Friday
May252012

Congratulations Space-X!

Brought to you by..... antennas.  Turning RF signals into electrical currents for over 100 years.

 

Good Work Space-X!

 

 

Friday
Sep302011

An SBIR Success Story and a Win for the Warfighter!

U.S. Military Special Forces Buy Unique Satellite Receive Suite From Nashua-Based Windmill International, Inc.

Windmill International, Inc. announced that it has received a $9 million order for their KA-10 Suitcase Portable Receive Suite (SPRS) for Central Command Special Forces in Afghanistan.  The order included KA-10s, training, and product support. Windmill's KA-10 SPRS is a highly-portable, rugged satellite receiver system developed to support Special Operations forces deployed overseas.  The battle-ready KA-10 conveniently brings crucial command center information and data to the in-field warfighter, substantially improving mission success probabilities and saving lives. [...more]

 

In September of 2004, Mr. David Martin (Windmill International, Inc.) and I, along with our AFRL sponsor Mr. John Turtle briefed the Special Operations Command on the results of our SBIR Phase I and Phase II program.  The briefing was entitled "AFRL SBIR contract to develop lightweight GBS Rx antenna for Special Forces".  The press release above is the culmination of that briefing and the result of hard work by a fantastic engineering team that simply did not understand that it couldn't be done.

Dave Martin (R) and myself (L) at Wahiawa, HI demonstrating the "Iron Maiden" in July 2004

In the picture above, our prototype weighed 65 lbs., but replaced about 400 lbs. of equipment, and ran on batteries.  The then-current system is shown in the background; it is a 1-meter dish antenna.  The "Iron Maiden" was the precursor of the new KA-10 system which weighs about 40 lbs. 

The working prototype KA-10, with Dave and I on our way to a demo in Washington DC, December 2005

The original goal of the system was to be sized for airline carry-on.  It struck us that we had achieved our goal when we were on a commuter jet bound for DC from Manchester NH.  (Photo by Dave Martin)

Upon arriving at Dulles, we were reminded by the Arrivals Board that while the hardware may be working perfectly, there were things that could still go wrong.  One challenge was that it was FREEZING in DC when we got there, and had to go to the mall to buy thermal underwear for the demo.  We demonstrated the unit on Pearl Harbor Day, December 7th, 2006.


The system worked perfectly, though the demo gremlins were quite active.  We had overcome magnetic anomolies, loose hardware, and a temperature-related sensor failure.  All these things were taken back to the engineering team as lessons learned and made the ultimate product better.  And, yes, we blamed some problems on software... unfairly.  Sorry, Dan.

There are many, many people to thank for having worked on this project, and I have not asked any of them permission to use their names.  But to them I say THANK YOU!!  It's been a wild ride.



At MILCOM 2006

(U.S. Patent 7,889,144 and other U.S. and International Patents both issued and pending.)


Friday
Aug122011

Antennuators


You can't win.
You can't break even.
You can't even get out of the game.

And playing the game is not optional.

-with apologies to C. P. Snow

 

Breaking the laws of physics is a popular pastime. Who would not want to get a thousand miles per gallon, lose weight by taking a pill, or receive a fortune in cash from a Nigerian prince by email? But, most of us understand it just ain't going to happen. And so it is with antennas. Previously, I wrote about the tradeoffs associated with antennas. The most contentious corner of our tradeoff triangle seems to be size; it's the easiest to measure, and the one in most visible conflict with other aspects of wireless product design.

You want your antenna small, admit it.

Small antennas have been around a long time. Remember pagers? They most often operated in the VHF band, where wavelengths were on the order of two meters. Yet, they were tiny (although the volume of cell phones is approaching what later model pagers were) and the antennas they contained were even tinier. The facts that enabled their good performance were: narrow frequency range and receive-only operation. You can have small size if you give up bandwidth and efficiency. No problem. For many years, AM radios operated from 510-1800 KHz (with wavelengths in the hundreds of meters) using a ferrite loopstick at the core of their antenna; narrow bandwidth, low efficiency, and completely successful.

But, these days who wants to give up bandwidth? If size is the easiest thing to measure, then impedance bandwidth is a close second. An RF network analyzer can measure impedance bandwidth in a few milliseconds, and generate a curve which can be put in a data sheet, or used to compare to an existing data sheet. So, let's make a small antenna with wide bandwidth!! If we're lucky, nobody will ask about the efficiency (which is pretty hard to measure, but easy to experience).

And so, some companies manufacture Antennuators. These are a cross between antennas and attenuators. Between good and evil. Between protein and carbohydrates. Between Apple and Microsoft. Let's radiate some energy and burn some energy as heat. That way, it's kind of usable, and the measured impedance bandwidth is glorious!

There are several ways to build an Antennuator. The easiest method is to build it with lossy materials in the parts that carry RF current. One example of such a material is stainless steel. Stainless steel is a rather poor conductor, but its nice to look at and resists corrosion. There are many whip antenna made from stainless steel, and often the environmental considerations outweigh the loss from the material selection. And in certain antenna types, which are inherently high impedance (such as the Kraus Helical) the difference in material losses will be insubstantial. But, when antennas are made physically small the currents can get rather high, and this is where lossy materials will rear their ugly head.

A few years ago I was working for a company that manufactured various specialized receivers and transmitters operating in the VHF, UHF and low-microwave range. I was the in-house antenna engineer as well as a product designer. A good friend of mine, John D., came to my office with a question. He ran the test department which had the responsibility of tuning and testing all the equipment before shipment. A group of "tactical repeaters" were being tested, and one was on his bench transmitting a few watts of RF at VHF into a six- or eight-inch "rubber duck" antenna (a helically wound short monopole).

"Spence, should the antennas get warm?"

I turned and looked at him incredulously. After careful consideration I answered with a incisive question of my own, "HUH?!!?"

"After about five minutes the antenna is warm, and I don't remember this happening before. Is it normal?" John asked.

"Hell, no!" was my measured response which began a careful investigation into this Antennuator.

I verified that, indeed, the antenna was warming up substantially, and that it was not some other portion of the system making heat and warming up the antenna. After some digging, John and I determined that there was a recent vendor change from vendor "C" to vendor "A". I grabbed some examples of the parts from each vendor and started cutting them open (this is a recurring theme, as you will see, Dear Reader). The helical conductor from vendor "C" was copper-colored, and that from vendor "A" was stainless-steel colored. Hmmmm.

Then I took scraps of the plastic that encased each antenna and carefully tested the dielectric loss using an UHF RF Thermal Conversion Exposure Cavity. Yep. . . a microwave oven. The difference in thermal dissapation between the two plastics was very significant: after about 15-20 seconds of exposure (with a cup of water at the opposite corner of the microwave for loading) the plastic from vendor "A" got hot whereas that from vendor "C" was not noticably warmer -- lossy dielectric confirmed. (While this test was conducted at about 2450 MHz, it is still indicative of losses at VHF.)

The Antennuator from vendor "A" looked great on the RF network analyzer, and even better in the purchasing department's scorecard. But, it was burning precious (battery powered) RF in the process. I immediately wrote an ECO (Engineering Change Order) eliminating vendor "A" as a supplier, and specifically naming vendor "C" as the supplier for this particular part. Problem solved. Purchasing had been credited for saving money on cheaper antennas, and Engineering looks evil throwing out inventory. The Earth continued turning on its axis.

More recently, one of my clients brought me a product from a Serious Defense Contractor, which was labelled "Antenna, Broadband, 50-2000 MHz". It was about twelve inches long, about 5/8-inch in diameter, with a BNC male connector on one end. It was flexible, black, and no doubt expensive. Since my client was taking a training class, I immediately used it as an example of an Antennuator. I started a discussion with the class as to how a twelve-inch long antenna can be rated for operation from 50-2000 MHz. After a healthy amount of discussion, one of the students put the antenna on an RF network analyzer and swept it from 50-2000 MHz, measuring the input impedance.

"It looks good to me!" he said, showing a respectable "knot" in the middle of the Smith Chart, indicating a VSWR of less than about 2:1 (ref. 50-ohms) over the range.

I said, "You'd think so, but remember the engineering rule: if you can't fix it, at least you can break it!". I extended the low end of the measurement down to 2 MHz. And do you know what we saw? It still had a VSWR of about 2:1. . . . but, it shouldn't have. It SHOULD have looked lousy at 2 MHz. Now think, what has a VSWR of 2:1 at 2-, 50- and 2000-MHz? That's right -- a RESISTOR!

I made a bold (and risky) statement to the class: "There's a 100-ohm resistor in parallel with the connector.", I confidently proclaimed, hoping like hell nobody would call me on it. Well, I was with a group that did not see any impediment to cutting open the antenna and finding out Right Now. And so, they did.

I was wrong. There were two, 200-ohm resistors (each rated 3-watts) in parallel across the connector. As I see it, when RF current flows in a conductor the only thing it can make is radiation, but when RF current flows in a resistor the only thing it can make is heat. So, the ONLY possible reasons those resistors were there were to make the transmitter happy over that frequecy range (no spurious oscillation, no VSWR alarms), to make the datasheet look magnificent (and meet procurement requirements), or to prevent the buildup of ice. Unfortunately, my clients are professional communicators and in need of their equipment actually... you know... communicating, and they were already acutely aware of the inefficiency of this product. Putting "50-2000 MHz" on the body of an antenna does not make it so. Notice the efficiency was not stated on the same placard. Pity.

Once I designed an antenna for a small surveillance product that had a resistor in it. The product was battery operated, and had a very high efficiency transmitter. Sometimes, however, if the antenna came too close to a conductor, it presented the transmitter with a very low impedance that caused a spurious oscillation. This was Very Bad in the intended application. I ended up using a small series resistor in the feed loop of the electrically small loop antenna. The cost of this resistor was a fraction of a dB of transmitted signal in normal operation, but it prevented the spurious oscillation completely and the associated loss of signal. This was a carefully weighed decision, it solved the problem, and the efficiency cost was calculated and acceptable. The problem could have been solved further upstream in the power amplifier, and there would have been an efficiency cost there, too.

Resistors turn current into heat. That's what they're supposed to do, and thank goodness they do it so well. But, when you find 'em in antennas, it's worth asking why they're there.

Sometimes, it's useful to burn undesired energy of the wrong polarization as in a Terminated Bifilar Antenna or a Log Spiral Antenna, for example. In that case, polarization purity (axial ratio) is more valuable than efficiency. That is an engineering decision, and a good one.

Sometimes, it's critical to keep the transmitter happy and invest in a bit of heat to do so.

Sometimes, it's to make the datasheet look miraculous and score a big order from the government.

Perhaps someone should ask the professional communicators if the solution to their very real problems is an Antennuator. I think not.

 

Saturday
Jul302011

The Antenna Tradeoff Triangle

Dusty sign seen on the wall in Ye Olde Machine Shop:

You can have it:

    Fast
    Accurate
    Cheap

Pick any two.

Every professional pursuit has its tradeoffs which must be managed.  In fact, I believe that it is the principal function of the engineer to manage tradeoffs.  We want airplanes to be strong, but light and affordable.  We want our favorite restaurant to be inexpensive, tasty and prompt.  We want our politicians to be honest, responsive and effective (OK... it's just theoretical). These competing desires are what we call the Tradeoff Triangle.  Sometimes the number of parameters we need to balance exceeds three, but for the purposes of our discussion today, the number shall be three... no more, no less.  Three shall be the number of thine tradeoffs, and the number of the tradeoffs shall be three. Four shalt thou not consider, neither ponder thou two, excepting that thou then proceed to three. Five is right out.

But, I digress.

Let's explore what the job of an antenna is, and where its tradeoffs can be found and thence managed.  And we are going to assume that antennas are reciprocal.  That means they can make radiation from RF current (transmitting), and they can make RF current from radiation (receiving).  In any wireless device, there is a receiver section, a transmitter section or both (transceiver).  These functional blocks are designed by a clever and talented RF guy, and generally interface to the antenna via a transmission line of a certain characteristic impedance; the most familiar values for this impedance are 50- and 75-ohms.  (The reason for the existance of these two values is a good subject for a future blog entry.  Anyone know the history of these choices?)

Usually, I hate it when someone tells me the punchline before I hear the joke.  Sorry, but here it is: Your antenna can be wideband, small or efficient.  BANDWIDTH, SIZE, EFFICIENCY.  Pick any two.  It is a sure sign of Antenna Snake Oil when you see tiny, wideband antennas boasting ultra-high efficiency.  Run the other way.  OK, let's take a closer look...

Antennas operate over limited bands of frequencies.  Sometimes these bands are smaller than we wish they were.  A useful way to think about bandwidths is called "fractional bandwidth" (FBW); for our purposes we'll define fractional bandwidth as the high frequency divided by the low frequency.

For example, modern cell phones generally require antennas that operate from 806 to 915 MHz (FBW=1.14 or 14%) AND 1710 to 1990 MHz (FBW=1.17 or 17%).  This covers all the GSM bands as well as the PCS bands.  Another familiar band is the 2.4 GHz ISM band which is where WiFi lives; this band is 2.4 to 2.5 GHz (FBW=1.04 or 4%).  Yet another example is the 900 MHz ISM band which is often used for wireless phones and other household and office devices; this band is 902 to 928 MHz (FBW=1.03 or 3%).  And finally, we are all familiar with GPS which needs about 10 MHz of bandwidth centered around 1575 MHz (FBW=1.006 or less than 1%).

So, antennas for each of these applications need to operate over the entirety of these bands.  This property, which is the first of our three tradeoffs is loosely called BANDWIDTH.  In the four examples above, note that the fractional bandwidth is representative of how "hard" it is to meet this requirement in light of our (soon to be illuminated) other tradeoffs.  GPS seems easy, and "Quad Band GSM" seems hard.  And so they are.  

Now, using the term "bandwidth" without any further qualification is Engineering Blasphemy (see also my rants about the use of "dB" without a reference).  The bandwidth of an antenna is completely dependent upon what is relevant to the application.  For cellphone applications, it may be the "efficiency bandwidth" or that bandwidth over which the total radiated power (TRP) or the total isotropic sensitivity (TIS) is north of a required value.  For GPS we may be bandwidth-limited by the axial ratio, or the quality of the circular polarization (RHCP in the case of GPS).

Frequently, the bandwidth of concern is the impedance bandwidth, which is the bandwidth over which the antenna's impedance remains within a certain "distance" (on the Smith Chart) of the ideal impedance.  Often this is expressed as Return Loss (10 dB is the usual minimum value), or VSWR (Voltage Standing Wave Ratio) where 2:1 is the usual limit.  If someone uses the term "antenna bandwidth" without explicity saying which bandwidth they are referring to, it is probably the impedance bandwith.  And thereafter they shall be scolded.

The second tradeoff in our triangle is SIZE.  There's different ways to think about  size.  You care about physical size when you are trying to stuff ten pounds of stuff in a five pound bag: you want your consumer product to be as small as possible and the industrial designer has graciously given you a volume which would not host most DNA molecules.  The antenna designer is thinking in terms of wavelengths.  As the antenna volume starts becoming a smaller and smaller fraction of a wavelength, the impedance bandwidth starts shrinking, and the ability to remain efficient with real-world materials starts disappearing.  

In December 1948, Lan Jen Chu published the paper "Physical Limitations of Omni-Directional Antennas", in which he derived a theoretical formula of the bandwidth of an electrically-small antenna.  In the interest of circumnavigating a soporific vortex, the conclusion is thus: the smaller the antenna, the narrower the bandwidth.  So there.

EFFICIENCY is the measure of how much of your RF power is going to be radiated, and how much is going to be turned into heat.  Assuming your goal is not de-icing, heat is an undesireable byproduct.  With real-world materials, especially as we shrink antennas, this becomes a significant concern.  A side-effect of shrinking the antenna is causing the antenna's RF currents to become large enough to make the radiation happen.  These high currents make the material losses which were previously ignorable a very real concern.  I have designed electrically-small loop antennas which have a radiation resistance (the good "resistance") measured in milliohms.  Suddenly the fact that the conductor is copper as opposed to aluminum becomes really important.  The dielectric materials used in trimmer and fixed capacitors for resonating such antennas become critical.

While it is pretty clear that losses in conductors and dielectrics are undesireable from an efficiency standpoint, there lurks in the shadows a side effect as enticing as it is detrimental.  These efficiency-robbing losses make the impedance bandwidth appear larger.  In fact, the higher the losses the wider the impedance bandwidth until the limit where ALL the energy is dissipated in losses and the bandwidth seems "infinite".  The ultimate example is a 50-ohm terminator: a perfect match over a huge bandwidth... and zero radiation.  The Dummy-Load Antenna.  The lesson is clear: When presented with an antenna with unexpectedly large bandwidth for its size, ask about the efficiency.  Oftentimes, this line of questioning is met with a stunned silence at best, or a complete change of topic at worst.  There is a tiny fraction of antenna companies operating today whose business plans depend upon your failing to inquire about efficiency.  I'll say it again: About the Efficiency -  Ask!

The product designer, antenna designer, industrial designer and marketing professional together must all cooperatively grapple with the antenna tradeoff triangle: BANDWIDTH, SIZE, EFFICIENCY.

Like it or not... Pick two.

Friday
Jul222011

Demystifying Antennas

´╗┐Since my last blog post I have been working on multiple projects with multiple, wonderful clients.

Most significantly, I taught a training course on antennas over the course of two weeks.  Eighty hours of training.  If you've ever done training, that may sound exhausting.  And you'd be right.  But, boy, was it rewarding!

Getting opportunities to teach what I have been learning over the last sixteen years is, for me, a lot of fun.  I have trained groups from the commercial, government and law-enforcement arenas.  The topic of antennas is often intimidating for those that use them, but don't design them.  It's seen as super-technical and borderline black-magic.  Perhaps it's because RF and antennas seem to the uninitiated what Einstein called "spukhafte Fernwirkung" or "spooky action at a distance".  You can't see it, but it works.  As one antenna guy put it, "If RF was visible, we'd be out of business."

So, in order to train people how antennas work, you have to make the RF visible.  And for two weeks, that's what we did.

We started with some physical demonstrations which explain how the dipole antenna works; and we make the claim that the dipole is the simplest resonant antenna you can construct.  We spend some quality time watching water slosh back and forth in carefully constructed clear tubes.  Then, once people have spent time developing a mental model for how the electrons slosh back and forth in a dipole (and have personally done the sloshing), they "get" resonance, radiation, and even impedance.

Next we get out a telescoping dipole antenna.  The very same one we use as a standard in the AntennaSys lab.  Add an RF network analyzer, a video projector and the strategic "laying on of the hands" and you have the next "ah-hah!" moment.   The students start connecting their simplified physical analogy to the actual electrical system they are playing with.  They can't see the electrons "sloshing", but now they believe it's happening based upon the real-time information the network analyzer is giving them.  Now, they start believing they can maybe understand a little of this stuff.

Then we predict what would happen if we.....

    shorten the antenna,

    lengthen the antenna,

    feed the antenna at a point offset from the center...

And then we back up their predictions and observations with computer simulations that introduce color-coded graphics which further confirm their observations.

Finally, we started actually building antennas.  Everyone in the class made their own dipole.  All materials came from the hardware store.  Everyone communicated over radios between their hand-made dipoles.

Suddenly..... magic happens.  The antenna is no longer a wonderful mystery.  Wonderful, yes.  Mystery, no.

We introduce more complex antennas: helicals, Yagis, corner reflectors, parabolic reflectors and more.  Add a healthy dose of decoupling, polarization, directivity, transmission lines, Smith Charts, link budgets, multipath and a slew of client-specific topics and the week goes by pretty fast.

When you take a person from the "antenna as mysterious object" phase to the "it's all about getting the slosh right", you've created a better antenna user, a better product engineer, a better systems engineer, a better communications professional.

And the spukhafte Fernwirkung ain't quite as spooky as it was. 

 

(Thank you Steve and Corinne for helping AntennaSys deliver a great course.  A special THANK YOU to Agilent and Anritsu for providing the RF Network Analyzers used in my course!)