Technical Articles

Antenna Traps—A Way to Cope With Limited Space

The downsizing of real estate these days reminds me of Halloween candy. Candy bars used to be big, but now they’ve shrunk to something they call “fun size.” Today’s properties have met the same fate–they’re often too small to satisfy a ham’s hunger for a decent antenna farm.

So how can you squeeze multiband HF antennas into a limited space? Choose antennas with parallel tuned circuits, known as traps. Over the years a number of multiband trap dipoles, Yagis, and verticals have been available for ham radio communication. Just think–you could be operating 80 meters on a relatively small lot.

How Do They Work?

Traps are one way to get multiband coverage without the need for an ATU. They act like a switch, electrically cutting off the rest of the antenna at the trap’s design frequency and functioning as a loading coil below the antenna’s resonant frequency. There may be several traps working together, allowing an antenna to be resonant on multiple frequencies.

For example, the Hustler 4BTV vertical antenna has three traps–10M, 15M, and 20M–with a capacity hat and tubing extension to add 40M coverage for a total of four bands.

When you’re operating on 10 meters, the 10M trap exhibits a high impedance and electrically cuts off the rest of the antenna. At 15M, the 10M trap becomes a loading coil, shortening the next section of antenna to the 15M trap. This now shows a high impedance and cuts off the next section and so on. When operating on 40 meters, all three of the traps are added into the system, along with the capacity hat and additional tubing.

Due to the loading effect of the traps, the 40-meter portion of the antenna will be somewhat shorter than a full-sized 40-meter vertical without traps. Total height of the 4BTV is only 19 feet, compared to 33 feet for a full-sized 40M vertical.

As you can see, adding traps shortens the overall length of an antenna significantly, especially if more than one set of traps is used. An antenna with traps only needs a single feedline and can cover multiple frequencies or bands. However, there are tradeoffs. Effective bandwidth on each band will be narrower than that of a standard quarter-wave vertical.

Dipole and Yagi

When it comes to traps, dipoles work on the same principle as described for the 4BTV vertical. The difference is they use pairs of traps, one on each leg of the dipole, with additional pairs added for multiple frequency coverage. The formula is always number of traps = number of bands –1.

Trap-style antennas are not quite as efficient as a full-size dipole, but if the traps are well designed, the losses are not significant. Most hams consider the losses a fair tradeoff for the convenience of having a shorter antenna that presents a 50-ohm match to coax on several bands.

A variation of the trap dipole is manufactured by Alpha Delta. Their design doesn’t use traps in the conventional sense. The DX-DD 80M/40M antenna manual describes the loading coils as ISO-RES, which appear to be wire inductors made from enamel-coated magnet wire. Operational details of the coils in the circuit are not detailed in the manual, but it does claim that they are not traps. The result is essentially the same as the conventional trap, with a compact 82 foot total wingspan that’s 33% shorter than a full-sized 80M dipole.

Using coils like this for multiband dipole antennas is an idea that has been around for a while. Alpha Delta has adopted this design for its 80M/40M antenna and two of its sloper models. Alpha Delta also has a 160M/80M/40M version, the DX-LB. The Diamond W8010 also uses inductors rather than traditional traps.

Besides the conventional wire trap dipole, there are some interesting variations that fit in small spaces. Comet H-422 and Diamond Antenna HFV-5  rotatable trapped dipole antennas can be used as a horizontal dipole or in a “V” formation to save space. Like the Cushcraft D4, these are rigid antennas constructed with aluminum tubing. All can be turned to null out noise and QRM, and to optimize the radiation pattern.

Yagi antennas consist of a dipole and additional parasitic elements. The dipole in the array is driven, and another slightly longer element operates as a reflector. Other shorter parasitic elements can be added in front of the dipole as directors. This arrangement gives the antenna directionality that a conventional dipole lacks.

On triband Yagi antennas like the Cushcraft A3S or Hy-Gain TH-3MK4, each element has two pairs of traps located near the ends. The first trap follows the 10M section, the next follows the 15M section, with the 20M section as the remaining part of the element.

On the A3S Yagi above, the traps block RF at a specific frequency or allow it to pass. From an electrical standpoint, this effectively lengthens or shortens the antenna.

Lossy Traps?

Traditionally, traps have been considered lossy devices, and there are those who believe they should be avoided at all costs. Do traps create some loss?  Yes, but not as extreme as some would like to make you believe; in the real world you’re talking about fractions of a dB. Consider that ATUs are also lossy devices and the correct use of traps will eliminate the use of an ATU. Feedline is also lossy, but you can control the amount of loss with your choice of coax or ladder line.

Don’t worry about traps–they are, for the most, part efficient if designed and built correctly. Traps are similar to loading coils (with a capacitor added), and loading coils are used on virtually every HF mobile antenna and some vertical antennas, but nobody complains about them in these applications. There’s a lot of anti-trap propaganda out there. Don’t let it sway you if you’re considering a multiband antenna that fits perfectly in your yard and allows you to use additional bands that you weren’t able to use before.

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