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Frequently Asked Questions...
Antenna Gain Calculations?
I have an antenna with 4dBi gain. So does that effectively take the emitted signal up to 24dBm in terms of power. As I'm starting with 20dBm at 100mW?
If that's the case where does this gain come from? Because if it's an isotropic gain then there's nowhere to take the power away from in order to increase it elsewhere and you can't create power from nothing.
The power into the antenna remains the same.
What happens is that the signal is 4dB stronger in a specified direction than if it was transmitted by an isotropic or point source antenna.
(A point source antenna doesn't exist, but if it did, it would transmit uniformly in all directions.)
The signal would be the same strength in the chosen direction as a signal with 4dB more power into a point source antenna..
In some other directions, the transmitted signal will be less than a point source antenna. This is where the improvement is achieved. Power is just redistributed, not created or destroyed.
The ultimate example of this is the dish antenna. In a cone shaped region of a few degrees, the signal is a lot stronger than an isotropic antenna but at all other angles the signal is a lot weaker. But the total power stays the same.
How to make a high gain wireless antenna
To see more related products on Shortwave Receivers, please check out the following:
The Cellular Yagi Antenna Explained
Explained: Frequency Gain, Cell Phone Signal Distance, Cell Phone Frequencies, Finding your Cell Phone Tower Location
The Cellular Yagi Antenna
One of the most frequent questions we get is about the cellular yagi antenna, which is used in rural/remote locations to get a strong cell phone signal, extend cell phone signal, and works with most cellular wireless repeater systems for buildings.
Like a lot of things on the internet it gets confusing going from site to site with each one seemingly giving different information and claiming there's is the best. We've taken the time to explain it, without trying to make an engineer out of the reader.
This will allow you to compare yagi's "apples to apples".
The yagi antenna : The most common directional cell phone signal booster antenna is known as just the yagi or yagi cell phone antenna. It is named after its inventor, who created it in 1908. It is the antenna of choice with most cellular signal professionals when needing high, consistent performance.
Yagi Antennas are generally used as outside, or "donor" antennas. Yagi's for cell phone use are normally single band, meaning they are engineered and made to work for 800 MHz or 1900 MHz, but not both. Some Internet sellers claim to have dual band yagi antennas, but the ones tested by two major antenna manufacturers we deal with have found that one of the two signals suffers. This is because antennas for lower frequencies need more surface area than higher frequencies.
A yagi antenna is made with a number of vertical bars, or elements mounted onto a longer horizontal bar. A drawing of a typical yagi antenna is shown below:
Yagi antennas cannot receive or transmit a signal from the side or rear, only the front.
dBi vs. dB: These are designations used in describing the amount of gain an antenna offers above the current signal strength of the device to be used, like a cell phone. dBi is always two numbers higher than dB. Therefore, a 13 dBi antenna has the same gain as an 11 dB antenna. The designation used is simply at the discretion of the user. This is important to know so that you can compare "apples to apples" when comparison shopping.
Gain: Because of the nature of radio frequency signals most yagi antennas generate a maximum of 13 dBi gain. Additional gain achievement quickly reaches a point of diminishing returns.
Example: A typical 800 MHz 13 dBi yagi is about 32 inches long. To achieve a 2 dBi increase, to 15dBi, the surface area (length) has to be approximately doubled.
Therefore, beware of claims of more than 13 dBi for yagi antennas made for use at the 800 MHz and 1900 MHz frequencies.
Applications: Yagi antennas are generally pole mounted and elevated, to get over obstacles that block the line-of-sight. One reason they are so highly recommended is that the high gain allows them to be elevated without causing significant gain loss do to the use of longer lengths of coax cable usually needed to pole mount the antenna.
Example: LMR400 coax cable (Wilson 9913) loses four tenths of a dB per ten feet at 850 MHz. So a fifty foot length loses a total of 2 dB's. 9 dB is still available to the connected device (cell phone, etc.) as the net gain from a 11 dB gain antenna.
The extra gain of the yagi is necessary for many rural and remote applications. Most repeater systems use this cable to get as much signal to the amplifier as possible.
Your Frequency : To determine whether you need a 800 or 1900 MHz yagi, go to UnwiredSignal.com and click "Find Your Frequency".
Your Tower Location: Since the yagi antenna needs to point towards your service providers tower, you need to know where it is. There are three easy ways to find the cell phone tower location. They are:
1. If the location where the yagi is to be used is not blocked by trees or taller buildings around it, just walk around the building's perimeter with your cell phone. The side of the building with the strongest signal reading is usually the direction of your tower.
2. Call your service provider's customer service and ask. If they won't give it to you, don't argue. Simply hang up and radial. The next person is likely to say they are happy to give the tower location to you.
3. Mount the antenna, connect it into the cell phone or laptop air card's external antenna port, if it is equipped with one. Then turn the yagi in 10 degree increments until you have the strongest signal. To do this you will need the correct adapter that allows the antenna's cable to plug into the cell phone or air card.
To find the correct antenna adapter or to view examples of cell phone external antenna port locations Click Here.
NOTE: You cannot hold a yagi antenna to attempt to tune it in. To tune the yagi antenna, it should be mounted on a metal pole where you plan to permanently use it. You must completely remove your hand each time you turn the yagi and wait 5-10 seconds before looking for a signal strength change. Using the phone test mode can help you to find your cell phone tower location / direction as well.
Phone Test Modes : Many cell phones can be put into "field test" mode for signal strength testing. This allows you to read the signal strength in decibels instead of bars. You can get to "field test" mode with just a few key strokes.
NOTE : The signal strength is shown as a negative number; it will have a minus sign in front. Therefore, the lower the number, the stronger the signal. So -75 indicates a stronger signal than -85.
To find the menu to put your cell phone or PDA into field test mode go to UnwiredSignal.com then click "Technical Support".
Signal Distance : If mounted properly, with line-of-sight to the signal (not necessarily the tower) you seek a high gain yagi antenna will send and receive signals for miles.
You can expect even higher performance when a high gain yagi antenna, like the 13 dBi gain Wilson 301111 is used with a Wilson direct connect 3 watt amplifier .
Example: Our customers successfully use the type applications 25-30 miles offshore in the Gulf of Mexico and for numerous other remote applications.
Yagi antennas are often the antenna of choice when using a wireless repeater system like the Wilson 801245 Small Office Home Office System and the Wilson 801106 wireless repeater system for large and/or weaker outside signal applications .
Type of Coax Cable Needed: The type and length of coax cable that should be used with any cell phone signal booster antenna, air card and/or amplifier is dependent upon the frequency that the antenna operates. The factors to determining the type and length of coax cable that should be used are as follows:
1. RG8X Coax should not be used in 1900 MHz applications. It should not be used in lengths of over forty feet when used at 800 MHz. It is double shielded, is considered low loss cable, but it still loses 7.7 dB per one hundred feet at 800 MHz.
LMR400 Coax should be used with all 1900 MHz applications, most wireless repeater (booster) systems and lengths of over forty feet at 800MHz.
About the Author
UnwiredSignal.com Questions? Call us at the store in Dallas, TX anytime from 9 AM to 6 PM Monday-Friday. We are happy to help you. 214-774-2588 or toll free 888-713-1243.
If you have questions about cellular signal booster antennas, amplifiers or wireless repeater systems go to UnwiredSignal.com or call us at our store, located at 1902A Greenville Avenue, Dallas, Texas 75206 214-774-2588 or toll free 888-713-1243.
By David Jones
214-744-2588 CST - M-F
888-713-1243 Toll Free