# Antenna question

1. Nov 19, 2009

### muh_j18

hi all,
what exactly is the gain of an antenna, and what is the difference between the gain and the directivity?
also, what is the EIRP, is it the same as the radiated power? and how can the radiated power be greater than the transmitted power? (EIRP= transmitted power+gain-losses in dB)

any help to explain these concepts is appreciated.

2. Nov 19, 2009

### Bob S

3. Nov 19, 2009

### vk6kro

what exactly is the gain of an antenna, and what is the difference between the gain and the directivity?

An antenna, such as a yagi, concentrates the available transmit power mostly in one direction. In that direction only, the signal transmitted will be stronger than if it was coming from a dipole in the same position ( and radiating at right angles to the dipole wire). This difference is called the gain of the antenna and is always given as gain over a dipole, in dBd.

In all other directions, the signal will be weaker than it would be from a dipole.

The more gain an antenna has, the more concentrated the radiation pattern is. This is measured by moving a little off to the sides of the main beam of energy and noting the directions where the power is reduced by 3 dB on each side. The angle between these directions is the beamwidth of the antenna, or the directivity.

also, what is the EIRP, is it the same as the radiated power? and how can the radiated power be greater than the transmitted power? (EIRP= transmitted power+gain-losses in dB)

The radiated power can never be more than the power supplied by the transmitter. However as above, it can be concentrated into one direction in ways that make it appear more powerful than the same power from a diplole would be.
Sometime this extra strength is achieved by taking power that would radiate upwards from a dipole and concentrating it horizontally. This is done with TV transmitting antennas where 360 degree coverage is required horizontally, but there is no need for power to radiate upwards.

4. Nov 19, 2009

### Born2bwire

The difference between directivity and gain is that directivity is the theoretical gain of the antenna. Gain is the realizable gain of the antenna when you take into account the fact that not all power sent to the antenna is radiated. You have efficiency losses due to finite conductivity of the metals in the antenna and due to lossy behavior of the antenna's dielectrics. So if the antenna had perfectly lossless materials, it would have 100% efficiency for the gain and the gain and directivity would be the same.

I would point out though that we often choose an isotropic radiator as our reference for the gain/directivity. In this case the numbers are given in dBi.