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Oii'piit Fanei

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Chassis wave ground plane wires. These can be attached to the vertical using "T" fittings and 45-degree elbows.

ft would be interesting to see some practical miniature beams built using TWAs. The effect of mutual coupling between elements would need to be looked into. However, TWAs would be ideal in constructing the high-performance but mechanically difficult W8JK type two-clement driven beams with theoretical 5.2 dB gain over a dipole. This antenna can be set up based on resonance and symmetry.

Fig, 1 shows a suggested starting point. A typical design lor 20 meters would have element lengths of 10 feet and a boom of 8 to 10 feet. Close-spaced beams like the above put severe demands on loss reduction since the radiation resistance is only a fraction of the radiation resistance of a single dipole. The TWA method is just the ticket. Rough calculations show that it should be possible to hold losses to about 1 dB with some end loading applied. This still leaves a nei gain of 4 dB over a dipoic, with full beam performance in all other aspects. When capacitive end loading is applied. antenna efficiency will increase over calculated values, and would make an easy way of resonating an antenna wound a little too short. End loading can be applied by using plumbing Ts and placing copper foil on new short sections of pipe arranged at right angles to the dipole. These can then be soldered to the helical and adjusted for precise resonance, as shown in the photo for a 5-foot-long 14 MHz resonator. This is an extreme design with only about 50 kHz bandwidth. However because of end loading it does achieve a more uniform current distribution and a re!atively high radiation resistance which is estimated as about 2.5 ohms. Skin loss resistance is calculated as 1 ohm. Estimated efficiency is therefore still an astounding 1\% (about i.2 dB loss against a lossless dipole). Using a 3-inch pipe, losses could be halved but bandwidth would not significantly change.

A second method for enhancing radiation resistance and efficiency is by using wider tape in the middle of the dipole and transitioning to a smaller width toward the ends, This would be particularly effective with light capacitive loading, The cunrem distribution would be flattened and resistive losses would be even lower.

A good rule of thumb is a physical tape length of one wavelength to achieve resonance for a uniformly loaded half-wave dipole. This rule gave very close results in all examples tried.

Multiband TWAs are also a possibility, The most straightforward approach would be to run two or more helical resonators in parallel, each wound for a particular frequency. The dipoles can be slightly turned between each other to avoid coupling, or be spaced a few inches apart. Unused dipole*. simply become high Q chokes or capacitive elements. The antennas can be stacked using PVC plumbing components and cemented. It might be worthwhile to explore coaxial TWAs. Interaction between dipoles does not appear to create significant additional losses, a principle which can also be applied to the design of TWA multiband beams.

Letters

Continued from page 25

summer in New England,

The telecommunications industry is growing very fast, and our company is expanding rapidly, Wc are hoping to make a breakthrough into the PCS market, which looks like a S5 trillion industry. It will make cellular telephones obsolete, and revolutioni/e all telecommunications. It was a combination of education, experience, and amateur radio which helped me land this position, Reading 73 magazine back in Germany from 1976-78 convinced me to go on for my electrical engineering degree, and to continue with amateur radio. Using your tapes and study mate-rid helped me earn my Extra Class license in 18 months. And that was before Bash and without any prior amateur radio training.

This electronic knowledge and my enthusiasm for technical writing made me into a salable commodity'.

It is difficult for the industr\ to find any competent telecommunications engineers who also have ilie ability to write a coherent declarative sentence. I urge you to continue to editorialize on the necessity for technical people to improve their w riling skills,

Most of us have read about the excellent educational system in Japan, where the> leach integral calculus to seventh graders, and have superb scientific training. What most Westerners overlook is that Japanese secondary school students spend an average of three hours a week studying liberal arts, including expository writing, calligraphy, and interpretive dance and music. This liberal education, coupled with ihcir superior

Remote tuning can be accomplished by moving an axial capacitive stub. The capacitive stub can be a flexible copper or steel wire wound and unwound on the inside center ol (he pipe. The wire is wound onto a metal pulley and shaft which is grounded to the center of the TWA. Little current flows so the arrangement is not vei y critical in terms of contact resistance. The PVC pipe provides a good seal to the environment. Either use a small motor or bring the tuning wire directly to the operating position. With a return spring only a single mechanical control wire is needed. Teleflcx type cables could be used. Fig, 2 shows the functional arrangements. For small inband changes, symmetry is not required. For symmetry and larger frequency changes, more elaborate tuning mechanisms can be dc\ tsed.

The adhesive copper lape is readily available in major cities. I used 3M™ #M8L rbis tape has conductive adhesive, A non-conductive one is #II94. Both tapes have a 1.4 mil copper thickness, Skin depth at 14 Ml I/, is 0.7 mils. Skin depth is inversely proportional to the square root of frequency so the tape could be used down to 7 MHz.

References:

]. ARRL Antenna Handbook 1W4-

2, Reference Data for Engineers, Eighth Edition, Prentice Hall 1993.

3, L.AMoxon G6XN, Hf Antennas (Handbook), 1986 RSGB.

4, Glenn S. Smith, "Effects in Systems of Parallel Conductors" Journal of Applied Physics, Vol. 43, No, 5, May 1972.

technical education, has helped make Lheir nation a leader We would do well to emulate their l \ample.

Again, I wish to express my graliiude This is the best job 1 ever hoped to have. Your generosity and kindness helped to make it possible.

An\ shucks.^Wavne tr

Michael Nie KB8YMX* Wayne: I read your "Welcome Mew comers1 piece w ith interest. Many amateur radio publications have printed op-ed pieces or articles bemoaning the lack of activity in the higher classes of licenses. The two words at the top of the page. "Radio Fun/' sum it all up. Amateur radio is a hobby. It is supposed to he fun. I can only speak tor m>self, but I feel

Gonttaued on page 82

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