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 Founded in 1988
This online version reaches 215 countries!

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~ May 2012 ~ Monthly Online Issue No. 181 ~

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Read Our Monthly Columns!

Antenna Modeling From The Shack
This was a regular and popular monthly column by L.B. Cebik, W4RNL (SK)Because computerized antenna modeling has become widespread, and its popularity as a design tool continues to increase, this series is devoted to helping readers get the most from the design software used. The articles focus upon the use of NEC and MININEC, along with useful adjunct software as well.

This column is primarily for "Guest Editorials" to provide a podium for our readers to voice their opinions to the rest of the world too. This is a chance for readers to get on their "soapbox" and speak about antenna and radio-related subjects. Don't miss these interesting views about anything and everything about radio and antenna systems! Now, what have YOU to say??

Ham WorkShop

Stone's Throw!

Ham WorkShop, is also another regular monthly column  filled with a variety of "RADIO-STUFF" of value to almost everyone in amateur radio from Novice to Extra and those just beginning to take up this special hobby. This includes subjects, but not limited to: VHF, choosing the right antenna, coax cable, small to mid-scale construction projects in a practical manner, use of test equipment, etc. It is also meant to help readers become more familiar with the technical jargon and the fun side of radio. jls.jpg (2372 bytes)Stone' Throw! a monthly column by antenneX publisher, Jack L. Stone, among other things, is to keep the readers informed about our progress, new developments, plans for the future, and to introduce the authors and their subjects each month. Also, our main slogan around here is "we aim to please", so this serves as a place for the readers to tell the publisher what is wanted or at least make suggestions. Just remember, the publisher is only a Stone's Throw away! Go in for a visit and read this month's column.

Propagation

Antenna Design & Use

marcel.jpg (2142 bytes)Propagation another monthly column by Marcel H. De Canck, ON5AU of Belgium. Signal propagation is a subject that is one of the most basic ingredients of radio and is something everyone in radio should know about in order to maximize communications in the most effective way. It’s not enough to have the best equipment and the best antenna if you are trying to send out a signal against a brick wall. Conversely, one may possess a very crude rig, running low power, but yet transmit/receive a signal to great distances with ease, simply by making use of a thorough knowledge about how signal propagation works within the environment. Follow this column and learn more about propagation! The author of the monthly column Antenna Design & Use is Justin Johnson, G0KSC who does in fact design and install antennas in many parts of the world. He�s a real hands-on expert with years of experience in this field. Following this column will be helpful especially to those pondering a new antenna project. Examples of how to model the designs will appear at times as well. There is something here for just about everyone wishing to know more about how antennas are created, tested and used.

FEATURE ARTICLES THIS MONTH IN THE LIBRARY
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A Reference Dipole
by Grant Bingeman, KM5KG

I imagine many amateur radio enthusiasts choose a simple horizontal dipole as their first shortwave antenna. Therefore it is natural for them to compare any subsequent antennas with that trusty dipole. Physicists and antenna engineers tend to compare all antennas to an isotropic radiator, which is a fictional beast that creates a spherical radiation pattern. That is, the RF signal level is the same in all directions. But since none of us has actually operated an isotropic radiator, we never developed a gut sense for its behavior. Instead, we are more familiar with a backyard dipole, even though few of us can describe its 3D pattern in detail.

Practical Antennas: Part 5.07
By Marcel H. De Canck, ON5AU

Many times the use of a balun is recommended for proper performance of an antenna system. Within the amateur radio community baluns have three main functions.

o First, they convert an antenna feedpoint or a parallel transmission line from a balanced circuit to a single-ended unbalanced configuration. The single-ended unbalanced configuration is necessary for cables and equipment using coaxial connectors in which the outer conductor is connected to ground somewhere in the system.

o Second, often we employ baluns to attenuate or avoid common-mode currents to keep them out of equipment and off the surface of coaxial cable sheaths.

o Third, we employ some baluns to transform load impedance values to an alternative value. There are many designs for baluns capable for various impedance ratios from 1:1 upward, whereas the most usual impedance ratio is 4:1. But depending on the need and circumstances, other transformation ratios are available such as 6:1 and 9:1.

More than often baluns are also described as a choke, current or voltage type. Some explanation is needed here to see the forest from the trees. First of all, what is a balun? The name balun is short for Balanced to Unbalanced and each part is pronounced the same way as in the separated words and not as balloon more than often heard. A balun is a device which somehow connects a balanced load to an unbalanced transmission line. This article explains.

NewcomerNotes: Special Propagation Conditions
By Robert Gulley, AK3Q

This month I want to examine special propagation conditions which bring about unusual openings on bands which might otherwise be closed, and I want to discuss solar activity and its impact on communications in general.

Last time I discussed some basics concerning propagation, including the components of the ionosphere which allow signals to be reflected around the earth, seasonal variations, and variable band conditions. These topics form a basis for understanding propagation because they are relatively constant. The ionosphere reacts predictably most of the time, seasonal variations follow similar paths each year, and band characteristics remain fairly consistent, again in the most general of terms.

Relationship of Vertical Height vs Optimum Radial Length
By Rudy Severns N6LF

The suggestion the length of radials in a vertical ground system is related to the height of the vertical has long been part of amateur antenna lore. The idea is that with a 1/4-wave antenna you use 1/4- wave radials and with an 1/8-wave vertical 1/8-wave radials, etc. While you can make arguments why this might be, I thought it would be interesting to conduct a modeling study using NEC (EZNEC Pro 5.0 w/NEC 4) to see if I could find any support for this idea.

Hidden Momentum in a Coaxial Cable
By Kirk T. McDonald
Joseph Henry Laboratories, Princeton University

Here we calculate the electromagnetic momentum and identify the "hidden" mechanical momentum in a coaxial cable of length L, inner radius a, outer radius b, when a battery of voltage V is connected to one end and a load resistor R0 is connected to the other. The current may be taken as uniformly distributed over the inner conductor, which has resistivity p. The outer conductor has negligible resistivity, and the current flows on it in a thin sheet at radius b. The battery shows negligible internal resistance.

Then, we shall deduce the charge per unit length on the outer surface of the inner conductor. Then, suppose the battery can be turned off in such a way the current in the cable falls to zero with some time dependence I(t). We calculate the impulse on the charge on the surface of the inner conductor due to the electric field induced by the transient current
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Intermediate-sized loop Antennas
By David J. Jefferies, PhD

A loop antenna has a continuous conducting path leading from one conductor of a two-wire transmission line, "the feed", to the other conductor. You may think of it as a "coil that radiates". The coil may have only a single turn. It may have arbitrary shaped perimeter, but the essence of a coil is that the defining wire encloses an area. Thus, a folded dipole is not a loop antenna in this sense, since the area inside the conductor path is vanishingly small. In a loop antenna, the magnetic field (generated by the loop current) threads the area of the loop and this provides the loop with inductance.
 


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