HIGH PERFORMANCE WIRE ANTENNAS

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Figure 1 is a treasure map. Contained within this diagram is one of Mother Nature's important secrets. The mystery of wonderfully loud signals emanating from insignificant looking antennas strung between two cooperative trees is explained in this simple chart. This map to the "Treasure Island" of incredible antenna performance was lifted from the 14th edition of the ARRL Antenna Book, page 7-1.

Popularity doesn't guarantee high antenna performance -

The all time favorite wire antenna is the half-wavelength, center-fed, dipole. It is easy to build and it performs adequately. The dipole is the reference used to measure the performance of the other antennas. It's the base line. The dipole is a suitable reference for amateur radio purposes because you can actually build one. In contrast, all the antenna patterns used in this publication are referenced against an 'isotropic antenna.' The results are expressed in dBi.


Isotropic Antenna


An Isotropic Antenna is a hypothetical antenna radiating or receiving equally in all directions. Such antennas do not exist physically, but represent convenient reference antennas for expressing directive properties of actual antennas.

One band just isn't enough, most hams want to be operational on as many bands as possible. This requires a separate dipole for each band. Since we now have eight HF bands, that means that eight different coax fed dipoles are required to adequately cover all the bands from 80 through 10 meters. Eight separate antennas is unacceptable to the average Ham. Most of us want 80 through 10 meters coverage with a single antenna.

There are several ways to achieve this goal of a single antenna for 80 - 10 meters. The most popular technique for achieving multiband operation with a single antenna is to divide the antenna into appropriate lengths with 'traps.' Traps act like electronic switches, which isolate the correct portions of the antenna wire to achieve half-wavelength resonance within each desired band. A five band trap antenna can use as few as two traps or as many as eight. It all depends the design. In general, the more traps, the better behaved the antenna is in terms of operating bandwidth and acceptable SWR.

There is a high price to pay for the convenience of traps. First, trap antennas are usually expensive. Second, there is a loss of operating bandwidth if a low SWR is a requirement. In addition to the bandwidth loss, there may be a slight loss in efficiency caused by the traps themselves. This loss may not be significant if the traps are well designed.

Disadvantages of trap antennas

Advantages of trap antennas

1. Dipole performance at best 1. No tuner required, if you are willing to operate within narrow portions of the bands.
2. Trap antennas often require trimming 2. Slightly shorter than full size dipoles
3. Traps fail 3. One antenna can cover many bands
4. High cost of traps
5. Hard to troubleshoot
6. Detuning effects of nearby objects
7. They are heavy and often difficult to support

It is my argument that trap antennas are not an efficient use of available antenna space.

If you put up a 120' long 80 - 10 meter trap antenna, why use only 33' of it on 20 meters? There are significant advantages to using the full antenna length on all bands. Let me show you some interesting data.

Figure 3 is the radiation pattern of and 'Extended Double Zepp' or 'EDZ.' An EDZ' built for 20 meters is about 85' long. Notice the difference in the pattern of this antenna and the curve for the dipole in figure 2. The narrowing of the lobes concentrates the energy radiated by the antenna. Concentrating energy is another way of saying the antenna develops gain. The Extended Double-Zepp produces between two and three dB gain. A 3 dB gain is the equivalent of doubling your output power. Note the four 'minor lobes.' Operation is not limited to the direction of the major lobes.

The point of comparing these two curves is to show that it is possible to build better antennas than dipoles. This is not the end to the story. In fact, let's go back to the beginning. Figure 1 gives us the most important reason for using the full length of 80 or 40 meter antennas on higher bands where antenna length becomes long.

Notice the curve, marked "gain." It rises as the length of the antenna increases beyond ½ wavelength. These gain figures are not large. For example, on 10 meters, the gain of an 80 meter dipole will be only 3 dB. But, that's three dB you wouldn't have if you were using a dipole. It's the same as doubling your power. Remember, every dB counts. 3 dB here, another 3 dB, they add up to a big signal. Figure 1 proves a very important point. Put up a lot of wire, get a lot of gain. It's that simple. Combine the advantage of a long antenna with techniques that lower the radiation angle and the result is unbeatable. This is the secret of high performance wire antennas. It's not magic, it just taking advantage of physical laws.

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The RADIO WORKS specializes in high performance wire antennas that take advantage of these principles. Take a close look at the CAROLINA WINDOM®, CAROLINAWINDOM Short®, CAROLINA BEAM™, SuperLoop™, VRD™ (Vertically Radiating Dipole™) and G5RV Plus.

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