ARES District 3 Bulletins 2017


WOOD COUNTY BULLETIN NR 17.08          DATE: February 19, 2017

What is antenna gain?  Antenna gain is a theoretical measurement of power that an antenna is radiating over a reference antenna that is suspended in free space.  Everything has to have a reference point, and from there other items are compared.

Some manufactures of antennas use a 1/4 wave length antenna as a reference point, while others uses a 1/2 wave length antenna as a reference point.  Antenna gain is measured in decibels, or db's for short.  Every 3 db of gain is equivalent to approximately doubling the power.  So if you have an antenna that has 3 db of gain, and you are transmitting 25 watts of power, the gain of the antenna would make it look like you were transmitting 50 watts.

If you have 6 db of gain you would have roughly four times the amount of power, or 100 watts.  Nine db of gain would be about eight times the amount of power or approximately 200 watts.  When you reach 10 db, the amount of power would be ten times, or 250 watts.  Twenty db of gain is about the same as having 100 times the power and 30 db is having 1000 times the power.  Your theoretical power, using 25 watts and a 20 db antenna would be 2500 watts and 30 db would be 25,000 watts.  An easy way to remember how many times to multiply is to use the first number of the amount of gain.

For instance 20 db has a 2 as the first digit, so you take a 1 and add two zeros, or 100.  Thirty db has a 3 for the first digit so you put a 1 and three zeros, and you have the number 1000. Forty db has a 1 and four zeros or 10,000.  If someone says they have an antenna that has either 20 or 30 db of gain, it's the same as saying that it has either a gain of 100 or 1000, that's a big difference.

Remember some antennas use a 1/4 wave reference, while others use a 1/2 wave reference.  The difference between 1/4 and 1/2 wave is about 2 times.  If your antenna has a gain of 10 db over a 1/2 wave reference antenna, it would have roughly twice as much gain as an antenna with 10 db that uses a 1/4 wave antenna as a reference.

Here is a list of gains, or losses of various antennas.  A Dummy Load, all power is absorbed and no power should be transmitted. A Walkie/Talkie with a Rubber Ducky antenna has about a minus (-) gain of about 6 db.  In other words if your H/T has an output power of 5 watts, then your theoretical transmitting power is a little over 1 watt.  A 1/4 wave antenna has a gain of zero (0), or about the same as the output of your radio, if the antenna is cut to the radio's frequency.  A J-Pole antenna also has zero (0) gain, but doesn't have to be cut to a particular frequency.

A 5/8 wave base loaded mobile antenna has a gain of about 3 db. A three element yagi type beam antenna has a gain of about 10 db. An eleven element beam antenna has a gain of about 20 db.  All of these gains are using a 1/4 wave antenna as a reference point. If they were using a 1/2 wave antenna as a reference point they would be twice as good.

Ken Harris WA8LLM
Wood County WV


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