After Hours

RF interference: Just how important are antennas

Being able to co-locate multiple RF systems within millimeters of each other in modern wireless devices is in large part due to reducing RF interference. Many of these "interference-busting" innovations are the result of engineers making improvements to antenna systems.

I'm sure everyone has had the pleasure of their 802.11 network randomly dropping connections. I might even guess that a cordless phone or microwave caused some of the aggravation. Learning that one device's useful Radio Frequency (RF) signals can be another device's RF interference is an important lesson.

Pehaps your smartphone was one of the devices that lost its connection when someone used the cordless phone. Now wait a minute, how's that possible? The smartphone is crammed full of radios, and it works just fine. Come to think of it, so does a wireless notebook. The fact that Bluetooth, WiMAX, 3G, and Wi-Fi can politely coexist in the same device is indeed amazing. How's it possible to create a friendly RF environment when all those radios are so close to each other?

New antenna technology to the rescue

Being able to co-locate multiple RF systems within millimeters of each other in modern wireless devices is in large part due to reducing RF interference. Many of these "interference-busting" innovations are the result of RF engineers making improvements to antenna systems in the following critical areas:

Antenna efficiency, considered the most important parameter, is the ratio of radiated power to input power. The percentage is a measure of the antenna's dissipative losses, which are due mainly to mismatched Electromagnetic Field (EMF) characteristics. New materials and engineering expertise have significantly improved this percentage. I remember being happy with 20-30 percent efficiency. Today, engineers are looking at a minimum of 50-60 percent efficiency. This is especially important with wireless devices getting smaller and the demand for longer battery life getting bigger. RF isolation is all about antenna coupling and is a good thing when antenna coupling occurs between radios that are supposed to be communicating. Conversely, inadvertent antenna coupling between radios that aren't trying to communicate diverts valuable RF energy to the wrong receiver. For example, if the multiple antennas of an 802.11n device are not properly isolated, a portion of the RF signal meant for a remote device may be captured by another of its own antennas. A normal rule of thumb is to have 15-20 dB of isolation between adjacent antennas.

To further explain this concept, I'd suggest reading an excellent paper by Antenova and the University of London, "Antenna designs for MIMO systems" in which the researchers are trying to maximize RF isolation in different form factors. For example, the following excerpt (courtesy of Antenova) is the test fixture used to mimic possible locations for a MIMO antenna system in a typical notebook lid.

antenna.JPG

The graph below (also courtesy of Antenova) portrays the results from the tests run on the above fixture and the relationships created between the individual antennas.

isolation.JPG

As can be seen, there's definitely interaction between antennas, with the least amount of isolation or greatest antenna coupling occurring between antennas one and two.

Ruckus Wireless is another company that's doing some exciting work with respect to RF isolation by using beam-forming technology. I felt it important enough to write an article called "802.11n: MIMO really needs smart antennas." The following image from Ruckus Wireless depicts one of their beam-forming antenna designs that warp RF signals directionally, creating an optimal RF link with the remote device. This process has the added advantage of heightened RF signal rejection from unwanted sources.

beamflex-antenna.png

Antenna selectivity, or Q, is a critical parameter when a wireless device uses multiple radios. Q is a quantifiable number that signifies tuned bandwidth of an antenna. A lower Q will allow an antenna to have more bandwidth or a wider working frequency range. In many cases that's good, but in devices that have multiple radios, each antenna should be very selective or have a high Q, meaning the antenna is tuned to resonate at the design frequency of the radio it's attached to and that's all. By doing so, the antenna acts like a filter, rejecting unwanted RF signals that could overload the receiver. So what's it all mean?

Equipment developers are painfully aware of the fact that every design is a compromise. They make decisions on whether function, form, or performance is most important at every step of the process. My goal with this article is to promote awareness of that subjective process, as doing so allows everyone to decide on whether they agree with the designer's choice.

I doubt seriously if you'll easily find vendor literature written about this subject, but it's available if you're persistent. I ask for these specifications every time I read about a new phone. For instance, I happen to use the Samsung Blackjack II, and SkyCross developed the antenna systems for that particular phone. I've even written an article, "iMAT: MIMO without multiple antennas," about their use of metamaterials.

Final thoughts

Ultimately, I'd like to see antenna system performance become a key selling point rather than an afterthought or a never thought of. This would especially benefit purchasing departments in enterprise settings where lost time and money from inefficient wireless device operation becomes significant due to the sheer number of users.

About

Information is my field...Writing is my passion...Coupling the two is my mission.

28 comments
Jacky Howe
Jacky Howe

I once managed to pick up and tune in to another masthead that was directly in line with the one that I was trying to connect to. It was't picked up until we had some inclememnt weather. The same company was providing the Pay TV service through both mastheads.

seanferd
seanferd

I've missed these. Very cool.

fahroni
fahroni

please back again devilXXhantu I AM DONT CHEAT WHY ME THE BANNED

Allen Pitts
Allen Pitts

It is interesting to see how many of the engineers in this are also Amateur Radio operators or got their early, hands-on experience in Amateur Radio. Mike himself has a page on the new ARRL technoilogy campaign website at http://tinyurl.com/675m22 It is my hope to keep expanding the topics on this website by adding new pages about technologies in wireless communications and showing how hams are often involved the creativity even if it may not be directly a "ham radio" application. Thanks Mike !!! APitts@arrl.org

mjd420nova
mjd420nova

I have seen what is really being referenced here but perhaps it could best be described as a PHASED ARRAY. Directional receiving and transmiting both RF and audio signals with non-moving elements is essential in microwave and biomedical devices. One of the most important break-thrus in medicine has been the development of the GAMMAKNIFE devices that focus gamma ray beams that are harmless individually but very useful when focused at points within the body to do selective cell destruction to enhance cancer research and gene therapy. An application within my home is the effects caused by true surround sound using just four speakers. Focus points between elements create beat notes that are not present in the actual audio emmitted but have the presentation of sub-woofer events without having to have active sub-woofer elements present in the environment. In RF engineering, rhombic arrays fed using phase adjustments can achieve incredible ranges without using E2 layer bounces to reach beyond the horizon independent of atmospheric effects or to create ionization of various E layers to reach beyond normal ranges and using polarization elements can even reach around the world on only single and double skips regardless of the time of day or position of the sun.

SD ITman
SD ITman

Great article.. In our manufacturing facility I have a few dozen RF barcode scanners, a small wireless network inside my larger multi-building wired network and I'm just gathering information and quotes on a facilities HVAC control system for our buildings and one of the systems uses wireless temp sensors. I was concerned that all the radio frequencies could somehow interfere with each other but am assured that it won't. Each of the barcode scanners uses a different channel and I'm presuming that the temp sensors work the same way. I am not that knowledgeable in the down and dirty ways that all the wrieless systems communicate with each other and don't have time to sit through a half dozen tech manuals to dig into it. Reading your articles help me to get the information that I need without having to put a lot of time in research. I don't need to know all there is to know but I do need to have basic understanding. Thanks for all that you do....

speleg7
speleg7

Actually, Ruckus is not doing beam forming. It uses antenna selection between 6 directional antennas, which is different from "true" beam forming.

mjd420nova
mjd420nova

The antenna is the most important part of any wireless/cordless device. It doesn't matter how much power the transmitter has or how sensitve the receiver is, if the antenna isn't tuned properly or positioned correctly, it isn't going to acheive good communications. Proper placement makes up about 75% of the formula, height and unhindered placement can allow even the lowest powered units to reach long distances. Using parasitic elements to create directionality and block out interfering signals can also help to boost signal strength within a confined area and prevent multiple devices from interfering with each other during usage. Boosting range may be desirable but can also create interference with others and if not properly done will damage the transmitter stages of many devices. Improperly tuned antennas will also damage units and this also applies to the surrounding environment when working inside of facilities and buildings. Neon signs and displays can create enough harmonics that they can overload reciever stages and make many devices useless. This is desirable in some cases such as movie theathers, churches and court rooms but may not be apparent in some commerical areas such as retail stores and warehouses.

dawgit
dawgit

- You just keep them comming, don't you? Well I, for one, am glad of it. While most of that is still above my head, I just don't have enough time, or the the reason, anymore, I still learn from your articles. It's good that's there's still some tech here in TR. I really do like to, and need to, keep info like this fresh in that gray stuff that fills my head cavity. I don't know of any other source for this kind of info, compact and readable, outside of AARL. (IEEE is good, but I did say readable.) Thanks. -d

Michael Kassner
Michael Kassner

I'm sensing that I might be sharing comments with a fellow Ham, could it be? 73's

Michael Kassner
Michael Kassner

One other thing to consider is that the sensors are being polled by the PLC. If that's the case there shouldn't be any interference. As for the scanners, they are transmitting so infrequently that I don't see collisions as being a problem either. If you've the time and inclination, I'd appreciate an update on your system when you have it completed.

Michael Kassner
Michael Kassner

It's important to make sure we're on the same page about beamforming. This definition from Wiki is what most professionals understand beamforming to be: "Beamforming is a signal processing technique used in sensor arrays for directional signal transmission or reception. This spatial selectivity is achieved by using adaptive or fixed receive/transmit beampattern. The improvement compared with an omni-directional reception/transmission is known as the receive/transmit gain (or loss). Beamforming can be used for both radio or sound waves. It has found numerous applications in radar, sonar, seismology, wireless communications, radio astronomy, speech, and biomedicine. Adaptive beamforming is used to detect and estimate the signal-of-interest at the output of a sensor array by means of data-adaptive spatial filtering and interference rejection." This next definition from Spectrum Signal takes a slightly different approach and helps explain what Ruckus is trying to accomplish: "Beamforming is the combination of radio signals from a set of small non-directional antennas to simulate a large directional antenna. The simulated antenna can be pointed electronically, although the antenna does not physically move. In communications, beamforming is used to point an antenna at the signal source to reduce interference and improve communication quality. In direction finding applications, beamforming can be used to steer an antenna to determine the direction of the signal source." I also have linked a very interesting site by the University of Texas that has great information on beamforming as it applies to various portions of the EM spectrum: http://users.ece.utexas.edu/~allen/Beamforming/ Ruckus Wireless has BeamFlex which is their interpretation of beamforming and the following quote is from one of their white papers that describes their smart antenna systems: http://www.ruckuswireless.com/technology/whitepapers/ "Central to BeamFlex is an agile antenna system with multiple antenna elements that can be combined in real time to offer an exponential increase in diversity order. With N number of high-gain, directional antenna elements, a BeamFlex antenna array provides 2N-1 unique radiating patterns to maxi?mize range and coverage in a home. A Diversity Combiner composed of low cost, software-controlled circuitry allows the BeamFlex software to manage antenna combining in real time. The core of the BeamFlex software is an expert system that constantly learns the environment ? the RF conditions, communicating devices, network performance and application flows. A Path Control module selects optimum antenna com?binations on a per packet basis to ensure a quality signal path to each receiving device. The Transmission Control module sets the transmission policies including data rate and queuing strategy based on application and station knowledge. (See fig?ure 4). The BeamFlex software interfaces to the 802.11 MAC layer and is compatible with standard 802.11 chipsets. Resid?ing in the host processor, it adds minimal incremental CPU load and memory utilization." I could have defined beamforming in my own words, but my debate background suggests that does not prove a case, hence all of the definitions. I apologize if this is all understood. It is just critical to begin a discussion on the same page if you will. The two key components of beamforming are the sensor array (antennas) and the hardware/software/application algorithm that controls the RF. In my mind, beamforming is the signal processing intelligence that determines how the RF wave front needs to be warped to promote the optimal signal link with a remote device. All of the definitions refer to this process as well as Ruckus with their Diversity Combiner. I hope that I explained my position better and I would enjoy discussing this fascinating topic more if you're so inclined.

Michael Kassner
Michael Kassner

I agree totally and that's why I'm so impressed that the RF engineers can get all of this to work so well in such a small package.

Michael Kassner
Michael Kassner

I appreciate the comments immensely. If you have any subject matter that you'd like to see written about, please let me know. Also, perchance are you an amateur radio op?

RFink
RFink

From one ham to another. Don't let the call sign fool you, I'm a tech plus. Your thread reminded me that it's time to upgrade my ticket. I kept my novice call sign because I liked the last three letters. :) I use the words "Just Not Receiving" for contests and they work well for getting through pile ups. Now that there's the codeless General I'll go for it. I used to be active on 10m, field day and skywarn. Those days are gone besides in my six years of skywarn in Michigan I only saw one tornado. :( If I move to AZ I guess my call sign will become KB8JNR/7. That'll take getting used to. :) 73 Edited: Forgot the 73.

mjd420nova
mjd420nova

I've held many grades but presently not active due to work constraints and homeowner association rules against any outside antennas. Working strictly mobile is the pits and as yet haven't installed any equipment in my new "plastic" car. Was once on the logs as W6BZ, prior to that as WH6BZ and 6 years under maritime mobile as NVUV.

speleg7
speleg7

I totally agree that we can warp our minds (similar to the RF waves ;-)) for ever around the definition of beam forming. My only claim is that the beam forming technologies have different flavors, each with its complexity and gain. Antenna selection can't reach the optimal solution for both diversity and directional gain as an electronically generated beam. Beam forming from omni directional antennas require channel knowledge and allows much better channel condition tracking. In addition, such beam forming requires Phy layer algorithms and can not rely solely on MAC layer algorithms and RF switching and combining. BTW, Spectrum Signal's approach mentions combination of radio signals from a set of small >>non-directional

Michael Kassner
Michael Kassner

I'm spending a week in San Jose at Cisco for a TelePresence seminar and it's my first time here. (I've finally made it to Silicon Valley.) I can truly see why so many people want to live there. The climate and landscape is amazing. Warm but not hot like back in Minnesota. I guess I must be missing something.

The Scummy One
The Scummy One

I was thinking along those lines as well. AZ is just a state away, and I wouldnt need to deal with the BS that is California.

Michael Kassner
Michael Kassner

I'm not a water sports person either, I just like the amazing beauty of the Great Lakes. Sorry to hear about the economy there. I'm not sure if that kind of governmental official is specific to your location. I wish you the best in all of your endeavors.

RFink
RFink

I think so, I'm not a water sport person. It's mostly for my wife, she likes dry heat and Sedona. I don't like Michigan's lack of an economy and the clueless idiots leading the government. As long as I have a job, I'll stay. AZ is a layoff away. :)

Michael Kassner
Michael Kassner

Saint Clair Shores is a beautiful place. Are you going to be able to leave the big lakes area for deserts? Glad to hear from another ham. The ticket does not make the ham. I have many friends with Tech licenses and they can walk circles around me. I've just been doing it so long that the tests were a natural progression. Several of my club members are on SkyWarn as well and have been busy this year. I just wish there was more time for radio, as it is how I cut my teeth on technology. If I ever get back down to the lower bands, I'll keep a listen for your call. 73's

mjd420nova
mjd420nova

Would have been nice to be that close to home but I'd just about had enough of the cold weather. Can't imagine a winter aboard any ship in that cold country. The joke in the regular navy was that you had to be at least six foot tall to join the Coast Guard so that if your ship sank you could walk to shore. Things have changed a lot since then. My youngest son joined the Navy in '02 and served four years active duty aboard the Carl Vinson (CVN-70) for two tours to the gulf and was the primary air support carrier for Iraqi Freedom. After his discharge in '06 he tried to join the Coast Guard and they turned him down because he had tattoos. He's now in the California National Guard. The ship I was aboard was a new generation destroyer escort, 468 feet long, 48 feet wide, one screw and one rudder, couldn't manuever unless you were going 15 knots. Had a five inch 54 caliber cannon and an asroc launcher. Built in Seattle and commissioned in '70. The entire class (DE1052 to DE1096) was decommissioned by '96 and most were scraped but some were sold to Taiwan and South Korea. All were part of the Tonkin Gulf Yacht Club and served as plane guard and shore bomb assignments.

Michael Kassner
Michael Kassner

I haven't heard "plank owner" for a long time. You'd also probably give me a great deal of grief as I was just a 'shallow water sailor" on the Great Lakes (1970 -75). It did get a bit spooky a few times. Especially the night we left to look for the Edmund Fitzgerald when contact was lost with it. I was on the WoodRush, a 180 ft buoy tender out of Duluth.

mjd420nova
mjd420nova

I used to have so much fun in the woods with only a hundred milliwatts. My secret was fine wire and a bow and arrow to put up a couple miles of wires over the tree tops. Used to play with a dozen different wires over a quarter mile long to choose from and make a crude beam antenna. Could only do that in the wilderness where there are no power lines that could spell doom to me and the equipment. The Boundary Waters is the only true wilderness I know of left in the lower 48. Used to spend two weeks there and often went a week without seeing anyone else. Just the bears, moose and beavers around. My birthday was pulled number three in '68 and would have been drafted in January of '69 so I joined in September of '68. At least I qualified for some great schools after serving in country for six months. Then got assigned to a new ship before commissioning and remained aboard for 4 years. Left as the last remaining plank owner when I was discharged.

Michael Kassner
Michael Kassner

That?s neat stuff to know. I?m of a similar persuasion. Sounds like I?m just a bit younger though. The birthday draft got me, with a number of 5, I joined the Coast Guard instead of the Army though. I?m on my way to San Jose today to take a class in TelePresence from Cisco for the rest of the week. I?m excited about that as it is the next big thing and I?m slated to be installing the virtual conference rooms around the country. Great audio-visual play toys. As for the Boundary Waters, that is an amazing place. I?ve been there once. I?m more of a city guy, hate bugs. A golf course is wilderness enough for me. I remember QRP and CW as being my favorite part of being a Ham. A funny story is that as a Novice, I built a small tube transmitter for 40m and the best confirmed connection I got was a warning ticket from the FCC in Seattle as I had a bad harmonic on 20m and they were letting me know that I needed to clean it up. I was amazed that the rig had enough power to get that far. The following link is what I?m interested in now: www.14567.org/

mjd420nova
mjd420nova

I grew up in northeast MPLS, left in '68 to join the Navy when it was join or be drafted. Spent 5 years homeported in Pearl Harbor and left in September of '74 and couldn't face a winter in the cold country and settled in sunny California. I held Extra and Advanced with Radar endorsment while in the service. Kept active until '78 when computers took over my primary interest and began working in the industry. Have kept up with the advances along with the newest equipment but just can't seen to find the time to support it anymore. I'm working a graveyard shift now providing service and support for other allnight users in production facilities in the San Fran Bay area. Much running around and early mornings are hectic getting daytime relief for things I couldn't finish at night. Sleeping is a luxury and free time is comes on rare occasions. Vacations are rare but wilderness camping is a real escape and provides the contrasting environment to keep ones head on straight. Presently planning a two week trip to the Boundary Waters area in August as soon as they approve my visit. Spent many summers there in my youth so getting the okay should be simple as they don't let just anyone in there anymore. Ran a few QRP contests from there in '65 and '66 when I first got my general ticket as a teenager. Seems so long ago now.

Michael Kassner
Michael Kassner

I'm K0PBX, have been a ham for 43 years now. Have my Extra and work pretty much in the vhf and uhf ranges. Packet and satellite are my favorite areas. Glad to know of more members that are hams.

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