All Questions
16
questions
15
votes
7
answers
2k
views
Why does the length of an antenna matter when electromagnetic waves propagate perpendicular to the antenna?
The optimum length for a dipole antenna is a multiple of half the wavelength that it is designed to receive or emit. Why is this? If an electromagnetic wave has E in the x-axis, B in the y-axis, and ...
- 161
1
vote
2
answers
52
views
Minimum frequency for a dipole antenna to work
Does a dipole antenna need a minimum frequency of AC to make the electric field detach and propagate?
- 156
0
votes
2
answers
70
views
Why do parabolic antennas need to be the same width as their wavelength?
I am reading the wikipedia page for parabolic antennas, and have a question about the below quote:
In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the ...
- 137
1
vote
1
answer
63
views
If the speed of light is a constant value and the wavelength is constant (antenna), how can one antenna transit/receive different frequencies?
The speed of light is about 300,000,000 m/s, and the wavelength is determined by the design of the antenna, how can one antenna transmit different frequencies other than just the one defined by
f = c /...
- 13
6
votes
3
answers
743
views
When designing antennas which speed of light should be used?
When calculating "antenna element length" should we use the speed of light in the "medium between transmitter and receiver (antenna)" or the speed of light in the "element ...
- 613
2
votes
1
answer
122
views
Antennas and Wavelength
I'm clearly missing something here, but I'm trying to grasp basics of how antennas work.
Relating it to standing waves on a string of length L, the lowest frequency possible is a wavelength of 2L, due ...
- 111
1
vote
2
answers
405
views
Why is a receiving antenna optimal at $\frac{\lambda}{2}$, mathematically?
I can understand the qualitative argument of a recieving antenna becoming resonant: An external $E$-field causes the charges to move in a conductor and bunch up, creating a voltage. If the driving $E$-...
- 173
0
votes
0
answers
77
views
EM scattering cross section, antenna directivity and its interpretation
The formula (35.25), see Slater, $\bar A_r = \frac{\lambda^2}{4\pi}$ is probably the most important formula in radar and/or antenna engineering. It says that the average absorbing cross section of ...
- 19.8k
0
votes
0
answers
105
views
Why half-wave dipole is the most efficient one in comparison to other electrical lengths?
I have a problem on the understanding of a half-wavelength antenna. Namely, why is the half-wave antenna has a highest power emission. Considering Far Field factor F($\theta$), which is known from the ...
- 591
0
votes
0
answers
91
views
Walkie talkie antenna
how is it possible that the antennas of the walkie talkies are so small, if the frequencies they receive have wavelengths of more than 30 cm? Maybe the why's are quarter wave antennas? and for pulsed ...
- 11
1
vote
2
answers
728
views
Why aren't extremely-low-frequency (ELF) radio waves used for underwater radar?
Since extremely-low-frequency radio waves are used by submarines for some simple, low-transmission-rate communications, why can't those same wavelengths be used for submarine radar? It may not be ...
- 4,509
4
votes
1
answer
537
views
How far do electrons go in an antenna?
Do you know what is the distance electrons cover in an antenna during the production of a radio wave?
Does the extension of the oscillation vary with the frequency or the power of the radiation? Is ...
user137879
1
vote
1
answer
400
views
How is it that 62" is the best length for 72Mhz receiver?
According the this article (which was published somewhere in 2005-2008 I think, but it's still comes up first in Google) the best length for a 72Mhz receiver antenna is 62" (~157.5cm). This puzzles me ...
- 158
33
votes
7
answers
23k
views
Radio antennas that are much shorter than the wavelength
From my limited experience with ham radio when I was a kid, I expect transmitting and receiving antennas to have lengths that are on the same order of magnitude as the wavelength, and in fact I recall ...
user4552
17
votes
2
answers
42k
views
Why is Near Field Communication (NFC) range limited to about 20cm?
Near Field Communication (NFC) operates at 13.56 MHz.
Near Field is the region situated at a distance r << λ
λ = c/f
...
- 313