Thursday, July 8, 2010

Is it true that you can use a 40-meter dipole antenna on 15 meters as well?

Q Is it true that you can use a 40-meter dipole antenna on 15 meters as well?

A In many cases, yes. Dipoles have harmonic resonances at odd multiples of their fundamental resonant frequencies. Because 21 MHz is the third harmonic of 7 MHz, 7-MHz dipoles are harmonically resonant in at least a portion of 15 meters. This is attractive because it allows you to install a 40-meter dipole, feed it with coax, and use it without an antenna tuner on both 40 and 15 meters.

There is a catch, though. This idea works if you cut the 40-meter dipole for use in the CW portion of the band, say around 7010 kHz. Such a dipole should also be resonant in the CW portion of 15 meters at about 21030 kHz (7010 x 3 = 21030). But what if the 40-meter dipole is cut for phone work at 7250 kHz?

7250 × 3 = 21,750

Oops! The 15-meter resonant frequency is out of the band. The solution is to capacitively load the antenna. The simple loading wires known as capacitive hats shown in Figure 3 lower the antenna’s resonant frequency on 15 meters without substantially affecting resonance on 40 meters.

To put this loading scheme to work, first measure, cut and adjust the 40-meter dipole to resonance at your desired frequency. Then, cut two 2-foot long pieces of stiff wire (such as #12 or #14 house wiring) and solder the ends of each piece together to form two loops. Now twist the loops in the middle to create two figure 8s. Solder the twisted centers of your figure 8s to each leg of your 40-meter antenna at a point about a third of the way out from the feed point (placement isn’t critical). Adjust the loop shapes and take measurements on 15 meters until you reach an acceptable SWR on your chosen frequency. When you check the SWR on 40 meters you should only see a minor variation.

Figure 3—Figure-8-shaped capacitance hats made and placed as described in the text and make your 40-meter dipole antenna resonate anywhere in the 15-meter band.

From QST February 2000