Archive for March, 2010

Freezing cold antenna in the desert (SKA 4)

Thursday, March 25th, 2010
Sun beating down on KAT radio telescope

Sun beating down on KAT radio telescope

Imagine you are in the middle of the desert with the scorching sun beating down on you with no shade-cover, no water, no electricity and temperatures rising above 50 degrees Celsius? This is more or less the circumstances under which the elements of the Karroo Array Telescope (KAT) have to operate. The only difference is that the antenna feed needs to be cooled down to 20K ( -253 deg C), which is potentially a difference of more than 300 degrees. The cooling of the feed is necessary to reduce total RF noise in the system as every 7 degrees adds 0.1 dB RF noise.

I spoke with one of the project engineers who said that the easy part is to cool the feed to 50 K. They’re using a Gifford-McMahon helium gas cooler for the first cooling stage which can absorb up to 50 Watt of heat energy. The tricky part is cooling it down from 50 to 20 K. This has to be done inside a vacuum otherwise the smoke and dust particles would freeze and clog up the cavity. This creates another challenge, because gasses behave differently under very low temperatures and particles don’t collide as usual. This is overcome by using an ion pump cooling system which uses a strong magnetic field to positively charge all the particles, accelerating them to sputter against the cathode plates and extract the molecules to form a vacuum. See http://www.gammavacuum.com/operation.asp for a more detailed, graphic explanation.

To view previous blogs on the SKA project, please click here.

Author: Robert Kellerman

Charging your phone with ambient electromagnetic radiation ? they can?t be serious!

Friday, March 19th, 2010

Iphone charging

Apparently Nokia claimed that by 2013 it will be possible to charge your cell phone from the ambient electromagnetic radiation (AER) emitted by Wi-Fi transmitters, mobile phone antennas and TV masts. This sounds amazing! Who wouldn’t want a phone that never needs to be charged? But, when making the sums I can’t see how they’re going to pull this one off.

Most cell phones consume about 2W of peak power during a conversation that works on a +- 20% duty cycle. This means that to talk for one minute your phone needs about 6.6 mW-h of battery power.

Typically the signal levels of radio, TV and cell phones are quite low unless you stand right in front of the base station antenna. I just spoke to a friend of mine who is an expert on base station safety and compliance. Their company has done over 65 000 measurements for Vodacom (a large service provider in South Africa) and most of their measurements show that the total signal strength in public areas is more than 10 000 times below the ICNIRP safety compliance standard. Thus the maximum AER power in most places is less than 1 mW/m2.

If we assume the surface of the antenna is the same size as the phone (+- 40 cm2) and receives all the power that hits the antenna (i.e. 100% absorption, no loss) it will receive 0.4% of 1 mW which is equal to 4 uW.

This means you have to charge your phone for 1650 hours (almost 70 days) to be able to talk for one minute!

Here’s another interesting blog article, http://boingboing.net/2010/01/12/rcas-wifi-power-harv.html where they determined that it would take 34.5 years to charge a Blackberry when holding it 5 feet from a Wi-Fi router.

Author: Robert Kellerman

Traveling wave slotted guide array ? with frequency dependant squint angle

Friday, March 12th, 2010
Generic image of a Traveling wave slotted guide array antenna

Generic image of a Traveling wave slotted guide array antenna

We recently added the Traveling wave slotted guide array to the Antenna Magus database. Compared with the previously added Resonant waveguide slot array, I must say I was impressed and amazed with some of the Traveling wave antenna?s advantages. It can handle higher power and operates over a much wider band (up to 25% bandwidth).

The slots of the Traveling wave slotted guide array are spaced equally on either side of the guide center moving further apart towards the load. Where the spacing is less than ?g/2 (and ?g is the wavelength in the waveguide), the beam squints towards the source, while a larger spacing results in a squint towards the load. Because the progressive phase shift between slots changes with frequency, the squint angle changes with frequency.

The image below shows the gain patterns of a 100 slot Traveling wave slotted guide array designed at 7 GHz, simulated over a wide frequency range. Note a total of 7 degrees shift in squint angle over a 10% change in center frequency. Being very popular for tracking radar applications, this could create a problem. For example when using a frequency sweep while tracking a small moving target the transmitted signal is constantly looking in different angles. I guess knowing the frequency of the returned signal; one can determine the squint angle and then calculate the exact position of the target? Maybe this can actually be seen as a useful ?feature? of this antenna.

Showing squint angle change for different frequencies.

Showing squint angle change for different frequencies.


Typical S11 of the Traveling wave slotted guide array

Typical S11 of the Traveling wave slotted guide array


Author: Robert Kellerman

Antenna towers ? ideal for base jumping

Friday, March 5th, 2010

Who would have thought that antennas and extreme sports have something in common? In the past two decades, base jumping has become more and more popular and apparently high antenna towers make excellent launch platforms. The sport evolved so that base jumpers can also wear special jumpsuits (or wingsuits) that shapes the human body into an airfoil which can create lift and allows them to travel horizontally at ridiculous speeds.

Base jumpers who live in the city far away from cliffs and mountains often have to make use of manmade structures to satisfy their need for an adrenalin rush. Usually jumping off buildings is illegal and jumpers mostly have to jump at night, facing the risk of being arrested. Since 2001 Kuala Lumpur launched an annual international event where jumpers from all over the world take part in jumping off the famous KL tower. This allows base jumpers to do several consecutive jumps during bright daylight with no police handcuffing them at their landing spots.

I?m wondering how many base jumpers become technicians (or maybe vice versa) who service antennas just to get access to ideal base jumping destinations? Has anyone taught them about ICNIRP compliance and the dangers of RF overexposure?

Free falling next to KL tower.

Free falling next to KL tower.


Author: Robert Kellerman