feeding a 20m and 40m dipole form one coax -- is called a "fan dipole" (for obvious reasons) and actually is better than a trap dipole but may have resonance or deployment problems. For example the multi-band fan dipole shown below is a stone bitch to deploy and tune without getting all of the bands set up so they don't "de-tune" adjacent bands.

In fact, I live in "CC&R hell" and cannot have the t*ri-band Yagi on the 70 foot tower we all lust after so a fan dipole in the attic had to do. Fortunately, I was able to lay it out so 40m, 30m, 20m, and 10m played nicely off of one coax but some re-arrangement was necessary. However, in the sense that one coax line and one run of wire can be deployed meets our needs most easily, then the question arises as to how this can be accomplished.

We could, of course, put up a run of 66 feet -- 33 feet on a side -- and put a switch in the middle of each side. When we wanted to work 20m, we'd just disconnect the extra 16 feet by opening the switch and Bob's your uncle, as the Brits say. Of course, this would become a monumental pain in the ass getting out the ladder or crawling up in the attic each time we wanted to change bands.

So what could we do? What if we had a magic device that would shut off the extra 16 feet when we worked 20m but did not when we worked 40m?

This is where traps come in.

It is a characteristic of a parallel LC circuit that, at resonance, impedance is almost infinite. Not to go into the hairy math in this discussion (it is discussed here) we can see that this is as good as the switch we wished to employ.

For example, if we configured our antenna as shown in the picture below, we would have the single run radiating element we desired and, when we transmitted on 20m the parallel resonant traps would offer an [almost] infinite impedance and the antenna would appear to be electrically resonant on 20m. Of course, the traps would not be resonant and the entire antenna length would be employed and, hence, resonant on 40m.

There's an added benefit to this as well. The inductor in the traps serve ot electrically add to the length of the antenna so therefore the overall length required for 40m is reduced by about 65 percent.

"Great", you say, "When do we start?"

..Well, that's for the next post on building traps.