- #1
Noudi
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Why antenna needs to be smaller in size with higher frequencies (EM waves)? please
Smaller Antennas, Higher Frequencies: The Need for Size Reductions
- Thread starter Noudi
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- Antennas Frequencies
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Discussion Overview
The discussion revolves around the relationship between antenna size and frequency, specifically why antennas need to be smaller at higher frequencies (shorter wavelengths) and larger at lower frequencies (longer wavelengths). Participants explore the implications of this relationship for various applications, including radio transmission and mobile communication.
Discussion Character
- Technical explanation
- Conceptual clarification
- Debate/contested
Main Points Raised
- Some participants assert that antenna dimensions are proportional to wavelength, with the formula wavelength = speed of light / frequency being central to this understanding.
- One participant argues that antennas for high frequencies can be small, while those for low frequencies require larger sizes, emphasizing the need for matching the transmitter to the antenna for effective power radiation.
- A participant elaborates on the concept of radiation resistance, noting that as antennas become a smaller fraction of a wavelength, their radiation resistance decreases, complicating power transmission.
- Examples are provided, such as the large T-shaped antenna used for low-frequency transmissions like BBC Radio 4, contrasting it with the compact antennas used in mobile phones that operate at much higher frequencies.
- Discussion includes the need for antennas to achieve good directivity, which may require sizes that span several wavelengths, referencing the large dishes used in radio astronomy.
Areas of Agreement / Disagreement
Participants express differing views on the relationship between antenna size and frequency, with some supporting the proportionality of size to wavelength while others challenge this notion, leading to an unresolved debate on the topic.
Contextual Notes
Participants discuss various assumptions related to antenna efficiency, power transmission, and the physical characteristics of antennas without reaching a consensus on the implications of these factors.
- #2
Baluncore
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Because antenna dimension is proportional to wavelength.
Wavelength = speed of light / frequency
Wavelength = speed of light / frequency
- #3
berkeman
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Noudi said:
Welcome to the PF.
This introductory article should also help: http://en.wikipedia.org/wiki/Antenna_(radio )
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- #4
Noudi
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Thanks Dear.
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sophiecentaur
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Actually, it's rather the other way round. The antenna for a high frequency (short wavelength) can be small. For low frequency, they need to be big.Noudi said:
When you want to launch an EM wave into space, you need to 'match' your transmitter to the antenna - in order to get the power out into space. The mechanism that causes power to be radiated (which is what the whole thing is about) ls the equivalent to a resistance for the transmitter to drive into. As the antenna gets to be a small fraction of a wavelength, this (radiation) resistance gets lower and lower - the limit being a short circuit. It is very difficult to put much power into an extremely low resistance because all the power gets dissipated in the transmitter and the connecting cable. Hence, to transmit Radio Signals at 198kHz (BBC Radio 4 Long Wave @ Droitwich) the transmitting antenna (the actual wires that radiate - not just the tower height) is a T shape with the vertical section about 100m high. Even with this massive structure, the antenna is not very efficient and a significant amount of power just warms up the metalwork and the ground nearby. For your Mobile phone, which uses wavelengths of much less than 1m, the antenna can be made to fit inside the case quite easily and is a pretty efficient design (for battery like).
To get good directivity, you need antennae that take up several (or even hundred) wavelengths. Hence the massive 'dishes' for radio astronomy.
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