Archive for the ‘Antenna related’ Category

Blog follow up – Have we reached the tip of the ice berg?

Wednesday, May 25th, 2011

A while ago I blogged on finding a simple antenna solution for radiating down a tunnel in both directions and that one of our engineers already came up with the perfect solution. Interestingly enough one of our blog readers came up with exactly the same idea – a dual bore-sight horn. He actually created a FEKO model and calculated the gain patterns shown below. Our engineer replied, “I see? yeah, that is pretty much spot on, although I tweaked mine a bit to have a square flare and reduced length.” So he claimed that it can be improved.

FEKO model preview

FEKO model preview

Calculated gain pattern

Calculated gain pattern


However shortly after that one of our blog readers actually came up with an even simpler idea – a back-to-back axial mode helix using the same ground plane, fed by a power divider. The blog reader who originaly told us about his problem really liked that idea and decided to go for this option. The photo below is his first prototype and he is currently busy optimising his design for better performance and simpler manufacturing.

Back-to-back axial mode helix prototype

Back-to-back axial mode helix prototype

Thank you to everyone who responded with their ideas. In the end we could help our engineer friend to make an excellent choice!

Author: Robert Kellerman

Have we reached the tip of the ice berg?

Monday, February 14th, 2011

iceberg

With almost 160 antennas in the Antenna Magus database we have heard people saying, ?Are you still planning on adding more antennas to Magus or do you think you?ve almost reached the tip of the ice berg?? and our response has always been, “we’re not even close”.

Recently one of our blog readers was looking for an antenna with about 10 dB gain in two opposite directions, for use in a tunnel (Narrow band). Even though there are so many antennas in Antenna Magus, the only bi-directional ones are the spirals. The spirals don?t have the gain, and require baluns, which add to manufacturing costs. Of course he could use an array, but this is going to be too big, and the feed network adds a lot of manufacturing complexity. This just proves that there are still more antennas that need to be included in our database!

I thought this is actually a nice opportunity for a challenge. Lets see if anyone can come up with a better (but simpler) type of antenna to achieve this (pattern below). Actually one of our antenna engineers came up with a great idea which I will share with you in an upcoming blog.

More or less the pattern he is looking for.

More or less the pattern he is looking for.

Author: Robert Kellerman

Antenna Magus Evaluation version corner reflector improves home internet reception by 12 dBi.

Monday, January 17th, 2011

900 MHz corner reflector designed in Antenna Magus

900 MHz corner reflector designed in Antenna Magus

I was quite chuffed when I saw this post on mybroadband about a corner reflector that was designed using the free Antenna Magus Evaluation version.

He explains how he designed and built a corner reflector from an old printer box and aluminium foil. It is used to boost the 900 MHz UMTS link between his home 3G modem and internet base station which increased the signal by 12.5 dB!

Obviously this will save a lot of money compared to commercial options which will cost over 150 US$ for a > 10 dB gain antenna and coupler which still has to be mounted on the roof (see http://www.poyntingdirect.co.za/c22/Antennas.aspx). I just don?t know how I will convince my wife that we have to place this beautiful foil covered box in the living room (coincidently the position of best reception).

Wife: ?Under no circumstances! Our internet works fine.?
Me: ?I know it is not an oil painting honey but this will greatly improve our internet speed?and we save 150 dollars!?

Author: Robert Kellerman

Antenna made from salt water!

Friday, November 26th, 2010
Sea water antenna

Sea water antenna

I love it when engineers think outside the box. When asked to design an antenna that can be mechanically adjusted for different frequencies most engineers would think of metal structures with some sort of adjustable length (like that ancient TV antenna which I blogged about some months ago). But who would think of making an antenna from salt water? When someone told me about it my first reaction was, no ways, this is probably some joke or something, but it’s not. Follow this link or watch the video clip below for a detailed explanation. The principle is quite simple – a salt water fountain with variable height acts as the resonant structure and a magnetic current probe picks up the induced fields from the water stream positioned in the center of the probe which is connected to a receiver.

I’m sure we’re going to see more of these salt water fountain antennas in the future. Can I have one for my iphone please!

Author: Robert Kellerman

The isotropic radiator

Thursday, November 18th, 2010

head

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:

Isotropic radiator, A hypothetical, lossless antenna having equal radiation intensity in all directions.
Note: An isotropic radiator represents a convenient reference for expressing the directive properties of actual antennas.

Omnidirectional antenna, An antenna having an essentially non-directional pattern in a given plane of the antenna and a directional pattern in any orthogonal plane.

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

NASA radio telescope photography at Goldstone, Mojave Desert

Friday, November 5th, 2010
230 ft NASA radio telescope.

230 ft NASA radio telescope.

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

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

Tuesday, October 19th, 2010

Tapered coax to parallel wire transition

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

Comparing S11 for various tapers

Author: Robert Kellerman

Microwave holography – Calibrating radio telescopes from space

Thursday, 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.

Microwave holography example of reflector surface error.

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

Author: Robert Kellerman

Investigating the Feature Selective Validation (FSV) technique

Friday, 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.

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).

GDM software screenshot  for comparison between measured data and original simulated data.

Screenshot for comparison between measured and original simulated data.


  GDM software screenshot for comparison between measured data and smoothed data.

Screenshot for comparison between measured data and smoothed data.

Author: Sam Clarke

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Stairway to heaven

Thursday, 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!

Amazing tower climb

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

Author: Robert Kellerman