How to Pick the Right Antenna.

By Marlon K. Schafer
Sales Engineer: www.electro-comm.com
Owner: Odessa Office Equipment

 

     When I was asked to write this article I jumped at the opportunity.  It would give me a great excuse to do some in depth research on antennas.  How they work, what makes them do what they do etc.  Here I am 3 months later with a 1" high stack of reference materials and a mind swimming in barely comprehendible calculations!  Many thanks to Bill Drach (Antenna Product Manager from Radio Frequency Systems), Seth Hanson and Bud Bayer (Gabriel Electronics Inc.), Linda D'Evelyn (Applications Engineering Manager Ball Commercial Products & Technologies) and Daniel Warren (www.borg.com/~warrend).  Without those guys (and gal) I could never have gotten this stuff translated into layman's terms!

     First, what is an antenna?  According to Webster there are three definitions. We only care about one of them:  "a usually metallic device (as a rod or wire) for radiating or receiving radio waves".  And the FCC often calls them radiators (go figure ;-).  For the purpose of this paper we'll think of antennas as lenses and reflectors though (I know that sounds strange, bear with me here....).  For more accurate definitions please check out http://www.odessaoffice.com/wireless/definitions.htm.

    All antennas are shown as dBi.  All RF (in relation to antenna gain) starts out as the power that would be put out by a perfect point in space that would radiate the signal in all directions and create a perfect sphere of energy and is called an isotropic radiator.  That theoretical device (you can't really build one as the energy has to get into it some how so you loose the perfect shape of your isotropic antenna) has a gain of 0 dB.  An antenna has "gain" when you start to focus that energy into directions of your choice.  Don't forget that antennas have gain in both directions (sending and receiving).  They work like a lens for outgoing energy and a magnifying glass for incoming energy.

     When choosing an antenna it's important to first determine the area of coverage needed and the distance of the link.  You'll have to use an antenna with enough gain to get you a stable link but not so much that you'll overpower the other end or cause excessive reflections (multipath).

     Antennas are also used in different polarizations.  Circular (I'll not cover this one here as they are not popular for our uses), horizontal and vertical.  Vertical is often called "E-Plane" (for elevation I guess).  And Horizontal is often called "Azimuth."  RF energy is easiest to think of it you picture it as two dimensional.  Long (as it travels to it's destination) and tall (the wave height).  When you orient the wave vertically the antenna is vertically polarized, horizontal would be when you rotate it sideways.  Think of it like a ruler.  They don't care about 180 degree changes.  Cross polarizing your antennas will typically cost you 20db+ of lost power.  This can often be used to your advantage by allowing more radio systems to share the same air space.  Also, some polarizations work better for different links.

     There are different types of antennas out there.  Here is a list of the ones you are most likely to use and some sample radiation patterns for them (note: the patterns are examples only and should NOT be thought of as the actual patterns for the listed antennas).

  • Omni: It has a vertical (E-Plane) pattern of:and a horizontal pattern of a circle.
        

    A 3d view would look like:

 

  • Grid: (This one is horizontally polarized by the way.)

    The vertical pattern would look like:  
    Horizontal would look like:   and 3d:


     

  • Yagi:  
    Vertical would look like:  
    Horizontal would be:

    3d would be similar to a panel.

 

  • Patch and Panel:

    In this case Horizontal and Vertical would be the same (not always going to be that way though):   3d:

     If you have a long link you'll want a high gain (narrow focus) antenna.  These will usually be grid or solid dish antennas.  For short links you'll want to use just enough gain to give you the operating margin (extra power) that you need for likely weather in your area.  You may need to modify that if you have to go through a tight spot.  Use some smaller cable or a longer cable run to drop the signal down to where you need it.

     For the broadcast site (commonly called a WPOP (Wireless Point of Presence) or PtMP (Point to Multi-Point) use the type of antenna that covers the area you will have customers in.  If you will be putting your WPOP in or near the middle of your customer base and you'll only have one AP (Access Point or broadcast radio) at your WPOP an omni antenna should be fine.  Be careful that you don't use an antenna that has a vertical beam width that will shoot right over the top of your customers.  If you are on a 500' structure and your closest customer will only be a few hundred feet away a 15dBi omni with a 10deg vertical pattern (5deg above and 5deg below the centerline) would not likely be a good choice.  Also, make sure that you don't go too high on your power levels.  A 500mw amplifier is 27dB and if you add that to the 15dB of the antenna you'll be at 42db.  Just a shade over the 36dB allowed by the FCC!!!!!  NOTE: Every 3dB is 2 times or 1/2 the power (Watts).

     When using multiple APs at a tower or if your customers will all be in one direction you should use sectored antennas.  Think of them as panel antennas that have a very wide pattern in one plane and very narrow in another.  As an example look at this 19dB 120deg panel antenna.  The H-Plane is the pattern as you look down on the coverage area, E-Plane would be looking at the pattern from the side.  You can tilt down these antennas so that none of the coverage is wasted up in space where there are no customers.  If you have the budget for it these are a very nice way to go as they don't waste signal or put it where you don't want it to go.  They can be had with just about any dispersion pattern (E and H planes) you'll need and there are even adjustable ones.

     At the customer side (CPE or Customer Premises Equipment) the system is thought of as a Point to Point link.  There is a special formula to figure out exactly how much power is allowed but for all intents and purposes 48dBm ERP (Effective Radiated Power) is ok.  This allows us to more easily use many different antennas at this end.  The main concerns will not likely be power levels but aesthetics and Fresnel zones.  Many customers will not want big grid antennas on their houses so flat panel antennas are often used.  Panel antennas also have smaller side lobes and wider main lobes (a lobe would be a high power area, or the peaks on the charts above) making them better at eliminating multi-path and easier to aim.  Some come as a device that just looks like a plastic box.  These can be painted to match the structure helping to keep Mama happier with Dad's new high speed internet service.  Or, put your logo and phone number on them and do some advertising!  (NOTE:  Don't paint them with metallic paint!!!!!!)

     OK back to the technical stuff.  I'll not cover Fresnel zone issues as this has more to do with mounting location than antenna choice (but here is a screen shot from a calculator that is very helpful).  It also varies greatly with frequency used.  We do need to understand how to determine what power levels are needed so that we can get the right fade margins for our path.  Snow and hail are usually not a problem but rain does have a measurable effect.  According to my BreezeCOM manual rain of 6" per hour (150mm/h) will cause additional losses of about 0.02dB per .6 miles (1Km) for that link.  So in a hurricane you'll loose an extra 0.4dB on a 10 mile link (this is for 2.4 GHz)  Until you get into very long links or much higher frequencies rain is not really much of an issue but should be factored for.

     To calculate the actual loss in free space (a fancy term for air) contact your vendor.  I've done some testing and with normal 802.11 type equipment and the max 36dB at the WPOP I can have nice stable links with 16dB panel antennas out to 9 or 10 miles (15Km or so).  Past that I use high quality 24dB grids.  My longest link off of a 36dB WPOP out to an un amplified customer site is 16 miles.  We get about 256kbps using 2meg 802.11 DSSS equipment (third hop, total path is about 24 miles (38Km) long.  This link has stayed perfectly stable with snow on the ground or bare ground.  It's not been up during a hot spell so I don't yet know what effect heat inversions may have on the path (the antennas are not as high as they should be on either end).

     I recently had a new install that needed an amp.  It was on his crank up TV antenna tower and we were not able to point the antenna in the exact area that we thought the tower was (too far away to see).  We put on the amp and aimed the antenna for the best signal.  We were using a 24dB grid antenna and had a signal of -78dB.  We did notice that the signal got a little bit better when we came down a couple of feet with the tower (a common indication of multipath).  We had to build a special mounting bracket for his antenna so that it could be aimed right.  We mounted the antenna on the tower about 2 or 3 feet (about 1 meter) below where it had been and pointed it in the same direction it had been before.  We then Moved the antenna a little closer to where we thought the tower was.  To my surprise the signal went down!  After cranking the tower down a few times I finally climbed to the peak of that all too steep roof and took a looksee at where the antenna was pointed.  It was pointed at least 2 miles west of where the antenna had to be.  I then pointed it more in line with where the broadcast antenna had to have been and we ended up with better signal without the amp than we had before with it!  We'd been aiming the antenna on one of the side lobes!!!  That's just another reason to use higher quality antennas with low side lobes.

     In another location I have a short tower on a hill.  The tower is about 100 yards from a huge water tank that is solid all the way up to twice the height of my tower.  I hope to get on the water tank some day but for now I'm working fine on the tower.  I used sectored antennas and left a "gap" in coverage where the tank is to help prevent additional reflections.  I've got one customer at 11 miles (18 Km) and he gets about 60K/sec ftp transfers on the 2meg distribution system (second hop).  I also built a "shade" for my omni here in Odessa.  There is a cell phone PTP link that is within 1/2 mile of my site and they really hit my system hard with a lot of interference in the 2.4GHz range.  They kill everything below channel 8.  I sit on channel 11 but still needed to cut down on their signal so I put a piece of pipe between their antenna and mine.  It dropped the noise level by a noticeable (but not measured) amount.  Customers picked up a few K/sec of speed even though we should have been fine with our channel selection.  Go figure.

     For really close customers (2 miles or less) I've had pretty good luck with putting an 18dB patch antenna right inside the house.  There are some construction types that this will work well with and others that it won't.

     Don't be afraid to try things that the "experts" tell you you can't do.  It's all just a numbers game.  Different materials have different properties and as long as you don't give your link too little or too much power you can do some really fun things when you understand the basics.  Always keep safety at the top of the list, reliability second and price third and you'll have a lot of fun learning what works for your system.  They are all different in one way or another.  I don't have many trees here but most houses are surrounded by them.  Does a lot of good to get great signal to a point within 50' or so of where you need it!  sigh......  You may have a lot of water in your area.  Some people seem to like vertical polarization over water others like horizontal.  Try each one and see what works best with rest of the terrain you have in the area.  For lots of trees I've had better luck with a 45 degree angle on my antennas than either horizontal or vertical.  I've never seen a 45 degree omni though ;-).

     I hope that this will be of help to many of you.  The choice of antennas will make or break a wireless system just like the choice of speakers will make or break a stereo system.  Use the good stuff and even an inexpensive system will astound your customers!

Marlon K. Schafer

Copyright 2001 Marlon K. Schafer