Archive for December, 2009

Square Kilometer Array (1)

Wednesday, December 23rd, 2009
Artistic impression of SKA

Artists impression of SKA

I just had the most interesting conversation with one of the antenna engineers currently working on what is going to be the largest Radio astronomy telescope the world has ever seen, the Square Kilometer Array (SKA). There is so much that can be said about this topic so I decided to break it up into a few blog posts.

The image above is an artists impression of the planned telescope which will consist of a few thousand reflector antennas forming a huge array with a +- 0.01 degree beamwidth, The array will be constantly scanning the sky, ?listening? to whatever signals are coming from outer space. The data collected from each fraction of a degree will be processed to produce ultra-high resolution images of objects in space like in the ones shown below – these are images of the famous Crab Nebula taken by the Palomar radio telescope (left) and the optical Hubble Space telescope (right).

Crab Nebula taken by Palomar radio telescope (left) and Hubble Space telescope (right).

Crab Nebula taken by Palomar radio telescope (left) and Hubble Space telescope (right).

The main drive behind SKA is for scientific researchers to learn what lies beyond the borders of our galaxy, to study dark energy, dark matter and to try and solve the mysterys and questions like what happened ?in the beginning?.

South Africa and Australia are in the run to host the SKA and have started working on smaller demonstration projects like Meerkat and ASKAP. Both these countries?have great locations available for the SKA construction and the final decision of who the host country will beis primarily a political one (the announcement of the host country will be in 2011). Should South Africa win the draw, the center of the array – where most of the antennas will be located is in the Great Karoo (a semi-desert region well known in radio astronomy circles as a ?quiet? spot ? in terms of RF noise) and will stretch over nearly all countries in Africa, south of the equator.

Here are some of the tight specifications for each reflector antenna in the array:

  • 2:1 bandwidth
  • 1 degree main beamwidth
  • practically no backlobe (< 30 dB)

and

Each 2 ton antenna must be kept physically stable despite changes in the elements (Sun, Rain, Wind etc.) to reduce possible noise that might reflect from the ground.

There are numerous other challenges like cooling down the feed, creating affordable vacuums, manufacturing tolerances, data processing etc. which I will touch on in future posts.

Feed of the Meerkat reflector antenna.

Feed of the Meerkat reflector antenna.


12m radio telescope reflector antenna in the Karoo, South Africa.

12m radio telescope reflector antenna in the Karoo, South Africa.

Author: Robert Kellerman

Microstrip-Franklin array contradiction

Thursday, December 17th, 2009
Microstrip-Franklin array antenna with zoomed insert

Microstrip-Franklin array antenna with zoomed insert

The image above shows the layout of a printed microstrip Franklin array antenna as well as a zoomed in section focusing on a single element of the array (microstrip line with phasing stub). We are busy implementing this antenna in Antenna Magus and are trying to understand the physics behind how the antenna radiates.

The best reference we could find on this antenna is a paper by Klaus Solbach, ?Microstrip-Franklin antenna?, IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. AP-30, NO. 4, JULY 1982 but it is not clear how this antenna radiates. The Solback paper refers to the ?slot? formed by the phasing stubs (as indicated in the image below) as the radiating element, but the equation defining the radiation resistance, Rs, seems like it is referring only to the microstip line section of each element of the antenna. This suggests a microstrip analogy with the wire Franklin array, which is in effect a linear array of sequentially-fed dipole elements. So ? is this antenna an array of slots, or is it an array of sequentially-fed quarter-wave microstrip dipole elements?

The slot formed by the phasing stubs

The slot formed by the phasing stubs

If anyone has insight on this matter which you are willing to contribute please contact us!

Author: Robert Kellerman

Version 1.5 released!

Thursday, December 10th, 2009

To all our blog subscribers and those of you who might not be aware, we recently launched Antenna Magus 1.5.0. Antenna Magus now has more than 100 antennas in the database!

Version 1.5 includes 6 exciting new antennas and you can read more about?this release?in the latest Newsletter 1.5.

Newsletter 1.5 preview

Newsletter 1.5 preview

Author: Robert Kellerman

Making your own Bi-Quad (Wi-Fi) antenna

Tuesday, December 8th, 2009

The Bi-Quad antenna is one of the handiest, cheapest and easiest to build WiFi antennas.

This blog entry will explain how to easily build one like in the picture below using cheap materials and a TNC-RP (reverse polarity tiny-N) low cost Wi-Fi router connector.

Bi-Quad antenna with Wi-Fi router connector.

Bi-Quad antenna with Wi-Fi router connector.

The antenna in the above image was designed using Antenna Magus for the following design objectives:

  • Center Frequency: 2.45 GHz.
  • Gain: 10.5 dBi
  • Bandwidth: 20% (500 MHz)
  • Feed: Coaxial feed with TNC-RP connector (typically used in Wi-Fi routers.)

Here is a diagram of the antenna with physical parameters:

Top view of the Bi-Quad antenna. Click for a 1:1 scale view.

Top view of the Bi-Quad antenna. Click for a 1:1 scale view.

Front view of the Bi-Quad. Click for a 1:1 scale view.

Front view of the Bi-Quad. Click for a 1:1 scale view.

(Note that if you click on the above diagrams they will open as 1:1 scaled versions. I decided to make them available if you want to manufacture this antenna yourself. The easiest is to print them to 100% of the original size and use these as footprints/templates to construct the antenna.)

Manufacturing process explained in 6 steps:

Step1: Cut 2 pieces of 2 mm copper brazing wire and bend to follow the profile of the printed diagram. Solder the two pieces of copper wire to form the Bi-quad?s radiating structure as illustrated in the image below.

Step 2: Bend and solder copper wire.

Step 1: Bend and solder copper wire.

Step 2: Measure and cut the ground plane to the same size as shown on the printed footprint and drill a 6mm hole in the center – I used a piece of off-cut metal that I got from a local canned fruit factory for my ground plane.

Step 2: Measure and cut the ground plane.

Step 2: Measure and cut the ground plane.

Step 3: Cut a copper spacer with a length of 30.6mm and inner diameter of > 5.5 mm. This is only to support the Bi-quad copper wire and feed. The most important requirement is that the coaxial cable needs to fit through the middle – varying the thickness by a couple of millimetres should not greatly affect the antenna?s performance. I used a copper pipe with 7.8mm outer diameter and 5.7mm inner diameter which I got from a refrigerator repair shop for R 30/meter (<5 US$/meter).

Step 3: Cut copper pipe.

Step 3: Cut copper pipe.

By now you should have the following components:

Bi-Quad components

Bi-Quad components

Step 4: Solder the copper pipe to the ground plane. Note that both pieces should be sanded to have rough surfaces and preheated in order for the solder to take easily. This can be quite tricky as the copper is a good heat conductor and cools down very quickly. (We used two soldering irons ? one standing upright through the middle of the ground plane with the piece of copper on top over the solder point. This ensured constant heat to both objects and the other solder was used on the outside to apply the soldering agent.) Here is the result:

Step 4: Solder copper pipe onto the ground plane.

Step 4: Solder copper pipe onto the ground plane.

Step 5: Solder the coaxial cable?s braid to the top of the copper pipe. The easiest is to fold it slightly over and to solder it to the outside of the copper pipe as shown in the following images.

Step 4: Solder copper pipe onto ground.

Step 5: Solder coax outer conductor to the copper pipe.

Step 6 (final step): The final step is to connect the copper wire to the feed section. The following image shows the complete antenna with the male TNC-RP (reverse polarity tiny-N) connector. These connectors are very popular and used with wireless Wi-Fi routers. They are easy to obtain and cost only a few dollars.

Bi-Quad antenna with Wi-Fi router connector.

Bi-Quad antenna with Wi-Fi router connector.

Bi-Quad antenna connected to a Wi-Fi router.

Bi-Quad antenna connected to a Wi-Fi router.

Measurement vs simulation

The following graph shows the measured |S11| compared with the simulation.

S11 Measurement vs Simulation

S11 Measurement vs Simulation

The measurement shows that the antenna is well matched over the desired band (- 20 dBi) but has an unwanted ripple on the S11 measurement. I only realised the cable and connector that I got from an old Wi-Fi antenna (also designed for 50 ohm) actually uses a 75 ohm cable which caused unwanted reflections between the connector and the radiating copper wire. I ordered a new cable and will update the post as soon as I replaced the old with the new cable.

I hope this blog entry will be useful for those of you wanting to improve your Wi-Fi reception at home!

Author: Robert Kellerman

Google vs Magus

Tuesday, December 1st, 2009

Magus vs Google

How many of you can remember doing anything on the internet before Google? Google was founded by Larry Page and Sergey Brin and was first incorporated as a privately held company on September 4, 1998. That is only 11 years and two months ago! I cannot imagine using the internet without Google but there was a time when libraries were the only place where knowledge could be acquired.

I want to draw a few parallels between Google and Antenna Magus.

Google Antenna Magus
The first internet search engine of its kind. The first antenna design tool of its kind.
The easiest way to find information on the internet. The easiest way to find information on antennas.?
Founded by a small group of IT geeks. Founded by a small group of antenna geeks.
Has the largest database of information in the world Has the largest database of antenna designs in the world
Changed the way people use the internet Is?changing the way people design antennas

It took a couple of years before Google became well known and widely used. That applies to anything new ? people need to use it first to realize how much they like it and eventually realize they can?t live without it. Antenna Magus is still in the first phase where people are discovering how useful it is but there are already some users who are loving it!

Here is a quote from one of them:

Through Antenna Magus I’ve been exposed to a number of antennas that I knew nothing about! In addition Antenna Magus has given me a good feel for what they are good for, physical insight into how they work and perform, a grip on how to design them, and where to find the key references.


Author: Robert Kellerman