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The reasons I can think of are transmitting antenna has to be more directive also larger length will imply larger power and thus will help compensating for losses. But on contrary if receiving antenna is made larger and transmitting antenna smaller won't the advantage be still applicable due to reciprocity theorem?

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    \$\begingroup\$ Antenna length is related to the operating frequency rather than transmitter power. \$\endgroup\$ Dec 17, 2020 at 2:53

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This depends on the requirements of the application. In some cases, based on gain requirements, the transmitting antenna is larger in diameter than the receiver while in some cases, it is the opposite. You may look at some examples of link budget design from here: (the snaps are used only for illustrations here) g1 g2

For applications such as long distance LOS communications that use aperture antennas like parabolic dish/reflectors, the gain is dependent on the physical aperture. This is given by the relation

gain relation

As evident from the relation, if you want more gain, you can configure the aperture of the reflector. A large aperture implies high gain and also increases the order of the signal power level at the receiver. Deploying a high gain antenna will reduce the path loss too.

'also larger length will imply larger power' - The length of the antenna is related to the wavelength of the transmitted signal. In the context of parabolic reflectors, 'length' becomes irrelevant. Diameter of the reflector plays an important role. The available power for transmission at the source depends on characteristics of transmission medium such as its reflection coefficient. power

For ensuring maximum power in forward direction, feeding plays an important role in case of parabolic reflectors. Spillover of power can be mitigated by selecting the optimum feed. You can't use a dipole antenna as a feed for a parabolic reflector as the spillover will be large.

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There could be any number of reasons, but here is one.

  • Better signal to noise ratio.

The signal strength at the receiver is dependent on the product of the antenna gain for both the transmitter and sender. But the received noise is also proportional to the antenna gain at the receiver. For the same total antenna gain product, your SNR will be better if the transmitter antenna gain is higher.

Note that Larger antennas will typically have larger antenna gain. So it makes sense for the transmitter to be the larger one, if one was going to be larger.

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Why are transmitting antennas generally larger than receiving antennas?

This question would be applicable only to radio broadcast transmitters and receivers.

A full-size, one quarter or one half wavelength antenna would be first choice for a broadcast transmitter, to present the minimum microvolts signal required at the input of a distant receiver for good reception.

At the same time, convenience and aesthetics would demand an unobtrusive / concealed antenna for the distant receiver.

For example, a 600 kHz AM transmitter, though rated at hundreds of kilowatts, would ideally require a 125 m high vertical monopole antenna, having multiple ground radials, to be effective with distant receivers having concealed ferrite loopstick antennas.

Likewise, in the case of FM radio and TV broadcast stations the relatively smaller transmitting antennas would be located on tall structures for effective reach.

However two-way low power radio stations (e.g. ham radio) would require full size monopole / dipole antennas at either end.

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The "ideal" length of a simple antenna is either 1/4 or 1/2 of the wavelength of the signal. This works for both sending and receiving.

The commercial AM broadcast frequency band extends from 530kHz to 1700kHz (depending upon country). The shortest wavelength is at the highest frequency 1700kHz. That wavelength is 176 meters. An ideal quarter wave-length antenna would then be 44 meters. A 530kHz quarter wave-length antenna would be 142 meters.

Radio stations can afford to erect tall antenna towers to get ideal quarter wavelength antennas or something approximating to that. AM band broadcast listeners, must be content with something smaller and less "efficient".

FM radio and television work at much higher frequencies than AM broadcast radio. For those services, it is important to distinguish the antenna tower from the antenna itself. The antenna itself is relatively short, but the tower is there to improve coverage. FM radio and television signals travel primarily in "line-of-sight" paths, so the higher the tower, the better the coverage.

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  • \$\begingroup\$ On the other hand, when the transmitter is, for example, on a satellite in space, and the receiver is on the ground, it tends to be the other way around. \$\endgroup\$
    – The Photon
    Dec 17, 2020 at 3:23
  • \$\begingroup\$ @ThePhoton reflector dish of a satellite receiver may be larger than the reflector dish of the transmitter, but the antenna element is usually the same size, a quarter wave-length. Communications to and from satellites tend to be at wave-lengths such that a quarter-wave antenna is less than a meter. \$\endgroup\$ Dec 17, 2020 at 3:29
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Antenna gain essentially always comes at the price of directivity (due to phasing issues, even simply adding "more" antenna in any productive way adds directivity)

For a fixed receiver installation operated with care, directivity can be a possibility: eg, in the era of VHF/UHF broadcast television, some consumers installed Yagi/Uda array antennas in some cases ever on remote rotators so they could be pointed in the correct direction for each station's transmitter.

But most broadcast receivers are small and portable, so something compact and near omnidirectional is desireable - in many cases the orientation of the antenna may end up fairly random and variable in all three dimensions.

In contrast, a broadcast antenna can be large, because only one is needed. And for a broadcaster, essentially all of the customers are horizontally towards the horizon - there's no reason to throw energy up into space or down at the ground, so an antenna that's fairly directional in the sense of a horizontal circle (vs an omnidirectional sphere) is desirable.

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It shouldn't have different length than the receiving antenna because the modulated signal coming out of the transmitting antenna has the same carrier frequency with the modulated signal received by the receiving antenna.

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