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Old 4th March 2009, 10:19 AM
boggo's Avatar
boggo boggo is offline
Join Date: Jan 1970
Location: Nth beaches,Sydney
Posts: 230
Everything you ever wanted to know about gain!!


The Inside Scoop on Antennas: Double Your Transmission Range for $100

This article discusses antennas for two way radios, and how to get the most performance from your equipment by making sure you are using the best antennas for the job.

If you're just a humpty dumpty in a big organization and have nothing to do with your radio purchases, maintenance or whatever, you may not be able to act on any of what you learn here. But, if you're in a smaller department, a volunteer, or have a personal radio, then some of the information here may be of interest. In any event, it never hurts to understand how your equipment works.

Judging from the calls we get, the matter of antennas is one of the more misunderstood areas of a two way radio system. An antenna is a critical part of any two way system - not just something to tack onto a bid, or buy as an afterthought. No radio can outperform its antenna. No matter how much money you spend on a nice radio, if you can't couple its transmitter and receiver to the atmosphere, you're wasting at least a part of your money.

Many times we've been able to improve the performance of a communications system merely by improving the antennas. Antennas wear out, and it's not uncommon to find even new antennas from reputable manufacturers operating poorly.

One term you'll see a lot, and is misunderstood by most people, is the term GAIN. Gain can be considered, in an approximate way, to be the power of the antenna. More is not always better, as we shall see.

Antenna gain is measured in decibels, or dB. Note that we write this as a little "d" and an upper case "B". A decibel is one tenth of a Bel. 10 dB equals a Bel, but we never call it a Bel. The Bel, by the way, was named for Alexander Graham Bell. Decibels are not an absolute measurement by themselves. Decibels are a relative measurement. It's sort of like saying, "louder than" (louder than what?). Antennas are rated by dB gain. We can use some analogies to help us understand.

A 0 dB antenna (sometimes called a "unity" gain antenna), is an antenna with no gain. Picture a sphere, or sort of like a basketball, stuck down over your antenna. (This is not a technically accurate description, but it will help you visualize what we're talking about). This basketball represents the coverage pattern of a unity gain antenna. A 0 dB gain antenna will radiate (and receive) its signals equally from all directions. Most handheld radio antennas, and some mobiles, are of this type. We'll go into this more later. For now, keep the mental picture of the basketball in your cranium. It's important. If you're outside the U.S., you may prefer to visualize a soccer ball, or a beach ball if you live in California.

More gain on an antenna will, usually but not always, give you more range. How do we get more range? Simple. Picture the basketball around your antenna. It's squirting its signal in all directions. Up and down as well as sideways. But, usually, we're not too interested in talking to someone up in the sky. We're interested in range towards the "horizon", as we radio guys like to say. So a significant percentage of the power we're transmitting might be wasted going up in the sky. To get more gain, we squash this basketball pattern down some. A little bit of gain (say 3 db will flatten the pattern to more like a donut. More gain (5 db will give a pattern like a dinner plate. The more gain the flatter the pattern.

And the more power we concentrate out towards the horizon, the further we'll talk. Make sense?

Generally, to get more gain you need a bigger antenna. A simple "quarter wave" (0 dB, or unity gain) antenna for VHF is about 18 inches long. A 3 dB antenna for the same frequency would be about 4 feet long, while a 5 dB antenna is maybe 8 feet. For UHF, a quarter wave is six inches, and a 5 dB about 2 feet. The lower the frequency the longer the antenna.

Using the information above, you can tell by looking at the two way radio antenna on your car what band you're on, and how much gain you should be getting from your antenna. If you've got a straight piece of wire, you probably have a unity gain antenna. If there's a coil at the base maybe an inch in diameter and 3 or 4 inches long, you're on low band or VHF. If there's a coil halfway up the whip, you're on UHF or 800 Mhz. A cellular antenna is 800 MHz. Most of the ones you see for cellular phone are gain antennas. Since the frequency is so high, it's practical to have a small antenna with a lot of gain. But if you're still on low band (30-50 MHz), a gain antenna would be so large that you couldn't use it on your car.

If you have a unity gain antenna (the basketball shaped pattern), you can see that it's not too important that the antenna be exactly straight up and down. But, if you're using a high gain antenna with a very flat pattern, you now know that if the antenna is tilted, you're squirting a lot of your signal up into the air or down into the ground. Picture the dinner plate around your antenna, but tilted. If the tilt is severe you've got very little power actually going out to the horizon where you need it.

Keep the antenna tilt in mind the next time you see one of those little sports cars with a cellular phone antenna that's tilted to match the angle of the rear window. It may look cool, but it's working crummy. It's entirely possible to double your range just by straightening out a tilted antenna. Here's your good deed for the day: look around in the parking lot and see how many cell antennas are not straight up and down. Point this out to the operator, fix it and be a hero.

There are times when you want a tilted pattern. We have one repeater on top of a mountain providing communications into the valley below. By tilting the antenna down, we concentrate more of the power into the valley where it's needed, instead of scooting it over their heads with a flat pattern. You can play some other games with antenna design such as making patterns oblong instead of round, but we're not going to go into that here.

Don't write off all low gain antennas. If you work in the city with a lot of high rise buildings, a unity gain antenna with its round pattern may help. Lots of times your repeaters are on the tops of buildings, or you can reflect power off tall buildings to get better signal coverage. But if you're in a rural, mostly flat, area and need long haul, I can't think of many reasons why you wouldn't want to consider a high gain antenna with its flat pattern out toward the

Now here's where we can save money. Anything you do to increase your signal will give you more range. You can do that by running higher power. Or you can do it by improving your antenna. It is much cheaper to get more range by improving your antenna than by running more transmitter power. Plus, better antennas let you hear further as well as talk further.

How does power affect transmission range? Good question. We have to get a bit technical here, but bear with me. It will be worth it, and it's not that difficult anyway.

The higher your gain (more decibels), the stronger your radio signal will be.

Doubling your transmitter power, say from 5 watts to 10 watts, is a 3 dB increase. Going from 50 watts to 100 watts, or even 500 watts to 1000 watts, is the same 3 dB. Increasing your power by a factor of ten is 10 dB gain. So, going from one watt to ten watts is a 10 dB increase.

3 dB is twice (or half), and 10 dB is ten times (or one tenth).

What do we care? Well, doubling your transmit power (a 3 dB increase) increases your transmit range by the square root of two, which is 1.414 times the range. This will hold true if all other factors are equal, such as the path being the same, obstructions and antennas also the same. 6 dB of gain will double your range (1.414 times 1.414 = 1.9994 increase).

Now, get this: system gain adds no matter where it is in the system. 3 dB at the transmit end and 3 dB at the receive end equals 6 dB of overall system gain. And, 6 dB gives you twice the range.

We can see that merely by changing from unity gain to 3 dB antennas at each end will, for the most part, double your range. Typical prices for quality 3 dB VHF mobile antennas are $50 each. So, for $100 you may be able to double the range of a simple two way radio system, as promised in the title of this article. 5 dB antennas for UHF are about the same price, and will do even better.

Adding 3 dB of antenna gain by changing one antenna will have exactly the same effect as doubling your transmitter power. A free for nothing additional benefit is that a better antenna will let you hear further as well as talk further. And, the antenna is much cheaper than a new radio.

In some operations, you don't have much control over the transmit antenna. This could be true in the case of a base station, where it would be major drama to swap out an antenna. But nothing here should stop you from looking at anything you can do to your mobiles.

Another situation might be in a body mic system. You can't do much with the antenna on the body mic transmitter (actually, you can, but that's beyond the scope of this article). But, on the receive end you might have more flexibility. Going from a simple whip to a gain antenna will give you better range on your body mic system. As most body mics are at VHF and antenna gain at VHF is limited by practical considerations of size, a 3 dB antenna is about the most you can do. But, 3 dB is 40% (1.414) times the range, so it's worth the $50 investment.

If you need even more range in a surveillance scenario, you can go with extremely high gain directional antennas. These antennas are called Yagis, or sometimes, beams. A television antenna is a typical example of a certain type of beam. For long haul stuff, where you don't particularly need to conceal the receive antenna, you can use a Yagi with gains as high as 15 dB or more are available. Referring back to the paragraphs above, 15 dB should give
approximately seven times the range as a simple whip antenna. but, be aware that high gain antennas are large, especially at the lower frequencies. These large sizes may mean that high gain antennas are not always practical.

You can get antennas at any two way radio supply company. You get what you pay for; the higher priced antennas will last longer. The best pricing is from the large distributors. Check the two way radio magazines for sources. You need to order antennas for the proper frequencies, and be sure they will fit whatever mounts you are using already. The supplier should be able to help you make the proper choice. Or call us if you're not sure.

Most of what we've learned above refers to mobile and base station antennas. But what about portable radios? Sometimes, this is where we need the most help.

Most of the time, there's not much we can do to increase the size of an antenna on our portables. In fact, many portable antennas actually are "negative gain". Your VHF walkie talkie antenna is not 18 inches long (I hope). To work properly, the radio must think it has the proper antenna. Since we can't use the full length antenna much of the time, we must artificially shorten the antenna. This "coil loading" reduces the efficiency considerably. About all you can do is replace the antenna when it gets chewed up. Or, sometimes, a different brand of antenna will work noticeably better. I haven't found one brand that seems to work consistently better on all radios, so you'll just have to experiment if you're having problems in this area. Sooner or later you'll have to replace your portable antennas anyway, so you could try different ones when you replace them, and compare to antennas on other portables.

If you're using the 3 inch stubbies on a UHF portable, you might see a difference going to the six inch rubber duck. Theoretically the longer antenna is better. I personally can't tell the difference, but others have reported a noticeable improvement. The longer antenna is more annoying to me, so I stick with the short ducks on UHF even if they don't work quite as well.

When you're using a portable radio, you'll get the best range if you hold the radio vertically, not tilted over your shoulder. If you have the option, move outside or towards a window. Don't stand behind or near anything large and metal that might absorb the signal. In most cases, if you move around to where you hear the best, that's also where you'll talk the best. If you use a speaker mike, you can even hold the radio up over your head to get a bit more height. Keep in mind, also, that when the portable is on your belt your body is soaking up something like 80% of the signal. So take the radio off your belt and stick it up in the air if you're in a weak signal area.

If you're in a fringe area in your car, wait to transmit until you're on the crest of a hill. Avoid transmitting from under bridges, in tunnels, alleys or behind high buildings. Of course, this all is academic if you're getting your ass whooped and have to scream for help. Speaking of which, hollering at your radio, mobile microphone or telephone will not make you more intelligible at the other end. Just the opposite is the case. It's a human tendency to scream at the radio when you're excited, but try to train yourself not to do that. In the heat of battle is when you most need to be heard. Speaking slowly and distinctly, in a normal tone of voice, is the best way to communicate effectively.

All for now. If this information was helpful, let me or the magazine know. In the meantime, feel free to call my office if we can help you with anything.

Copyright (C) June 1992 by Steve Uhrig, SWS Security
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