By Howard E. Cann, Jr., KA3MRX
Tuners are a
compromise to fool your radio into thinking it is seeing a resonant antenna. It can be an
inefficient way of working radio, since some of your precious power is being wasted in the
tuner as heat. The amount varies, depending on the antenna and the frequency. Although
there may not be a great amount wasted, you will want all the power to get to the antenna
and out into the airwaves as you intend
Certainly, there are times and places when the use of an antenna tuner is necessary. At a portable location of mobile operation an antenna tuner could be very helpful, but for the ultimate day to day operation, a resonant antenna performs simpler and better.
The all-band vertical is an antenna preferred by many amateurs. It's main advantage is that it requires little space and the installation is quite simple. The main disadvantages are its cost, lack of gain, and the need for ground radial wires to operate properly (in spite of the marketing claims of "radials not required"). I notice that many of the amateurs who use the all-band vertical seem to abandon them in favor of other systems. That could be because of their experimental instincts and not because of the "poor" vertical. The manufactured vertical provides little outlet for our experimental instincts. I also do not think that the saying "A vertical radiates equally poorly in all directions" is a fair statement about verticals. I have worked many vertical long distance operators with tremendous signals.
Let us look at the common dipole, something all of us can build if we have the space. If we don't have the space, maybe we can borrow, steal or sneak it! Its advantages are that it is cheap, easy to build, parts are easy to find and it works best.
The dipole matches most radios and feed lines. The impedance of the dipole is 50 ohms. Our radios are designed for 50 ohms. Also it is favorable that 50 ohm coax provides perfect matching connections between radio and antenna. Now vertical antennas are 20-30 ohms, loops are 100 ohms and inverted vees are 75 ohms. They may vary slightly from this depending on height, other adjacent materials and ground characteristics.
Baluns are matching devices that are used to take care of these differences. However, many who have used baluns have had problems and discarded them. The dipole is the simple, direct and trouble free solution.
THE HALF-WAVE DIPOLE
The half wave dipole will cover about one band with SWR less than 2 to 1. Except that on the 75-80 meter band (3.5 MHz to 4 MHz) the SWR will be more than 2:1 in some parts of the band. But by calculating for one frequency in the center of your phone band and another in the CW band, then cutting one half of the antenna for the phone frequency and the other for the CW frequency, the SWR is acceptable throughout the end of the entire band.
After doing the rough cutting and connections, raise the antenna and check the SWR. If the SWR is not correct, lower the antenna and either shorten the antenna by pulling more wire through the insulator and rewrapping it onto itself or letting out more wire. By checking the SWR at several points in the band you can determine if the wires need to be shortened or lengthened for minimum SWR.
Now with the single band dipole properly adjusted, we are ready to create a multi-band antenna out of the 75-80 meter dipole. Calculate the length of a 20 meter dipole, then simply attach the wires to the center insulator of the 75-80 meter dipole as shown in the drawing. Check the SWR and adjust if necessary. It is possible to tie the 20 meter dipole insulators on the 75-80 meter dipole and let the 20 meter drop slightly below the 75-80 elements. By tying them elsewhere there is less interference and less chance of tangles caused by the wind. Some hams make spacers out of acrylic plastic to prevent tangles. With a tuner, this antenna will work all bands.
Next, build another dipole for 40 meters and add to it one at a time, dipoles for 15 meters and ten meters. Take time as you go and test each dipole as you go. Don't try to build them all at once, since tuning will be difficult with the interactions of the various elements. Now with two antennas and two feed lines you will be able to work all bands without a tuner. An antenna switch could be helpful, but it is not too much effort to change coax if they are well marked.
KA3NIL, Bob Cappers of Princess Anne Md., has been faithfully using the no tuner antenna system for three years now. It is installed about 20 feet above sea level and Bob normally runs about 20 watts out of his Icom 730. He has worked over 100 countries and has the 73 Dynasty award in hand.
#14 insulated single strand copper wire in any color desired can be purchased at local electrical supply stores for around $15.00 per 500 ft. roll. It will make many dipoles and serve as grounding wire too. Here on the coast, crab pot rope is best and cheap for hanging up dipoles. It is made of nylon and lasts better than the clothesline or polyprolene rope.
Insulators can be purchased for 50 cents to a dollar and look nice, but other materials will work. Glass jug tops, acrylic pieces, PVC pipe or even 100 lb. test monofilament fishing line will work.
An amateur can still have fun building things. Maybe radios have gotten too complicated to build and parts hard to find. But antennas are still waiting to be built with easily available materials. There is no reason to settle for manufactured products and miss all the fun. So have fun. -30-
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Last modified: December 31, 2010