May 2008 Issue

Folks: The new antenneX online issue #133 for the month of May 2008 is ready to read at your pleasure!

IN THIS ISSUE
We again include many fine articles by our global writing team. Now, please allow me to introduce this month's line-up of content:

OUR MONTHLY COLUMNS:

Antenna Modeling By L. B. Cebik, W4RNL (A Posthumous Publication)
Radiating and Transmission-Line Currents
In episode 100 of this series, we examined the modeling work-around often used to test a coaxial-cable-fed antenna for common mode currents. However, we seem to have no comparable work-around for detecting common-mode currents when we use parallel transmission lines. In fact, the help screens that accompany EZNEC record the following statement: "I don't know of any way to accurately model common-mode effects on a two-wire transmission line (that is, how to model a radiating two-wire line). If it's necessary to do this, the line will have to be modeled as two parallel wires."

Interestingly, if we model a resonant dipole and a resonant folded dipole at the same frequency, we may examine the current tables and discover that the current magnitudes and phase angles that we encounter seem to have very little in common. We can perform the same test with a folded monopole and a single-wire monopole. The results will be the same. Both of the single-wire antennas will show a near-cosine-wave decrease in current magnitude as we move from the feedpoint to the wire end, and the phase angle will change by only a few degrees. The folded versions show current values very different from the single-wire models.

From the Shack By Bob Cerreto, WA1FXT
LB Cebik - My Mentor, My Friend
To discuss LB as a mentor here would repeat the same words that have been spoken and are now being spoken by many of the people LB helped. If you ever sought LB’s help then you all know the words to say. Surely, his gifts and talents were and are shared with all of us. LB gave to all of us everything that was given to him. LB’s hope and philosophy was for each and every one of you was to pass the same knowledge on to others in the same passionate, humble, and kindly manner. So, the next time you can help someone with a technical problem, remember how LB helped you. Pass the knowledge on with humbleness and unbounded enthusiasm.

Propagation By Marcel H. de Canck, ON5AU
Space Weather and Solar Properties - Part 6
More than often, I have mentioned in previous issues, and we all know, that the sun is the engine to propagation properties and conditions. The sun’s role in these propagation properties might be in the better or worse sense. With this series of Space Weather and Solar Properties, I shall discuss and explain the impact of the sun on our radio communications. In this first issue, I shall make a start with a brief introduction of some general sun facts and parts of the sun. Later I shall dig more deeply into all of them with the different impacts most solar behaviors might have to our propagation conditions. Once you have insight and knowledge into space weather conditions, you may be able to do some predictions or foresee how propagation may become better or worse. Take one thing for sure; it’s a most violent environment up there!
Stone's Throw! By Jack L. Stone, Publisher
Our Friend, LB Cebik W4RNL (SK) - A Tribute
A monthly column covering breaking news, new concepts and products, people making news and introduction of the current month's issue articles and its authors—although not limited to this only.

FEATURE ARTICLES IN THE LIBRARY OF NEW ISSUES:

The Dual-Element Wideband Dipole:
Some Preliminary Notes
By L. B. Cebik, W4RNL
(A Posthumous Publication)

Occasionally, one finds an antenna design with fascinating potentials. Such is the case with the dual-element wideband dipole (DEWD), the first sample of which comes from Nikolay Kudryavchenko, UR0GT. He developed a relatively simple wire antenna that would cover the entire 80-meter band with a 50-Ω SWR of less than 2:1 without the need for special matching systems. How or why the antenna works as it does is subject to some discussion. Our interest will be in better describing the behavior patterns as it works. It is only a dipole, with a typical bi-directional pattern when set horizontally over ground. Still, it has some very unique features.

What Does an AC Voltmeter Measure?
By Kirk T. McDonald, PhD
Joseph Henry Laboratories, Princeton University

An AC voltmeter is a device that measures the oscillating I₀ across a large resistor R₀ that is attached to leads whose tips, 1 and 2 may be connected to some other circuit. The reading of the voltmeter is V_meter = I₀R₀. AC voltmeters typically report the root-mean-square voltage V_rms = I₀R₀/√2 rather than the I₀R₀.

Then, discuss the relation of the meter reading to the different V₁ - V₂ in the scalar potential V between points 1 and 2 in the absence of the voltmeter.

Inverted Amos Antenna as Linear Feed
for Cylindrical Parabolic Reflector
By Dragoslav Dobričić, YU1AW

In this paper I will try to examine the parameters which are leading to the optimum efficiency of a cylindrical parabolic antenna illuminated by collinear dipole array in front of plane reflector. In the article of October 2007 issue of antenneX, there are explanations about different types of parabolic reflectors which are produced by different slicing of surface created by rotation of parabolic curve around its axis.

Another type of parabolic surface reflector can be created by extruding (drawing) parabolic curve along the line which is perpendicular to the plane in which parabolic curve lies in. Because the focus point is also drawing along the line, focus line is created instead of focus point.

As a result we have cylindrical parabolic surface with focus line. This type of reflector surface needs specific linear feed for good illumination efficiency.

Taking AIM: Part III
By Bob Cerreto, WA1FXT

So far, we have discussed hardware descriptions, initial setup, basic scan features, advanced scan features and some of the antenna related Functions available for the AIM 4170. This part of the paper will discuss the remaining antenna and non-antenna related utility functions.

My test antenna continues to be a simple 2M dipole. The test feed line is a 6-foot length of RG58A.

We will show you a crystal filter design application to demonstrate the Measure Crystal function. If you want to design your own crystal filter with us, you will need some crystals (all the same marked frequency), small inductors, capacitors, and a test fixture similar to this one.

Frequency Division and Dividers
By David Jefferies

Most are happy with the idea of harmonic distortion in non-linear circuits, in which multiples of the original frequencies are generated. Fewer people are happy with the idea of subharmonic generation, where the non-linearities produce lower frequencies than those originally applied.

In a system which is sufficiently non-linear there is frequently a route to chaos involving successive period doublings in the response to a sinusoidal drive, as the amplitude of the drive is increased in relation to the non-linearity.

In this article, examples are described, and proposals to exploit this process for the provision of phase locked frequency division are presented, various systems are considered, and it is suggested that a diode having charge storage time of 10 picoseconds an injection voltage of about a volt, and a charge storage capacity of about 10^5 electrons would provide an ideal device to divide a 50GHz signal at a power level of around 10mW.

Experiments are presented which demonstrate, for modulated signals with 100 MHz carriers, phase locked division in which the amplitude and frequency excursions of a carrier are faithfully followed at speeds representing a fractional bandwidth of at least 10%.

Designing a 50-Ohm 2-Element Beam the Hard Way
By Morris Jones, AD6ZH

I was taught to design a beam by picking a constraint such as boom length, front to back ratio, forward gain or number of elements; and then use a table or chart in a reference such as the National Bureau of Standards (NBS) 688 or an antenna engineering handbook to select a compromise configuration that best meets design goals. This often results in an antenna with a feed line impedance mismatch at the end of the design process. A matching network such as a gamma, hairpin, “T”, coax transformer, or L/C network is then attached to the antenna. This experiment's goal was to find a set of design charts where the impedance is picked first, and then the “best” beam is selected within a 50 ohm constraint.

For paid subscriber access go to the login page:
http://www.antennex.com/library/library.htm
....or
http://www.antennex.com/premium.html
...and select Library of New Issues from the top section #1.

Non-subscriber access, go to the Guest Rooms:
http://www.antennex.com/guests.html

For the premium articles, be sure you have a current subscription and don't miss this new issue. It's in your best interest to know these things.

To subscribe:
http://www.antennex.com/signup.htm

For Login help:
http://www.antennex.com/help/need_help.html

• Do you have a question? •

Hope you enjoy the new issue!

Jack L. Stone, Publisher
antenneX Online Magazine
http://www.antennex.com
jack@antennex.com