Archive for September, 2010

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

Prof Hidetsugu Yagi ? a true legend

Thursday, September 9th, 2010

Prof Hidetsugu Yagi

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

Validating the Skeletal wire biconical antenna

Thursday, September 2nd, 2010

We recently investigated the ?Skeletal wire biconical antenna? (often refered to as the wire biconical) as a useful EMC antenna for Antenna Magus. As part of the validation process we wanted to compare the simulation data with measured data but we faced a slight problem. In order to balance the feed of the physical antenna a balun is required which has to be placed in the center between the two cones, physically separating them. This cannot be accounted for in the simulation as it has to be fed using a single port. We wanted the physical model and the simulation model to be as similar as possible. Another alternative that we considered was to feed the antenna from the side through the center of one of the arms – but that introduced other uncertainties and non-symmetry issues.

Image of the Skeletal wire biconical antenna

Image of the Skeletal wire biconical antenna

Eventually we decided to build a wire monocone using the same dimensions as the biconical antenna, replacing one of the cones with a ground plane. Skeletal wire biconical antennas are usually designed to operate between 30 ? 300 MHz and are used in EMC applications but we decided to frequency scale the design by 3 times in order to reduce it to a more practical size. Once we finished building the wire cone section, we relised that in spite of our efforts to reduce the antenna?s size we still needed a +- 3m diameter ground plate if we wanted to simulate an infinite ground. Someone came up with a clever plan to instead of using a solid metal ground to use six 10m wires which are placed in the symmetrical plane perpendicular to each element of the cone. This would have the same effect as using a solid ground plane. The following picture shows the final antenna being measured on the roof of the Stellenbosch engineering faculty.

A few risked their hands to help with constructing the antenna while I?m holding the blow torch. : )

A few risked their hands to help with constructing the antenna while I?m holding the blow torch : )


Rooftop measurement with the 10m wires running along the roof.

Rooftop measurement with the 10m wires running along the roof.

The conditions weren?t ideal as it was pouring with rain while we had to do the measurement. We were quite surprised how well the 6-wire ground plane worked! Perhaps the wet roof had enhancing ground plane effects?

The graph below shows measured and simulated s-parameters of the antenna. It was interesting to see the resonance at 850 MHz is not a computational discontinuity but is picked up by the measurement as well. Some investigation shows that the resonance can be moved out of band by adding a short between one of each cone?s wires and the center wire.

S-parameter comparison of the Skeletal monocone antenna.

S-parameter comparison of the Skeletal monocone antenna.

Author: Robert Kellerman

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