November 18th, 2010

I wonder how many engineers fully understand the term “isotropic radiator”. We recently had an interesting discussion about this. What is really interesting is the fact that although an isotropic radiator cannot exist in practice, it is used in so many antenna synthesis and theoretical applications that one commonly finds the term used as if such a device does in fact exist in reality!

In order to clear up any confusion around isotropic radiators, we need to first make sure about the definition of the term “isotropic radiator” (some people – even university lecturers and highly regarded academia – confuse this term with “omnidirectional radiator”). The IEEE standard defines these two terms quite clearly as:

Few if not none, antenna textbooks explain why an isotropic antenna is theoretically impossible. However Silver gave a simple proof more than six decades ago (S. Silver (Ed), Microwave antenna theory and design, MIT Rad Lab Series, McGraw-Hill, 1949, pp 78-79). Subsequently, in 1954, Mathis offers a more complicated proof after invoking an obscure mathematical theorem of Brouwer, 1909.

Isotropic radiators are commonly used in array synthesis to determine the antenna factor which is then multiplied by the vector field of the single element in an array to synthesise the array pattern.

I remember how I once spent quite a lot of time struggling to analyze an array of radiators in a full-3D EM simulation tool to determine the array factor of a base station antenna. The array patterns in my simulations all showed a ?glitch? (extremely high field value) in a specific direction and only after lots of investigation and ?debugging? I realized that the field vector orientation in the isotropic element patterns that I was trying to use as array elements was undefined (or rather ambiguous) at the poles (theta = 0 and theta = 180).

The analogy used by an antenna engineer I know describes the problem quite well: ?The direction of field vectors at the poles of an isotropic radiator are undefined, just like the direction of the hairs at the crown of your head ? it?s just something one has to make a peace with!?.

Author: Robert Kellerman

## Why 50 ohm?

November 10th, 2010

A couple of our engineers recently had a long discussion while trying to decide what range of impedances to include for the design of a coaxial feed for one of our antennas. The question that quickly arose was: ??why are most coaxial cables that are commonly available 50 ohm or 75 ohm??. I?m sure you must have asked yourself the same question before.

While looking into this, we discovered a 1955 IEE paper, titled ?THE CHOICE OF IMPEDANCE FOR COAXIAL RADIO-FREQUENCY CABLES? by WT Blackband. It is an excellent study considering various factors like attenuation, voltage, cable thickness and thermal characteristics of coaxial cables of various impedances and made with various materials. Though each performance measure for coaxial cables suggests a different optimal impedance, the general conclusion is basically: ??The best choice of impedance is 75 ohms for low-loss air-spaced cables, and 50 ohms for general-purpose thermoplastic cables.?

A definite must read if you ever wondered what the best coaxial cable impedance might be for a specific application ? and why 50 Ohms and 75 Ohms seem to be so popular!

Author: Robert Kellerman

## NASA radio telescope photography at Goldstone, Mojave Desert

November 5th, 2010

One of my friends recently sent me a link that he thought I would like and he was right. Dave Bullock, a programmer, photographer and frequent contributor to Wired.com took some beautiful photos of the NASA radio telescopes at Goldstone in the Mojave Desert. There are some nice pictures with descriptions of the 230 feet dish, control center, horn feed, cooling and amplification components. Visit http://eecue.com/a/1581/Goldstone-NASA-Deep-Space-Network.html to view Dave?s photos.

Author: Robert Kellerman

## Birds eye view of Magus offices taken by 2 cameras mounted on an L-39 Albatros model aircraft

October 26th, 2010

One of my colleagues spend most of his spare time building gadgets from components that he imports directly from Hong Kong. He recently bought an L-39 Albatros 50mm EDF remote controlled aircraft and mounted two imported mini video cameras on the front and back of the aircraft to capture the front and rear facing views. He made a short movie (see below) of a bird?s eye video taken of our offices and surrounding area in Stellenbosch, South Africa.

I was very impressed by the quality so I asked him about the cameras. Apparently these mini cameras aren?t very expensive as they are basically reconfigured cell phones without the keypad and RF components and are mass produced in +- one million batches. The only drawback is that due to the nature of the cell phone industry the components change all the time so in 6 months one wouldn?t be able to get hold of those specific cameras any more.

Here’s the view taken from the ground, http://www.youtube.com/user/megatesla#p/a/u/2/pyGd3DKDlD0. Note how fast the aircraft is moving!

Author: Robert Kellerman

## Tapered coax to parallel wire transition with 100:1 performance bandwidth

October 19th, 2010

Tapered coax to parallel wire transition

While designing an antenna, one of our engineers needed a transition from a coaxial transmission line to a parallel wire transmission line. Because he needed a balun with wideband impedance transformation he had a look at the baluns described in the balun article in Antenna Magus. (You can read more about this article in Newsletter 2.1). The Marchant balun would have been an overkill so he chose the tapered coaxial balun and looked at various tapers to find a good impedance transformation across the band.

The tapered coax to parallel wire transition transforms a lower coax impedance to a higher impedance of a parallel wire transmission line. It also operates as a balun while obtaining ultra wide bandwidths depending on which type of continuous taper is used.

The outer of the coaxial cable is gradually removed along the length of the transition, until a parallel wire transmission line is realised. The exact taper to which the outer is cut, will determine its performance across the band.

The graph below shows comparisons of normalized reflection coefficients between different Klopfenstein tapers and (an easier to manufacture) linear taper. The low, medium and high Klopfenstein tapers refer to the matching level designed for and the higher (or tighter) the spec, the harder it becomes to manufacture the taper. With the right equipment, it is possible to achieve maximum performance bandwidths of 100:1!

Comparing S11 for various tapers

Author: Robert Kellerman

## Microwave holography – Calibrating radio telescopes from space

October 7th, 2010

I recently read an interesting article on Microwave holography ? a method used to do high precision calibration of large (> 30 m) radio telescopes. The maximum frequency of operation is determined by how well the surface can be calibrated. Microwave holography, as applied to reflector antennas, is a technique that utilizes the Fourier transform relation between the complex farfield radiation pattern of an antenna and the complex aperture distribution [1]. The far-field amplitude and phase response of the antenna is measured by a geosynchronous satellite doing a high resolution raster-scan. This method can be compared to optical holography where both the light intensity and depth (or phase information) is recorded which gives the image a life-like perception.

The captured far-field data is used to calculate a surface error map which is used to adjust (or calibrate) the individual panels in an overall reflector. ?Here are some examples of improved performance taken from the reference article: ?At Ka band, by improving the root-mean-squared offset error of the surface relative to that of a ?perfect? dish from 0.67mm to 0.25mm, the gain is improved by 3dB (i.e. you get double the signal!) ?This is independent of the dish size ? and on a 34m diameter dish, improving the accuracy from 0.67 to 0.25mm is such a miniscule change, it is impressive that it has such an effect! These big dishes seem real simple, but are high-precision engineering on a grand scale.

The image below is processed far-field data captured by the Microwave holography technique. The red and blue colors represent regions that are deformed by a constant value of +- 0.2mm, respectively where the green color represents a perfect dish surface.

Microwave holography example of reflector surface error.

Reference:
[1] Microwave Antenna Holography by David J Rochblatt, Chapter 8.

Author: Robert Kellerman

## Antenna Magus Version 2.3 released!

October 7th, 2010

To all our blog subscribers and those of you who might not be aware, we recently launched Antenna Magus 2.3.0. This update features 10 new antennas, as opposed to the usual 6. Extensions have been made that allow Antenna Magus to export models comprising multiple files where needed. This feature enables Antenna Magus to export models that use impressed excitations defined in secondary files. You can read more about this release in the latest Newsletter 2.3

Author: Robert Kellerman

## Investigating the Feature Selective Validation (FSV) technique

October 1st, 2010

One of our blog subscribers asked us why we still eyeball results for comparison, rather than using the Feature Selective Validation (FSV) technique. A good question indeed, and to be honest, one that we had never thought about! It is amazingly easy to do the things in the way in which you are accustomed?. (a.k.a the old way!) FSV looked like an elegant technique that could simplify one of the most common tasks we undertake, so I decided to do a bit of investigating.

For those in the dark about FSV ? it is a technique that attempts to compare two sets of data points (usually traces on a graph), and classify the comparison as either ?Excellent?,?Very Good?, ?Good?,?Fair?,?Poor? or ?Very Poor? – where the same categorisation would be given, on average, by a group of experts. It has been around for some time, and research has been done to test it. The method has shown great promise in EMC – in fact, it has been adopted in IEEE standard 1597.1 ?IEEE standard for Validation of Computational Electromagnetics Computer Modeling and Simulations?.

In contrast to the EMC data typically used for FSV validation, antenna data is visually a lot simpler. I decided to test FSV by doing two FSV comparisons on the simulated and measured data of the skeletal wire monocone from an earlier blog post. In the first test, I used FSV to compare the measured and simulated results. In the second, I manipulated the simulation data to remove the cone resonance in the upper region of the band by smoothing the data in this region. I used the software tool published by Antonio Orlandi at the University of L?Aquila to do the FSV calculations for me.

Three data series used for testing the FSV technique.

Unfortunately it appears that the FSV technique fails for this example ? both comparisons result in the validation being considered ?Fair?, with a neglibigle difference in the figure of goodness and confidence histograms. When modelling this antenna, the quality of the model is directly proportional to how accurately it can predict the unwanted cone resonance. As an antenna engineer (with some knowledge of the structure in question, what is important and what I am looking for), I would call the first comparison ?Good? and the other ?Poor? ? a two category jump!

FSV does seem to be a great tool for automatic validation ? especially for visually complex data that is typical of EMC measurements and when the person doing the comparison doesn?t have prior knowledge of what they should be looking for. We won?t make the call based on one test, but I don?t think that we will adopt FSV for our purposes any time soon. We do know prior information, which will lead to more intelligent comparisons, and more intelligent modelling adjustments.

Here are two screenshots from the software tool used for comparison between measured data and original simulated data (first image) and comparison between measured data and smoothed data (second image).

Screenshot for comparison between measured and original simulated data.

Screenshot for comparison between measured data and smoothed data.

Author: Sam Clarke

## Stairway to heaven

September 16th, 2010

I?m not generally afraid of heights but after seeing this clip of an antenna technician climbing a 1768 foot tower, I much rather prefer designing antennas in my office than assembling them!

Click here for an explanation where this video came from (and why the above Video link may stop working!)

Author: Robert Kellerman

## Prof Hidetsugu Yagi ? a true legend

September 9th, 2010

Prof Hidetsugu Yagi

In every era in almost every category one will find geniuses, like the Mozart?s of classical music, the Einstein?s of science and the Michelangelo?s of art. In the last 100 years there have been a few remarkable people whose contributions laid the foundations that would shape the antenna industry as we know it today. One of these antenna legends is a Japanese professor, Hidetsugu Yagi, well known as the father of the Yagi antenna.

I recently discovered a re-publication paying homage to a paper published by Mr Yagi in 1928 titled, ?BEAM TRANSMISSION OF ULTRA SHORT WAVES?. [Reference: James. E. Brittain, ?Yagi on a Microwave Communication System?, PROCEEDINGS OF THE IEEE. VOL. 72, NO. 5, M A Y 1984]. It was fascinating to see the discoveries they made almost a century ago long before the existence of transistors. Prof Yagi and his student Shintaro Uda (hence the name ?Yagi-Uda antenna?) discovered that positioning an element having a slightly higher natural frequency next to a single driven element acts like a director shaping the radiation pattern to be more directive where a slightly lower resonant frequency element acts like a reflector. Up until then most experiments had to be done at > 100 cm wavelengths (lower than 300 MHz) simply because they couldn?t produce stable oscillations at higher frequencies. A quote from the same paper: ?Mr. K. Okabe, assistant professor at the Tohoku Imperial University has succeeded in generating exceedingly short sustained waves by introducing certain modifications in the so-called magnetron.? Could this be the first microwave signal generator?

These guys didn?t have the luxury of ordering components and connectors from a catalogue, design and build something and plug it in to a signal generator, simply turning the knob to the preferred frequency. They probably had to start preparing for a measurement by making the cable!

As Isaac Newton once famously remarked in a letter to his rival Robert Hooke in 1676:

“If I have seen a little further it is by standing on the shoulders of Giants.”

How many things do we – as ?modern? antenna engineers – take for granted, when infact we are truly standing on the shoulders of Giants?

Author: Robert Kellerman