What exactly is (Horn) Antenna Gain

I was doing an experiment for finding the radiation pattern of Pyramidal Horn Antennas at RF frequencies(as part of my course curriculum). There I came across the Friis Transmission Equation



What I don't understand here is the two terms Gt and Gr. What does these gain terms refer to? Gain is something associated with active devices like amplifiers. But how come antennas have gain? They don't have any source of energy so that they can amplify the power, they are transmitting or receiving.

Bishal Santra said:

What does these gain terms refer to?

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The gain reference is an isotropic antenna, which radiates omnidirectional. If you use a directional antenna, you can direct more power into one specific direction (=> antenna gain) which means less radiated power in other directions. But as you have already mentioned, the overall radiated power is unchanged, of course.

https://en.wikipedia.org/wiki/Antenna_gain

They have effective power amplification
gain of an antenna usually relates back to one of 2 things
an isotropic radiator ( theoretical) dBi. or a halfwave dipole dBd
A halfwave dipole has 2.15 dBi

EVERY directional antenna has gain over a dipole. A Yagi, dish reflector, or horn antenna can have substantial gain
over a dipole. And it is very easy to get high gain as the frequency is increased and antennas become physically smaller

Dave

davenn said:

They have effective power amplification

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Dave, it really depends what we call "power amplification", and that's where the confusion came up.

What seems to confuse the OP is that antennas as passive devices obviously can't amplify power. If we feed 1W into a dipole, the total radiated power is 1W. If we feed 1W into an antenna with 20dB gain, the total radiated power is also 1W. No amplification of the total radiated power, of course.

So where's the gain? A directional antenna directs more power into a specific direction, and then we have "gain" in that direction, and at the same time we have "loss" in other directions. There is no conflict between this antenna gain (in one direction) and the constant radiated power (integrated over all directions) requirement.

The gain reference for Gt and Gr in the Friis equation is the isotropic radiator.

Just like a bugle or trumpet for audio will resonate according to the horn tubing length , aka the "Helholtz Resonator" , high velocity vibration with high pressure in is amplified by area of tube exit with impedance transformation and also gain by directing the sound pressure in one direction.,

For antenna, there is gain by three methods (lumped together) Resonance, Impedance transformation and Diversity ( directional focus)

Resonance comes from the natural frequency of its geometry for speed of light in that dielectric medium with impedance inversion at 1/4 wave and matching at 1/2 wave. The Q = f(center) /delta f ( -3dB)

Voltage for electric Field can also be optimized by transforming impedance to match source and load impedances for maximal power transfer, just like a magnetic transformer has a gain for voltage or inverse gain for current.

All antenna have some directional gain in 3D, if omnidirectional in 2D it is called isotropic and we use 0dBi as a reference for gain of this.

A horn has a depth and width which converts omnidirectional power to narrow beamwidth in each axis.

This diversity or directional gain is also bidirectional foir Rx and Tx as the other gain methods.

volker@muehlhaus said:

Dave, it really depends what we call "power amplification", and that's where the confusion came up.

What seems to confuse the OP is that antennas as passive devices obviously can't amplify power. If we feed 1W into a dipole, the total radiated power is 1W. If we feed 1W into an antenna with 20dB gain, the total radiated power is also 1W. No amplification of the total radiated power, of course.

So where's the gain? A directional antenna directs more power into a specific direction, and then we have "gain" in that direction, and at the same time we have "loss" in other directions. There is no conflict between this antenna gain (in one direction) and the constant radiated power (integrated over all directions) requirement.

The gain reference for Gt and Gr in the Friis equation is the isotropic radiator.

Click to expand...

and that's why I called it EFFECTIVE power amplification

D

if you are in the garden, and have a rubber hose flowing with water...the water comes out at say 1 gallon per minute, in a wide gushing flow.

You now screw on a nozzle at the end of the hose, and the pressure builds up, and the water starts to stream in a pencil thin beam, 20 feet away, but still at 1 gallon per minute...just more directional.

Same with a horn antenna. It just directs what energy there is coming out into one smaller beam