The "gain" of an antenna comes from an ability to squirt more energy in a selected direction, and less energy in an undesired direction. This ability is known as "directive gain," or "directivity."
The other property that affects antenna gain is the efficiency of the antenna. Some antennas have an efficiency near one, others have a much poorer efficiency. For example, the "loopstick" antenna in an inexpensive AM band (medium wave band) portable radio can have an efficiency of less than 0.01. Antenna gain is given by
Gain = efficiency X directivity.
You can increase the range of your wireless gadget by increasing antenna directivity and/or efficiency.
The range increase for an antenna gain increase occurs equally for both the transmit and the receive antenna. There is a further range increase for increasing the gain of both antennas in your wireless link.
The negative side...
In the US and Canada, for most frequency bands, the allowable transmit power is specified in terms of strength of the radiated electric field, not transmit power. Increasing the transmit antenna gain under these conditions can cause a violation of the rules if the transmit power is not reduced.
But, there is no restriction for receive antenna gain.
And, there are less transmit antenna gain restrictions for spread-spectrum users in the ISM bands. As this is written, the FCC (US) is planing a further easing of the spread-spectrum ISM band transmit antenna gain rules.
So you do not understand antennas. What to watch out for...
Many antennas for license-free wireless products are poorly thought-out and/or poorly implemented. Most of these antennas are an attempt to construct a half-wave dipole, or a quarter-wave monopole over a ground plane. Properly implemented, either of these antennas can be efficient and provide a small amount of "gain."
The implementation problems run in two general areas:
1. Not recognizing that a wire antenna, like a lamp or appliance, must have a two wire connection to the power source, and
2. Not recognizing that an antenna closely mounted to a "grounded" metal structure is acting as a transmission line, not a radiating antenna.
Both of these implementation errors cause very poor efficiencies. Both are usually very easy to correct.
When constructing an antenna as an integral part of a circuit, always ask "where are the radiating elements?" Include the ground structure near, or connected to, the antenna and transmitter. Then design accordingly.
When buying an antenna ask " what is the efficiency?" Alternatively, ask if the advertised gain is "measured gain," or "calculated gain." Many purchased antennas advertise a "calculated" or "theoretical" gain. The actual gain can be close to this value, or very much less than this value.
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