Antenna Notes©2002 Serge Stroobandt, ON4BAA |
Last page update: 14 June 2003
ON4BAA's Coaxial-Fed Multiband Windom Antenna
A 10GHz Dielectric Rod Antenna
The World Atlas of Ground Conductivity
Antenna Modelling Software
The Mechanical Design of Tapered Antenna Structures
The Electrical Modelling of Tapered Antenna Elements
My Opinion about the Crossed-Field Antenna (CFA)
Highly Recommended Books on Antennas
More Antenna Notes
Antenna Humour
ON4BAA's Coaxial-Fed Multiband Windom Antenna
A 10GHz Dielectric Rod Antenna
Can you believe this: a long-yagi, 20.5dBi gain, made out of plastic! Know all about it by downloading this full report (0.6MB PDF).
Summary
Dielectric rod antennas provide significant performance advantages and are a low cost alternative to free space high-gain antennas at millimeter-wave frequencies and the higher end of the microwave band.
The fundamental working principles of this type of antenna are explained and guidelines are given for a maximum gain design. These were applied to an X-band antenna design which resulted in a maximum end-fire gain of 20.5dBi for an antenna length of 11.18λo. E- and H-plane radiation patterns were measured as well, revealing high sidelobe levels, especially in the E-plane. This is about the only fundamental disadvantage of the dielectric rod antenna. However, some end-fire gain and main beam sharpness could be sacrificed to reduce the level of the sidelobes.
The tapered dielectric in waveguide feed configuration proved to be well matched over an extremely wide band; over 3GHz. The pattern bandwidth depends on the intended application of the antenna, but is in general also quite large. Not knowing the surface wave excitation efficiency of the feed was the only difficulty encountered during the design process. As a result, the maximum end-fire gain was achieved at a frequency different from the design frequency. This problem would not have existed if a computer code was available to model the transition from feed to antenna.
The World Atlas of Ground Conductivity
Important for HF antenna modelling is to know the conductivity of your soil. In literature you can find some ways to measure ground conductivity yourself. Nevertheless, it is a always bit of a hassle having to construct a measuring apparatus just for using it once or twice. Moreover, the area of land that most influences the radiation pattern is at about 10λ away from your antenna, i.e. often not accessible for measurements. I am by no means trying to discourage ground conductivity measurements, but being practical, I know that most people (including myself) prefer to spend their time on other things.
For people like us, the CCIR (now ITU-R) published quite some time ago Recommendation 832, containing a detailed world atlas of ground conductivities. The maps are for VLF and MF frequencies, though the MF maps still remain useful for the lower HF frequencies. An example MF conductivity map of my home country Belgium is shown below. Click here or on the map below to download the World Atlas of Ground Conductivity in PDF(4MB) to see a map of your country or region. Please remember that this file is for personal use only. Commercial and professional users should obtain a copy of the more recent ITU Digitized World Map (IDWM).
Figure 1: MF conductivity map of Belgium. Conductivities are expressed in mS/m and standarised to 1MHz.
Antenna Modelling Software
The software codes I use for HF antenna modelling are EZNEC by Roy Lewallen, W7EL, and Antenna Optimizer (AO) v6.5 by Brian Beezley, K6STI (click to order your copy by e-mail). However, there are many more codes around for a huge variety of different antenna configurations and purposes. If you want to know more, you can nose around on the site of the Applied Computational Electromagnetics Society (ACES).
Most important of all when using antenna modelling software, is knowing how to use each particular software code. The modelling and segmentation of structures the code was not designed for, will or sure produce erroneous results.
To learn the tricks of the trade, I invite you to take a look at the antenna-modelling website of L. B. Cebik, W4NRL. In my opinion it is the most complete, useful, scientific and knowledgable radio-amateur web site on antenna-modelling. If you are a serious radio-amateur or antenna professional it is a must-read, and for sure you will like it!
The Mechanical Design of Tapered Antenna Structures
Long horizontal antenna elements or vertical antennas require a sound mechanical design for the sake of everybody's safety and for your own convenience. After all, it is no fun having to miss your antenna after a heavy storm went by. The good news is that you won't need an engineering degree to get round the maths. Your humble servant made an universal Excel workbook for the mechanical design of horizontal and vertical tapered cylindrical or tubular structures of any material for you to download. Have fun and hopefully you will sleep better during those stormy nights!
This material has been provided as a guide only. The web master, Serge Stroobandt, ON4BAA is not responsible for accidents that may occur as a result of my or your miscalculations.
The Electrical Modelling of Tapered Antenna Elements
Click on the link to see L. B. Cebik's, W4NRL, "Tapering to Perfection" tutorial.
My Opinion about the Crossed-Field Antenna (CFA)
"It definitely is a hoax!" Click here to read an article that was published on antenneX in response to a letter thet I sent to the antenneX editors and that seemingly made them very suspicious about the CFA claims. Later some time, I will explain here in detail why the CFA only works in Egypt.
Highly Recommended Books on Antennas
In order of personal preference:
If you are living in the European Union it might be cheaper to order antenna books from the on-line shop of the Radio Society of Great Britain, hence avoiding high import taxes.
- John Devoldere, ON4UN, Low-Band DXing, ARRL
- Karl Rothammel, Y21BK, Antennenbuch, Franckh-Kosmos, 10. Auflage, 1991
- Robert Schetgen, KU7G, Vertical Antenna Classics, ARRL, 1995
- Les Moxon, G6XN, HF Antennas for All Locations, RSGB, 2nd Edition, 1993
- Jerry Sevick, W2FMI, Transmission Line Transformers, ARRL, 1996
More Antenna Notes
Antenna Humour
Inverted-V without coax loss!
Music extract: Antena by Isabelle Antena
