Friday, March 15, 2019

One wire, two (or more) bands: The HF trap dipole

Do you need a basic antenna that works well on a handful of frequencies? Just want a direct coax connection to the transceiver without twiddling an antenna coupler? Or maybe you're short of space and need something that's not quite full size?

If so then a trap dipole might be the answer. It's a dipole that can do multiple bands. How many? I've seen trap dipoles that claim five or six bands. But they get complicated with interaction between wire lengths and the component values in each trap.  Whereas a two band trap dipole is much easier to build. It would make an excellent second antenna project after you've succeeded with a single band dipole.

A two band trap dipole is shown above. Ignore all the parts from each trap outwards. What you're seeing is a regular half wavelength dipole operating on the higher frequency band.

If you were just to add more wire to it then that dipole's resonant frequency would move down. You don't want that. So you place a parallel tuned circuit, called a trap, on each leg. The traps act as  insulator on the higher frequency band. Then you can add more wire on the outside of each trap without spoiling performance on the higher frequency band.

Like all tuned circuits, the trap's behaviour changes with frequency. It no longer acts as an insulator on lower frequencies. If you get the value of the trap right, along with the length of the outer portion of the antenna element then the antenna will also resonate on a lower frequency. All while still operating perfectly on the higher frequency.

Another reason why you might use a trapped antenna is that because the inductor in the trap operates as a loading coil it allows the antenna to be shorter end-to-end than a full sized dipole. This entails a performance compromise including narrower bandwidth than a full-length dipole. However it may be the only way you can operate on a particular band if space is limited. Also, if you only operate CW or digital modes, the narrow bandwidth probably won't be a limitation.

You don't necessarily have to have your trap resonant inside an amateur band. Some designs specifically have traps resonant outside it. And with careful design it's possible to get more than two bands from one pair of traps.

How do you make a trap? There's several ways. You can buy them pre-wound so you don't have to. A common approach is to wind the required inductance on some plastic pipe. The capacitor required can be put inside the pipe. The capacitor needs to be a low loss high voltage type for best performance and power handling. Other trap designs use coaxial cable for both the inductor and capacitor.  Very small traps, such as used for portable QRP station antennas use iron powder toroids for the inductors. You'll find ideas in the links below.

* Online trap dipole coil calculator
* W8NX 1.8, 3.5 and 7 MHz trap dipole
* NU3E attic multiband trap dipole
* VK1HW 3.5 and 7 MHz trap dipole
* K0BXB 7 and 14 MHz trap inverted vee
* Trap dipole for 7 and 14 MHz (video)
* AD5X 18 and 24 MHz trap dipole (pdf)

PS: More antennas are covered in Hand-carried QRP Antennas. Available in electronic and paperback form (some countries) this well-reviewed book is a popular read amongst hams who go portable.

No comments:

Post a Comment

Upgraded website

Enjoy reading about diverse facets of amateur radio? Like building projects? Sometimes find my videos hard to find? If any of these applies ...