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~ July 2008 ~ Online Issue No. 135 ~

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~ In This Issue ~

The Practical Lindenblad Stack
Hidden Momentum in Coaxial
Coaxial Cable Current
Yagi Antenna Q Factor
Basic Op Needs of the Radio Shack
Bicycle Mobile J-Pole II

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Read Our Monthly Columns!

Antenna Modeling From The Shack
This is a regular and popular monthly column by L.B. Cebik, W4RNL 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

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!

FEATURE ARTICLES THIS MONTH IN THE LIBRARY
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The Practical Lindenblad Stack
By L. B. Cebik, W4RNL
(A Posthumous Publication)

Our examination of basic Lindenblad array theory and practice in the preceding issue of antenneX showed us the underpinnings of N. E. Lindenblad's 1942 development of an omni-directional antenna capable of circular polarization in the X-Y plane (that is, parallel to the surface of the ground). The advantage of the antenna system is that it communicates well on a point-to-point basis with either vertically or horizontally polarized antennas. I adapted the system to 6-meter and 2-meter antennas that provided full-band coverage. Such antennas are useful to anyone who wishes to use a single antenna for the diverse services that we find on those bands, ranging from horizontally polarized CW, digital, and SSB at the low end of the bands to vertically polarized FM in most of the upper part of the bands.

In typical amateur radio fashion, I began to wonder: if one antenna is good, might two be better? From that thought arose the idea of stacking Lindenblads vertically to increase omni-directional gain. This article will tackle the new question in some detail after redesigning the Lindenblad for that kind of service. Stacking is not a casual practice of simply setting two antennas in vertical alignment and then feeding them in phase.

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

The objective here is to 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 resister R0 is connected to the other. The current may be taken as uniformly distributed over the inner conductor, which has the sensitivity p. The outer conductor has negligible resistivity, and the current flows on it in a thin sheet at radius b. The battery has negligible internal resistance.

Then to 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). Next, calculate the impulse on the charge on the surface of the inner conductor due to the electric field induced by the transient current.

Yagi Antenna Q Factor
By
Dragoslav Dobričić, YU1AW

In one of my previous articles, in the April 2008 Issue of antenneX, I proposed use of the antenna SWR working bandwidth as contributor in evaluation of Yagi antenna figure of merit. Antenna VSWR bandwidth is important as relative measure of antenna Q factor and overall antenna reproducibility and sensitivity to presence of other objects in its near field. Because of that, VSWR bandwidth or Q factor contribute to overall characteristics of antenna.

The VSWR bandwidth is relative measure of antenna Q factor and it is very important factor for any evaluation of overall Yagi antenna quality. In this article I want to show how we can calculate antenna Q factor for use in antenna quality evaluation instead of previously proposed use of VSWR bandwidth.

Basic Operating Needs of the Radio Shack
By L. B. Cebik, W4RNL
(A Posthumous Publication)

Looking at the shack as a place, we can begin the process of transforming it into a center of activity. Amateur radio is a very broad name for many activities. To sort out what may be most relevant to each of us as individual hams, we must go back to basics. In this article, we will look at the basics from a perspective which differs somewhat from our usual view of Hamming: one of being deeply involved in one or another part of it. Perhaps the most fundamental aspect of amateur radio is communications, so let's start there in this series of articles.

Bicycle Mobile, Direct Feed 146MHz J-Pole: II
By Ed Lawrence, WA5SWD

This is an original design to address problems encountered with other bicycle antenna designs available. Concerns addressed included good RF behavior, effectiveness, safety, size, durability and ease of construction and use. Compromise was necessary in some areas to meet the overall objective. Further development efforts are continuing but the basic concept seems sound.

Few simple antennas outperform a simple ground plane but, for a bicycle mobile, radials are not acceptable due to safety concerns. A stub-matched half-wave design looked promising but excessive RF coupling to the shield made the coax ‘hot’ for RF and it was ruled out.

Any antenna that requires a good ground plane such as the bicycle frame will either be mounted too low for maximum signal or not have a good ground or both. No other antenna designs were found that did the job as well as a J-Pole described in this article.

Coaxial Cable Current
By Jacques Audet, VE2AZX

In this paper I analyze and measure coaxial lines when subjected to currents on the outer shield. The analysis may be done with simple models, no black magic being necessary. A lab grade VNA is used to precisely measure SWR and attenuation.

The first case deals with measurements done with the split 25 Ω – 25 Ω termination. This case was modeled at both low RF and higher frequencies, up to a coax length of two wavelengths.

Measurements and modeling results show that this method does not generate large shield currents as compared to feeding a half wave dipole with a coaxial cable.

Possible transformer action of a short length of semi-rigid coax was also investigated, by forcing current to flow longitudinally along the shield. It shows that only skin effect coupling can be detected below 5 MHz, using a very sensitive detector.
 


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