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ANTENTOP- 03- 2003, # 004

Current Distribution in the A.L.C.

 

 By Yuri Blanarovich, K3BU, VE3BMV, VE1BY

mailto: K3BU@aol.com

http://members.aol.com/ve3bmv/Razors.htm    http://www.K3BU.us/

 

A recent article on eHam.net by Alan Applegate, K0BG "In Search of 'The Perfect Mobile Antenna'" on Aug. 5, 2003 and posted comments, created some controversy and heated exchange between myself and the "well respected engineer" (according to Aaron, NN6O) Tom Rauch, W8JI. This article had some flaws and was far from approaching "perfect" mobile antenna. To which W8JI stated:

"By using a flawed and seriously over-simplified model, the results are totally misleading when applied to conventional antennas. Repeating misleading information in article after article does NOT make it correct. It certainly does not make our community more skilled or better informed about how things work."

and

"While I appreciate all your (K0BG) efforts, it is important that readers and writers fully understand why and how something works before reaching conclusions. Otherwise this all just wastes bandwidth, and people learn incorrect information. The goal of E-Ham and Internet should be to INCREASE technical skills through mass peer review and learning, not to repeat misinformation. "

In view of the above and in order to "INCREASE technical skills through mass peer review and learning"  I have summarized in my posting the most important items contributing to high efficiency of mobile or shortened antennas. W8JI commented that I was  wrong about the current distribution in the antenna loading coils, where I stated:

"4. Loading coils. Should be mounted as high as possible in the antenna mast to increase the current radiating portion of the whip. Wire or tubing should be rated to carry the power without melting the coil. Q of the coil is not that important, coil in this situation is the part of the radiating element, most of the current is at the base of the coil and surprisingly Q or form factor is not THAT important as measured and verified experimentally by W9UCW. Loading coils at the base or autotuners are the worst. Bugcatchers, single coils with no shorting, spider mounts for multiple coils are very efficient."

 

To which W8JI replied:

"The idea current is high in only the start of a coil is not correct.
Model an antenna with EZnec, and look at the load. Model a coil in any software, and look at current. Read any textbook, even beginner's textbooks, and see what they say. Measure a real antenna yourself!

 

You are like to call names, insult people, and argue rather than take the time to learn basic electronics. This is in any book, including the ARRL Handbook. If you look at HOW an inductor works, the current flowing in one terminal ALWAYS equals the current flowing out the other terminal. "

(The rest of the exchange can be seen at eHam.net following the K0BG article at http://www.eham.net/articles/5998)

This was quite an accusation and a challenge to reality and my knowledge. This was not the first time that W8JI "challenged" me and was proved wrong. Knowing what was the reality (uneven current distribution, coil gets hotter at the bottom) and what Barry Boothe, W9UCW measured, I was curious what was the source of W8JI misinformation. I ordered 19th edition of ARRL Antenna Book and followed chain of references that led to information on page 16-7 and Fig. 9 and 10. (see page 53)

Looking at those two pictures, we can see that the current across the radiator was "linearized" to be a nice straight line instead of actual cosine curve. Also, Fig. 10 is missing the important cross-hatched area, the current across the coil is shown as a "nice" linear current over h2 and coil apparently has zero physical length. This passage in the Antenna Book is written by Bruce Brown, W6TWW "Optimum Design of Short Coil-Loaded High-Frequency Mobile Antennas" first published in ARRL Antenna Compendium, Volume 1, page 108.

 

 

 

 

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