Steven S. Holland

This is, I think, our third or fourth snow day of the 2007-2008 school year.  It seems like it has been a long time since the weather has been this snowy.  Back in 1995-1996 or so, I recall having days off of grammar school due to it simply being too cold (-5F, etc), and then maybe a few times throughout high school.  So it has been a while.  Then again, not having any classes this semester means nothing really changes for me — I still spend today writing more of my thesis….just as I would be anyway. 

I have been using this book for the last 3 years as another source of some electromagnetic and antenna theory. It is an excellent book that is always being revised (similar to the electromagnetics book I posted about here) and is available in free pdf files. I highly recommend it to anyone looking for a good EM/Antenna reference.

…and you would have a population equal to all humans who have ever lived throughout history.

This came up in conversation the other day with a friend of mine, and was an estimate I was unaware and, to be honest, is incredible to think about.  The estimate says that of the entire human population throughout history, from all eras – 5.6% are alive today.   It makes sense when you think about the 6 billion population total as of today, but it’s still something that’s hard to wrap your brain around.  It’s essentially a product of exponential growth, although it is hard to know how exactly population growth has gone, as this article states, and plateaus of population numbers or sharp increases aren’t exactly accounted for.  All and all, it admits it is a guess, but it is fairly conservative, and seems like a plausible guess.

EDIT: The pcitures were huge and were getting cutoff, so I resized them quickly only to make them look really bad, and hard to read, so instead now they are thumbnails — click on them to see full size images.

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I came across this article on an antenna intended for MIMO (multiple input, multiple output) applications, that I feel is impressive from the standpoint of small size, but not for the dual feed system it purports as being revolutionary. The antenna is from the company Skymap, and the product is their iMAT antenna .

Now, for MIMO systems you want to be able to used different bands or spatial regions within which to send or receive signals, since this allows capacity of the system to be increased – for example, allocating different spatial regions for different channels allows the reuse of the same frequency band. A good example of this is the cell phone network — by dividing up geographical regions into “cells” with their own set of antennas, the same freqeuncy bands can be used at the same time by callers in nonadjacent cells, in contrast to having one large powerful transmitter cover all of the area.

Anyway, a lot of articles I find talk up the discovery that you could use two feeds on a single antenna to tap into either different polarizations or frequency tuning by placing feeds at voltage nulls of the excited mode of the other probe. This is a bit misleading, however, since this idea is not new, and has shown up in many types of antennas. Since I have spent a lot of time working with microstrip antennas, I threw together a quick example of an antenna that utilizes two feeds to tune to 2 separate frequencies, and each frequency also has a different polarization – this was just done to show two separate polarizations and two different tunings. With microstrip (patch) antennas, you can also tune multiple frequencies on the same polarization, or multiple polarizations on the same frequency, etc. Below is an image of the antenna, drawn up in Ansoft HFSS.

This is a simple rectangular patch on 30mil duroid substrate(relative permittivity of 2.2) that is 5cmX4cm, and is fed by coaxial probes from underneath along each axis, as shown below:

The center conductor of each probe is brought up through the ground plane and is terminated at the patch. They are positioned to match the antenna impedance to 50Ohms, and the coaxial cable feeding the antenna has a 50Ohm impedance. Each probe is along the center line of the rectangular patch, which places it at the voltage null for the mode the other probe is exciting. As a result, each probe can’t really “see” the other probe, since the other probe is located at a virtual short, and so each probe acts as though it is exciting it’s own antenna. This reduces coupling between probes and ensures each probe excites the dominant mode of the patch along it’s dimension and will have a linear polarization with very low crosspolarization (which can arise from asymmetric feeding under the patch). Below is a diagram showing how the field polarization and frequencies are orientated along the orthogonal x and y axis:

It should be noted that the x and y polarizations are themselves orthogonal, and would allow individual polarizations to send signals on the same frequency, and the separate frequencies could allow multiple channels on the same polarization. Anyway, just to show some technical specs, below is the Return loss of the antenna:

The red curve shows the response of the X axis coax probe, and the purple shows the Y axis coax probe. A return loss of -10dB is a match of 2:1 VSWR, which is a typical bandwidth spec, giving each band 10MHz and 18 MHz of bandwidth. What this shows is that each probe only tunes at one frequency, severly reducing coupling between the probes, which was below -30dB in this simulation (although isn’t shown). Shown below are the impedance loci on the smith chart:

Next art the gain patterns for each band, first the 1.962GHz band and then the 2.422GHz band.

Here Brown is the E-plane pattern, and Red is the H-plane pattern , with crosspol shown as the small circles down below -25dB, which is typical for a patch antenna.

Here Blue is the E-plane pattern, and Purple is the H-plane.

The bottom line is that this example is very simple, and has been exploited for lots of applications where dual frequency, dual polarization, etc, are desired. So feeding a single antenna with two feeds is not new as these articles make it sound. Don’t get me wrong, the antenna in the article is impressive, and is very small and has great performance, but I think they are wrong in saying this is a new idea.

There is a reason devices have a third prong: Dell’s new laptops don’t have a third prong (an earth ground). As the article mentions, users are reporting feeling slight shocks when touching the brushed aluminum casing…which is possible due to the absence of the third prong, which normally grounds the outer casing appliances (those with metal cases) or exposed metal pieces on an appliance. Without the earth ground, the AC power is converted to DC just fine, but the DC voltage is “floating” with respect to earth ground. For example, the AC mains could be converting to, say, 20Vdc, but this is simply 20Vdc between the plus and minus output of the power supply, and the minus side of the supply can be any voltage and is not necessarily the same as the earth ground potential. Normally one of two things happens: 1.) the minus side of the DC supply is connected to earth ground, or 2.) if no electrical connections are made to the outer casing, the outer case is connected to earth ground. This is done to ensure that anything the user touches will be at the same potential as the user, and no shock would result. If the case was not brushed aluminum, and instead plastic, the user would notice no difference, except when plugging in peripherals.

What I wonder is what the grounding of the case looks like. If the case has no electrical connections at all, and the case is developing a static charge that is discharging (which can develop thousands of volts – dragging your feet on the carpet and touching a door handle can easily build up tends of thousands of volts), there is no danger from these shocks, other than an annoyance. If, instead, the case is actually used as the ground reference for the computer, then this can potentially cause harmful shocks since power can be drawn out of the computer supply when the potential difference between the case and the earth ground is shunted through a user.

Today was the 50th anniversary of the obligatory children’s toy – LEGOs! The official LEGO site has information about it: LEGO. Also, a LEGO Timeline, which has these interesting facts:

• There are about 62 LEGO bricks for every one of the world’s 6 billion inhabitants.

• Children around the world spend 5 billion hours a year playing with LEGO bricks.

• More than 400 million people around the world have played with LEGO bricks.

• LEGO bricks are available in 53 different colors.

• 19 billion LEGO elements are produced every year.

• 2.16 million LEGO elements are molded every hour, or 36,000 per minute.

• More than 400 billion LEGO bricks have been produced since 1949.

• Two eight-stud LEGO bricks of the same color can be combined in 24 different ways.

• Three eight-stud bricks can be combined in 1,060 ways.

• There are more than 915 million combinations possible for six 2 x 4 LEGO bricks of the same color.

• 7 LEGO sets are sold by retailers every second around the world.

• The LEGO bricks sold in one year would circle the world 5 times.

• 40 billion LEGO bricks stacked on top of one another would connect the earth with the moon.

• LEGO bricks are so much more than just toys. They are used in classrooms from preschool to university level to teach everything from math, language skills and science to engineering and technology principles.

• The LEGO brick has inspired generations of innovators, like Jonathan Gay, inventor of Flash.

• World-renowned author Douglas Coupland believes the LEGO brick represents a “language in itself.”

Google had this awesome main page all day:

LEGOs were my favorite toy as a child — there are a number of those rubbermaid tubs in my parent’s basement full of LEGOs. My brothers and I spend a lot of time build lego cities with those road base plates, and it was the greatest toy of my childhood. The best part was — we had a lot of parts, and it’s not like parts go obsolete. I think most of my LEGOs were passes down, save for a few technics sets — I should find one of my cousins, or my nephew, who would enjoy getting our old LEGOs.

But I am always jealous that they didn’t have the Mindstorms LEGO sets when I was little — with those sets you can program microprocessors into your LEGO projects…

I came across a page detailing the past year in the life of the Hubble Telescope, and what’s in store for it’s future: Nasa’s news website on Hubble. It aslo talked, however, about Google Sky, which is a google earth feature that allows you to look at the sky in addition to what is on the earth. This is really neat, I am currently downloading Google earth to play with this a bit.

Has anyone used this before?

I found online a downloadable, free book on electromagnetic field theory, more from an electrodynamics standpoint (physics).  So far I have only skimmed through but it gives treatment of spectral analysis of an arbitrary source, radiation problems, relativistic electrodynamics, etc.  Even more important, is that it is an ongoing project — people are welcome (infact, highly encouraged) to submit ideas/work to be included in the book or to find errors in it’s current form.  This provides the benefit of at least some sort of checks and balances on what is in the book, in terms of people reading it and submitting feedback, and also it is a collaborative effort to provide a completely free source for this knowledge, which they remind us should be free always.

Welcome to my website. I am using a blog format since this is easy to post to quickly, and lends itself to the postings of an assortment of things, rather than having to organize pages coherently on a webpage, I can instead post and label it and allow users to sort by topic. As a result I will be changing my actual website soon to essentially function as a resume linked off of this site, since I never got around to making pages for my research or side projects…this will be that function.

So what can you expect to see here?

Topics

This blog is going to be basically a technical hodge-podge of things — articles I find interesting, code I am working on, simulations I have run that I found interesting. It is unlikely i will post anything directly research related, like results, since that wouldn’t bode well for trying to publish papers and such, but various things that I work on in my free time will be posted. It is my hope that this will be a place to post things and get feedback on, in the best of cases hopefully to help someone looking for information on a topic, and in the worst case simply forming as a log of my activity.

EDIT: I brought over posts I salvaged from my old blog, hence the large number of posts today!

Briefcase that “changed the world”

This BBC article tells the stort of the journey of one very important microwave component delivered to America during World War 2 –the magnetron.

During the war engineers in England developed a high power microwave source, but not having the money or industrial support to develop this new source, they brought it to the US. They gave it to the US — no catch, so long as they provided financial and industrial support. The source was developed and led to airborn radar as it was powerful and lightweight compared to the other sources available at the time.

Why should anyone care? Because it allowed radar to be exploited during the war. Oh, and it is also the high power source tucked in the back of every microwave oven. So next time you make popcorn or one of those breakfast bundle things with the sausage on the stick surrounded by a pancake….speaking in a British accent is encouraged.