April 2008 Issue

Folks: The new antenneX online issue #132 for the month of April 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
Reciprocity: Home on the Range
The ARRL Antenna Book (20th Ed., p. 2-1) contains a beginner's discursive explanation of reciprocity. "In the same fashion that a loudspeaker can act as a microphone, a radio antenna also follows the principle of reciprocity. In other words, an antenna can transmit as well as receive signals." This brief extract follows an explanation of an antenna as a "special transducer" capable of converting RF current into propagating electromagnetic waves and converting intercepted waves into electrical current. The context is the very beginning of a chapter called "Antenna Fundamentals." Hence, we should not expect mathematical sophistication.

From the Shack By L. B. Cebik, W4RNL
4:1 Toroidal Current Baluns: Some Preliminary Measurements
Past 4:1 balun tests have included a dual ferrite-bead current balun and a voltage balun. The present test focuses on a 4:1 Guanella current balun. The history of these baluns has been traced in many sources, including the many works of Jerry Sevick, W2FMI. The design predates the use of ferrite cores and includes air-core baluns. The basic design consists of two windings isolated from each other as well as possible. The windings should have a characteristic impedance that is the square root of the input and output impedances. A 50-Ω to 200-Ω balun dictates a winding impedance of 100 Ω. To achieve the step-up/step-down action, the lower impedance end connects the windings in parallel, while the higher-impedance end connects them in series. The advent of ferrite cores in ready supply allowed placing two windings on separate cores in close proximity. The limited permeability (μ) of early cores—still in wide use today—dictated bifilar windings with 15-18 turns per core.

Propagation By Marcel H. de Canck, ON5AU
Space Weather and Solar Properties - Part 5
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
Seek and Ye Shall Find
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:

Reinventing the (Big) Wheel
By L. B. Cebik, W4RNL

The search for a horizontally polarized VHF omni-directional antenna is as old as amateur use of their VHF allocations. In 1961, R. H. Mellen, W1IJD, and C. T. Milner, W1FVY, presented an improvement of the halo that has had enduring interest and provoked considerable construction frustration from time to time. They called their antenna “The Big Wheel on Two”. These notes will not so much reinvent the big wheel as they will revise the way in which we formulate its operation. Once we have taken that step, we shall discover that we may modify or revise the design in ways that will better meet our needs and simplify adjustments. In the process, we shall examine the process of modeling the 2-meter omni-directional antenna. All models of the antenna in these notes will use 146 MHz as the test frequency, because once we have the right view of the array and the right design for it, we shall discover that it easily covers all of the 2-meter band.

The Velocity Factor
of an Insulated Two-Wire Transmission Line
By Kirk T. McDonald, PhD
Joseph Henry Laboratories, Princeton University

Here we estimate the velocity factor F = v/c and the impedance Z of a two -wire transmission line made of cylindrical conductors of radius a whose centers are separated by distance d, when each wire is insulated by a layer of (relative) dielectric constant ε of thickness t, as shown by illustration in this paper. The thickness t is of the same order as radius a, but a + t << d. The space outside the insulated wires has unit (relative) dielectric constant. All the media in this solution have unit (relative) magnetic permeability.

VHF Antenna Noise Temperature
By Dragoslav Dobričić, YU1AW

One of the criteria of receiving system is its ability to receive very weak signals: its sensitivity. For given bandwidth, the sensitivity is determined by only two factors: Antenna Gain (G) and System Noise Temperature (Tsys). The system noise temperature comprises the antenna noise temperature (Ta), the cable losses converted into noise temperature (Tcable), and intrinsic noise of the receiver or preamplifier (Trx). These parameters determine signal to noise ratio (S/N) which one linear receiving system has at its output.

There is no universal recipe and the receiving system must be tailored in order to meet the prevailing conditions as seen at the antenna output terminals. The first factor determining these conditions is the noise arriving with the signal, which cannot be influenced by the operator but must be coped with by the receiver.

Taking AIM: Part II
By Bob Cerreto, WA1FXT

Over the years I have used a number of different methods to measure unknown impedances at antenna feed points. These methods involved the use of multiple types of test equipment and some set up time. Most methods were difficult to make portable and the data provided by the test equipment sometimes required further calculations. Gathering data at multiple frequencies was very tedious and while the results can be accurate, considerable time is required for project completion. The purchase of an AIM 4170 from Array Solutions has changed our way of performing these types of measurements. We now have a compact, portable and accurate testing method that will provide quick impedance measurements and other electrical parameters.

This multi-part series talks about my experiences with the 4170 and is targeted at the inexperienced user. In the first part of this series, we gave some hardware descriptions, described the initial setup and demonstrated basic scan features for the AIM 4170. This second part will complete use of the scan feature set and show some of the functions available. Future parts of this paper will show some non-antenna related utility features and customization of the configuration files. My test antenna continues to be a simple 2M dipole. The test feed line is a 6-foot length of RG58A. The article assumes the reader understands some basic feedline concepts.

An Extra Cheap Yagi Beam
By Robert Wilson, WE7ZKK, AL7KK

Yagi antennas are a tried and true antenna type; they work well and are widely available. Most are made from aluminum tube of stepped sizes. Well here are plans for making an extra cheap Yagi out of conventional copper wire. The latter part of this article will give generalized plans for any high frequency band. For the twenty-meter band it is possible to make an effective Yagi beam out of nothing but number 12 AWG copper wire, or even better steel core copper coated wire of the same size.

Proplab-Pro 3 - Part 2
By Marcel H. De Canck, ON5AU

As told in Part 1 of this Proplab-Pro 3 review, one important option has not yet been discussed: the Ray-Tracing engine. The power to model reality is through ray-tracing. This option was created more then a decade ago, in Proplab-Pro 2, unique in its kind. With Version 3 the 2-D and 3-D ray-tracing models are made even more accurate and with some additions. In particular the 3-D ray-tracing model was error prone in Version 2, but this is no longer true with Version 3. Pre-processing ionospheric profiles in no longer necessary, but computations of ionospheric electron densities and three-dimensional layer gradients occur directly. In addition the new 2007 IRI is substantially more complex than the old 1995 IRI. This results in a significantly easier operation, a definite boost in traced ray accuracy, but not faster in time. With other words, the authors considered that accuracy and ease of operation are more important in Version 3 than speed.

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