June 2009 Issue

Folks: The new antenneX online issue #146 for the month of June 2009 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)
AM BC Modeling with NEC
6. Grounds
We have essentially completed our journey through modeling broadcast towers with respect to the basic dimensions of modeling them. In many of the later episodes, we used NAB-recommended single-wire substitutes for full tower structures to maintain the clarity of the modeling suggestions in question. Of course, one may choose to model a full structure or a multi-leg alternative to the single-wire monopole. These notes only represent the barest of starts along the AM BC tower modeling task using NEC.

Nevertheless, as we look back over our work, we may harbor questions based on one facet of the modeling work: the use of a perfect ground for all models. Actual AM BC antennas use extensive radial fields, normally with each of 120 radials about 1/4-wavelength long at the assigned frequency. (Many actual fields include intervening shorter radials, but we shall not work with them here.) The radials are buried within the earth's surface in soils of highly variable quality as we move from one site to another. Hence, some folks may question the ability of a perfect ground to replicate accurately the conditions. In one sense, the questions are otiose, since standard practice is to refer such towers to perfect ground. However, there remain for some a few nagging questions about correlating site measured values of feedpoint current magnitude and phase and field-strength measures as well to modeled values using perfect ground.
From the Shack By David J Jefferies, PhD
Why use microwaves for satcoms?
Frequencies above about 30MHz can pass through the ionosphere and so are available for communicating with satellites and other extra-terrestrial sources. Frequencies below 30MHz are liable to be reflected by the ionosphere at certain stages of the sunspot cycle. The ionosphere consists of several layers of ionized gas which alter in height during the 24 hour daylight cycle. The ionosphere has an effect on satellite communications even if it does not completely prevent them.

Frequencies from about 100MHz to 2 GHz are used for communicating with low earth orbit satellites (LEOs). Since the range from ground station to satellite is only a few hundreds of km, it is not necessary to use high gain ground based antennas. Of course, there is a direct link between the beam divergence angle of an antenna and its directivity and its gain.

A highly directional antenna concentrates most of the radiated power along the antenna "boresight." There is no functional difference between receive and transmit modes of an antenna except that power flow is directed inwards to the receive antenna and outwards from the transmit antenna. An antenna has the same efficiency, directivity, and polarization characteristics in receive and transmit modes. This property is called "reciprocity" and occurs because of the symmetry of the electromagnetic equations when the direction of time is reversed.

A high gain antenna is therefore very directional and has to be pointed with correspondingly high precision. It is an advantage if it does not have to be moved in azimuth and elevation. This restricts the use of high gain antennas largely to geostationary satellite applications. Steerable high gain antennas are very specialized and costly.

Propagation By Marcel H. de Canck, ON5AU
Ham-Band propagation properties – Part 5 (and end of this column)
For decades experienced radio hams know very well how the different ham-bands behave related diurnal, seasonal and cyclical for their home location to make local and DX contacts. However this does not mean that this coming series about ham-band propagation properties should be of no use for them. Perhaps you plan to take some equipment with you on holiday and would like to know the communication possibilities while there. But the newcomers and not so experienced radio hams will certainly gain knowledge of how the different bands behave and how and when DX openings from various parts of the globe might be expected.

During this series I will make a study of how the propagation properties vary with the regular Ionospheric variations. A huge amount of area coverage maps were generated to illustrate these changes and behaviors. The results will mainly count for northern hemisphere mid-latitude locations in West Europe where I live. So to know how the propagations behave and develop for your location, you need to do some predictions yourself. To start with, you only need to make predictions with the SSN range of the moment. For one frequency band it only takes 24 coverage maps a month or a season and the task is not so time consuming. Doing so, at the end of the year you have a most informative “Prediction Atlas” you will consult more than often. As solar phase activity increases you have to follow up with new sets of prediction maps.

Stone's Throw! By Jack L. Stone, Publisher
Propagation to End
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:

Designing Multi-Band Parasitic Beams
Part 5: Alternative 15M, 10M Yagi Design Example
By L. B. Cebik, W4RNL
(A Posthumous Publication)

As we experimented with the 10-meter driver placement and the use of an additional director, we found some interesting trends associate with the Moxon-Yagi array. For example, the addition of an extra director appears to broaden the SWR curve, but also to lower the average front-to-back ratio. As well, the gain improvements wrought by an added director appear small and serve mostly to smooth gain performance across a specified passband. In addition, placing the 10-meter driver ahead of the Moxon 15-meter driver resulted in sharper SWR curves. What we cannot tell from using only the Moxon-Yagi dual-band antenna models is whether these are general trends or unique to the combination of Yagi upper-band elements and lower-band Moxon rectangle elements.

Therefore, we need to apply the same general exercise to the more complex Yagi-Yagi array composed of a 3-element wide-band Yagi for 15 meters and a comparable set of Yagi elements for 10 meters. Our initial designs used 4 elements on 10 meters, but the terms of the present work require us to try both 4- and 5-element 10-meter Yagis.

Energy Flow in a Waveguide below Cutoff
By Marco Moriconi
Instituto de Física, Universidade Federal Fluminense, Niterói, Brasil
and Kirk T. McDonald, PhD
Joseph Henry Laboratories, Princeton University

Electromagnetic waves of frequency below a cutoff cannot propagate down a hollow waveguide. One view of why this is so has been given where it is argued that constructive interference between the physical source of the waves and the infinite set of related image sources can only occur if the free-space wavelength is short enough. In this paper, we discuss the flow of energy, as described by the Poynting vector supposing that source currents in the plane z = 0 launch fields according to the TE₁₀ mode pattern of a rectangle air, vacuum waveguide of inner dimensions a in x and b in y.

NewcomerNotes: How Does an Antenna Work?
By Robert Gulley, AK3Q

As I have mentioned in previous columns, every antenna is a compromise of sorts. No one antenna can receive all signals equally well, and some signals simply cannot be heard without the right antenna. Antennas are by nature selective, or perhaps to be slightly more accurate, selectively receptive to the various RF signals racing through the atmosphere. Electro-mechanical elements within your TV and radio block some signals while passing others through a process of tuning and amplification. This keeps us from hearing all the signals at once—which of course would make them unintelligible. The antenna is the first step in this selection process, and so a basic understanding of how antennas work will increase our chances of using them effectively.

Starter Antennas for Six Meters
By Augie "Gus" Hansen, KB0YH

The summer VHF skip season is here. Don’t miss the fun in the “weak signal” portion of the six-meter band, which is the bottom end where CW, data, and SSB modes rule. Presented here are some ideas for simple antennas for this band that can be assembled from readily available materials and put in play quickly. And these antennas can be easily scaled for use on the other VHF bands in the range of 30-300 MHz.

Operating on six meters, the so called “Magic Band”, is an acquired taste. If you are one who wants to turn on your rig and make contacts at any time of the day or night, 40 meters is the place for you. Most of the time six meters seems to be dead, although it is usually good for local contacts. However, when the skip is in, the band comes alive and can provide lots of excitement and some great DX opportunities. The same can be said about two meters, except the openings are less frequent. Six-meter DX propagation is most often sporadic E skip (Es). It can happen at any time of year and during any part of the solar cycle. However, Es openings are most plentiful during the spring/summer transition and the fall/winter transition periods. F2 skip occurs infrequently and with rare exceptions only during the solar cycle peak periods.

Stacked Quad Beams
By Edward J. Shortridge, W4JOQ

Quad radiators have been used in a variety of beam antennas for a very long period of time and because of the performance it has been endeared to very many users. There are a large variety of shapes and feed points, all of which have merit, but in this article we will start by dealing with the most basic form and then show the progression of modeling of several Stacked Quad beams.

I have chosen to use one which is square with four equal sides and with the feed point at the center of the lower leg of the square. It is possible to use other aspect ratios, where the height and width are considerably different, and each has its advantages and disadvantages. With the feed point located in the center of the bottom of the square, the Quad radiator is horizontally polarized and fed in a balanced form, which is most applicable to the Quad Beams to be discussed in this article.

Boom Radius Influence on Yagi Antennas
By Dragoslav Dobričić, YU1AW (Serbia)

Six different Yagi antennas for the 2-meter band which have the same boom length and very similar gain but different sensitivity to environmental impacts due to their different average Q factors are used to investigate how a boom influences on Yagi antenna performances.

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