Archive for the ‘Antenna related’ Category

The Marchand balun is not square

Friday, June 4th, 2010
Marchand balun image from original published article.

Marchand balun image from original published article.

I recently had the privilege of seeing the inside of a real vintage Marchand Balun. One of the Professors at Stellenbosch University lent one to us that he built in the 70s so we could see how it looks (from the outside). Even seeing just the outside was quite informative as all Marchant balun sketches found in papers and text books makes it seem like these baluns are square (as shown in the image above, taken from the original paper “N. Marchand, Transmission-line conversion transformers, Electronics, vol. 17, December 1944, pp. 142-145. (Reprinted in J.L.B. Walker et al.”) but they’re not. They’re rectangular like the one shown in the image below.

Like with most engineers, curiosity often overrides the desire for preservation. So while I was busy writing this blog entry, a couple of the engineers in the office unsoldered, cut off the semi rigid and opened up the cavity to have a look inside. These baluns are actually quite impressive as they have very wide operation bands from 2:1 up to 10:1.

Real life Marchand balun

Real life Marchand balun

Author: Robert Kellerman

70% of published papers are a waste of time.

Thursday, May 27th, 2010

Most published antenna related articles are a waste of time when it comes to verifying the findings. When confronted with an unfamiliar antenna the first step is to find reliable published research and the problem is not finding papers. It’s like doing a Google search with a popular key phrase and trying to get the information you’re actually looking for. I remember a while ago one of our engineers was struggling with an antenna and in desperation he cried out and threw his hands in the air. Immediately the Marketing guy (that being me) said, “why don’t you just use Antenna Magus?”. Everyone in the office burst out laughing because he was busy working on a new antenna that still needed to be implemented.

Very few articles contain sufficient information in order to reproduce new findings. Surprisingly these are all papers that have been peer reviewed (sort of like an engineering stamp of approval) but most of them lack critical information like a physical parameter or material property or measurement distance. I’ve often wondered if it’s due to the authors negligence or is it done on purpose?

Here’s an example of an excellent reference article found by one of our engineers while implementing the Rectangular Capacitive-disc-fed Patch (which should be in version 2.1) G. Mayhew-Ridgers, J.W. Odendaal, and J. Joubert, “Efficient full-wave modeling of patch antenna arrays with new single-layer capacitive feed probes,” IEEE Transactions on Antennas and Propagation, Vol 53, No. 10, 2005, pp. 3219-3228.

It took him 10 minutes to figure out how to design and model the reference antenna design and reproduced the results in less than an hour.

Fortunately for our users Antenna Magus has all good published references listed in each antennas information document.

Looking for the needle in the haystack.

Looking for the needle in the haystack.

Author: Robert Kellerman

When measured and simulated results do not match?

Thursday, May 20th, 2010

Most engineers at some stage get that sinking feeling when their measured and simulated data do not match, especially after they’ve tried and tested everything to find the root of the problem. Some engineers just give up and write off the discrepancies due to cable/connector loss or some phase shift that is not accounted for in the simulation.

One of our engineers recently had a similar occurrence with the validation of the Yagi-Uda Monopole Array with Folded Feed antenna soon to be implemented in the Antenna Magus database. He manufactured the reference design in order to have a trusted measurement to use in the validation process. Unfortunately the impedance measurements and simulated results had some differences especially around the resonant frequency. After examining and comparing the parameters of the manufactured antenna with the simulation model using a vernier he found that the brazing rods weren’t exactly 3.2mm as specified and the thickness varied as well. Not expecting these differences to have a huge effect, the simulation model was modified to take these into account and surprisingly the new simulated results almost matched the measurements. Later one of the other engineers confessed that the rods fell off his motorcycle three times on his way back to the office.

So next time before blaming the computer or the network analyser have a closer look at the built model – you might be simulating something that doesn’t exist.

Simulation model and manufactured model of the antenna before any modifications.

Simulation model (a) and manufactured model (b) of the Monopole Yagi with folded feed array antenna before any modifications.

Comparison of S11 for measured vs original and new simulated results.

Comparison of S11 for the measured vs old and new simulation models.

Author: Robert Kellerman

New Coffee espresso antenna

Thursday, April 1st, 2010
New coffee espresso antenna

New coffee espresso antenna

Who would ever imagine ground coffee could make an excellent dielectric? We recently did some research and experimented with different, cheaper dielectric materials.? One of our engineers suggested that we should investigate ground coffee. We originally thought it was a ridiculous idea but after some research on the electrical properties of coffee, we did a few simulations and surprisingly found that a standard double espresso group packed with ground coffee makes an excellent cavity and dielectric for a circular patch antenna in the 3.5 to 4.5 GHz range. The image above is a photo of the new Circular coffee espresso antenna. Note how conveniently the cable fits through one of the spouts.

We have been experimenting with different types of coffees and so far it seems like Arabica coffees with a lot of flavor (like the ones mentioned in my previous roasting coffee blog) give the best performance. The image below shows the new coffee antenna with a typical gain pattern at 4 GHz.

Look out for the new Coffee espresso antenna soon to be included in the Antenna Magus database!

Coffee antenna gain pattern

Coffee antenna gain pattern

Author: Robert Kellerman

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

Interesting tunable TV antenna (2)

Wednesday, February 24th, 2010

This is probably one of the most interesting but impractical antennas I’ve come across. It might have been very useful in the days when TV had only one or two channels but how on earth would men be able to channel hop when each time you first have to get up and tune your antenna? Maybe in those days they did’?t know how to design wideband UHF antennas with > 6 dB gain?

However, it is still a very fascinating idea – having a tunable antenna, so after some investigation I think I might have figured out how it works.

I made a model and simulated the antenna with different tuning positions and the resonant frequency increased as I increased the tuning stub (similar to pulling it upward) as shown in the graph below.

S11 vs Frequency for different tuning lengths. The frequency increased as the antenna arms were shortened.

S11 vs Frequency for different tuning lengths. The frequency increased as the antenna arms were shortened.

Radiation pattern of the antenna tuned to 600 MHz with maximum gain of 6.2 dBi.

Radiation pattern of the antenna tuned to 600 MHz with maximum gain of 6.2 dBi.

The gain of the antenna is +- 6 dB as shown in the image above. This is quite high for a folded dipole but it seems like a very clever cross between a folded dipole and a V-dipole. Another interesting observation was that almost no currents flowed on the tuning stub which means it has almost no effect on radiation.

The length of the loop is about 3 lambda at the resonant frequency thus instead of using a standard folded dipole with one wave length circumference it operates in a different resonant mode with a V-dipole-like angle making it more directive.

If I had no choice but to use this antenna I would have probably built a remote control stepping motor to tune the antenna to the right frequency on each channel hop. Maybe I must still do this and invite a couple of friends over to come watch TV and channel hop at my house!

Read the previous interesting tunable TV antenna post here.

Author: Robert Kellerman

Interesting tunable TV antenna

Wednesday, February 17th, 2010
Interesting tunable TV antenna

Interesting tunable TV antenna

One of the engineers found this antenna lying in his garage. We had a quick chat about how it radiates and it turned out to be quite an interesting discussion. Someone said it’s a loop antenna, others said it’s a folded dipole. Another guy said the manufacturers probably didn’t have a clue how the thing works. They just took a bunny ear and told the manufacturers to make it tunable. So I thought lets blog about it and see what you (our readers) think.

The antenna is fed at the base as showed below.

Feed of the tunable TV antenna

Feed of the tunable TV antenna

TV antenna being tuned

TV antenna being tuned

The image above shows how the tuning part pulls in and out which seems like it’s a short circuited transmission line that varies the admittance while changing the resonant length of the antenna. Below is an equivalent circuit diagram of the antenna.

Equivalent circuit diagram.

Equivalent circuit diagram.

Please comment if you think you have a better idea how it works.

Author: Robert Kellerman

SKA – single element overview

Thursday, February 11th, 2010
Single Meerkat radio telescope element

Single Meerkat radio telescope element

The image above shows a single telescope forming part of the Meerkat project which is planned to form part of the future +- 500 telescope Square Kilometer Array.

The feed consists of two dual-polarised horn antennas, five conical deflector cones (where future horns could be inserted), cooling components, LNA’s and RF- to-fibre modulators. The struts are also designed to maintain strict tolerances on the feed position relative to the parabolic reflecting surface.

The 12m dish is made of a resin-foam composite and has a parabolic shape with 0.38 F/D ratio. It has to maintain its shape within millimeter tolerances and remain an almost perfect electromagnetic reflector while withstanding harsh environmental conditions like temperature variations, rain and strong winds etc.

The dish mount is an altitude-azimuth system and rests on a concrete pedestal. Another challenge is constantly stabilising the telescope, compensating for external forces like wind, gravity or metal expansion (and contraction due to large temperature changes) which can pull the whole structure off target or cause unwanted spillover.

In the next few blog entries I will discuss these interesting and potentially challenging topics in a bit more detail.

Author: Robert Kellerman