Monday, July 2, 2012

Simple Antennas for the Amateur Radio Operator

KH6JRM's Amateur Radio Blog

A simple 40 and 15 meter antenna you can build in just a few minutes

Sometimes it is possible to operate two or more bands with one antenna and a short run of good quality coaxial cable.  A simple 40 and 15 meter antenna, used either as a dipole or as an inverted vee, will provide hours of enjoyable contacts at modest power levels.

I've built several variants of this antenna, with the inverted vee configuration preferred because of my limited backyard space.  An antenna cut for 40 meter operation can be used on  15 meters because dipoles have harmonic resonances at odd multiples of their resonant frequencies.  Because 21 MHz is the third harmonic of 7 MHz, a simple 40 meter antenna (approximately 33 feet on each side of a center connector) can be used for both 15 and 40 meters.  There is one drawback to this wonderful plan.  The idea works if you cut the 40 meter dipole for use in the cw portion of the band, for example around 7.010 MHz.  As you move higher in the 40 meter band, the third harmonic will fall outside of the band.  How do you correct this problem?

According to an article on page 9-20 of the "Hints and Kinks for the Radio Amateur, 16th edition (ARRL publication),"  the use of capacitance "hats" will lower the antenna's resonant frequency of 15 meters without seriously affecting resonance on 40 meters.  The article states, "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 hte ends of each piece together in the middle to create two figure 8s.  Solder the twisted centers of your "hat" to each leg of the 40 meter antenna at a point about a third of the way out from the feed point.  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 shuld only see a minor variation."

With this in mind, I quickly assembed some extra wire, a 50-foot length of RG-6 with the proper connetors, and fed the 40 meter antenna through my trusty Drake MN-4.  Then, I made some crude loops out of #14 house wire and attached the loops as instructed.  SWR was acceptable on cw and SSB portions of both bands.  I strung this antenna as an inverted vee.  My SWR was between 1.6 to 1.8 to 1 on both 15 and 40 meters.  With proper prunning, I probably could get the SWR down a bit lower. But for now, this two-band vee works great!  You could also add a 20 meter section to the "array" with those wires running at right angles (90 degrees) to the 40 meter elements.

The antenna is cheap, easy to raise and lower, and even easier to store in the garage.  One should always have a backup antenna in case your main structure falls  victim to the weather or vandals.  This project would make an excellent antenna for shortwave listeners who prowl the 41 meter band.

Have a good day and stay safe!

Aloha es 73 de KH6JRM--BK29jx15