## Archive for November, 2010

### Antenna made from salt water!

Friday, November 26th, 2010

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

Thursday, 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?

Wednesday, 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

Friday, November 5th, 2010