# Electromagnetic waves: Antenna length

• ap_cycles
In summary, the length of an antenna is determined by the wavelength of the wave it receives, which is why the antennas on TV and radios are about the same length. The smaller wavelength of the waves detected by cellphones allows for smaller antennas, such as ferrite rods, to be used. The amplitude of the wave does not affect the size of the antenna, but the gain of the antenna can impact the efficiency of signal reception. For cellphones, the minimum desired amplitude of a received signal is roughly 10 times kTB, where k is Boltzmann's constant, T is temperature in kelvin, and B is bandwidth in Hz. Antennas play a crucial role in receiving signals and their design varies based on the frequency band
ap_cycles
Hi,

I come across these 2 questions on a website:

1. Why are the antenna on TV and Radios about the same length
2. Why is the antenna on the cellphone (i.e. the older versions) smaller than the Radio antenna?

My answers to these 2 questions are: Firstly, the waves that the cellphone detect are of a smaller wavelength than those detected by the radio or TV. If i am not wrong, the wavelength of an antenna is about one quarter the wavelength of the wave it receives.

Secondly, the amplitude of the waves. One can argue that the longer the antenna, the bigger will be the amplitude of the wave it receives. The max amplitude (i.e. max energy) the cellphone receives is smaller than that received by the TV or radio?

What do forummers think of my answers? I am quite confident of my 1st, though not so sure about the 2nd. Btw, i am a high school physics teacher about to deliver this topic on EM radiation soon. :shy:

(Off topic: do most modern cellphones have an antenna anyway? IF the answer is yes, won't hiding them within the chasis of the phone defeat the original purpose of a protruding antenna?)

ap_cycles said:
Hi,

I come across these 2 questions on a website:

1. Why are the antenna on TV and Radios about the same length
2. Why is the antenna on the cellphone (i.e. the older versions) smaller than the Radio antenna?

My answers to these 2 questions are: Firstly, the waves that the cellphone detect are of a smaller wavelength than those detected by the radio or TV. If i am not wrong, the wavelength of an antenna is about one quarter the wavelength of the wave it receives.

Secondly, the amplitude of the waves. One can argue that the longer the antenna, the bigger will be the amplitude of the wave it receives. The max amplitude (i.e. max energy) the cellphone receives is smaller than that received by the TV or radio?

What do forummers think of my answers? I am quite confident of my 1st, though not so sure about the 2nd. Btw, i am a high school physics teacher about to deliver this topic on EM radiation soon. :shy:

(Off topic: do most modern cellphones have an antenna anyway? IF the answer is yes, won't hiding them within the chasis of the phone defeat the original purpose of a protruding antenna?)

You are correct that a good antenna will be a quarter wavelength in length for a monopole (antenna above a ground plane), or two quarter wavelength elements as a dipole antenna (two quarter wave conductors opposed to each other). But for the TV versus radio point, it depends on what radio band you are asking about. The US FM band is indeed close in wavelength to the US TV bands, so antennas for those are similar in size. The US AM band is much lower frequency, around 1MHz, so the transmitting towers are large, and the receiving antennas are ferrite rods wrapped with coils. Those receiving antennas for AM are not very efficient, but that's okay because the AM transmitters blast their signals at high power levels.

For cell phones, yes, their frequency bands are near 1GHz (as opposed to the mid 100MHz range for FM and TV), so their antennas will be shorter. And in the smaller/newer cell phones, they are using minature ferrite antennas. There are also some cool new advances with high-dielectric-constant antennas for miniaturized applications -- great stuff.

You can probably learn a lot by just reading through the antenna page at wikipedia.org. There are lots of links out of that page that are also good.

The amplitude of the wave bears in no part to the size of the antenna. The amplitude of the electromagnetic wave is NOT a disturbance in physical space. The only factor that we might consider related to amplitude is the gain of the antenna. That is, we may wish to design a more efficient antenna (which usually requires a larger antenna) to receive a signal that is low in power due to various noise sources.

The minimum desired amplitude of a received signal is roughly 10 times kTB, where k is Boltzmann's constant, T is temperature in kelvin, and B is bandwidth in Hz. kTB is roughly -114 dBm per MHz receiver bandwidth. The noise figure of the receiver should be less than 3 dB (the noise of the receiver input should be < 3dB above kTB). The length of the antenna in proportion to the quarter wavelength is an important measure of the antenna sensitivity.

Bob S

Note:
dBm = dB milliwatts
0 dBm = 1 mW
10 dBm = 10 mW
20 dBm = 100 mW
etc.

Last edited:

I would like to address the questions and answers provided in this post.

Firstly, the length of an antenna is determined by the wavelength of the electromagnetic wave it is designed to receive. This is based on the principle of resonance, where the antenna length is one quarter of the wavelength of the wave. This allows for efficient reception of the wave.

In the case of TV and radio antennas, they are designed to receive longer wavelength waves, such as radio waves and microwaves. These waves have longer wavelengths, hence the antennas are longer in length. Cellphones, on the other hand, are designed to receive shorter wavelength waves, such as microwaves and infrared waves. Therefore, their antennas are shorter in length.

Secondly, the amplitude of a wave is not affected by the length of the antenna. The amplitude of a wave is determined by the source of the wave, not the receiving antenna. The purpose of an antenna is to receive the wave and convert it into an electrical signal, not amplify it.

In modern cellphones, the antenna is often hidden within the chassis of the phone. This is possible due to advancements in technology and miniaturization of components. The purpose of the antenna is still the same, to receive and convert the electromagnetic waves into an electrical signal for communication.

In conclusion, the length of an antenna is determined by the wavelength of the wave it is designed to receive. The amplitude of the wave is not affected by the length of the antenna. Advancements in technology have allowed for smaller and more compact antennas to be used, without compromising their efficiency.

## 1. How does the length of an antenna affect the reception of electromagnetic waves?

The length of an antenna determines the frequency of the electromagnetic waves it can receive. Generally, longer antennas are able to receive lower frequency waves, while shorter antennas are better for higher frequency waves.

## 2. What is the optimal length for an antenna to receive a specific frequency?

The optimal length of an antenna is typically half the wavelength of the electromagnetic wave it is intended to receive. This allows for the most efficient transfer of energy between the antenna and the wave.

## 3. Can the length of an antenna be adjusted to receive different frequencies?

Yes, the length of an antenna can be adjusted to receive different frequencies. However, this will require changing the physical length of the antenna, which may not always be feasible or practical.

## 4. How does the shape of an antenna affect its ability to receive electromagnetic waves?

The shape of an antenna can affect its ability to receive electromagnetic waves in various ways. For example, a dipole antenna with a straight shape is better for receiving horizontally polarized waves, while a loop antenna is better for receiving vertically polarized waves.

## 5. Can the length of an antenna impact the strength of the received signal?

Yes, the length of an antenna can impact the strength of the received signal. A longer antenna can receive a stronger signal, as it has a larger surface area to capture the electromagnetic waves. This is why longer antennas are often used for long-range communication.

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