If your radio can use different sized antennas, typically this is done so that you can switch out a whip antenna which will give you wider coverage with a stubby antenna, which will make the radio more compact. More important than the length of the radio antenna is the height of the antenna. The higher you can place the antenna, the further the range that you can successfully transmit and receive.
In this case, an externally-mounted antenna will give you better range. It is also possible to extend the range of a handheld device with an antenna if it is part of a desktop base station. A base station radio might have a built-in antenna, but it can be disconnected and instead be connected by cable to an external antenna.
Mobile radios and base station radios are far more powerful than handheld radios, sometimes as high as watts. Keep in mind that with certain frequency bands, there may be limits to how high an external antenna can be. For MURS radios, the FCC dictates that the antenna height cannot be greater than 20 feet above a structure or 60 feet above ground level, whichever is greater.
If possible, do both. Categories: Uncategorized. A dipole is basically a piece of metal that receives RF transmissions. A simple dipole looks like this:.
Basically a T-shaped piece of metal. The loop design makes it stronger. Cover that in plastic and you begin to get an idea of how an antenna could look like the one at the top of this article.
A yagi antenna combines dipoles of different sizes to create an antenna that is very good at getting a wide range of frequencies. The front part of that Televes antenna is a yagi, and behind it is a folded dipole. There are certain frequencies that are resonant in the antenna, which is when the efficiency of energy reception is highest.
This frequency is determined by the length of the antenna and the speed of light in the antenna material. In this resonant condition, the electrons' motion and the incoming electric field are always in the same direction, so every wavelength of the EM wave builds up more motion and puts more energy into the antenna. If the frequency of the EM wave is not at the correct frequency, then sometimes the electrons' motion and the electric field will be in opposite directions, leading to a loss of energy in the antenna.
Like pushing someone on a swing, each push has to be at the right time and in the right direction. The red represents density of charges as they are pushed back and forth. The condition for resonance in an antenna is that the wavelength of the standing wave is twice the length of the antenna see the above animated diagram. In fact, most simple antennas have a length close to half the wavelength of the signal they are built to receive.
As an analogy, consider a basin half-full of water. You can tilt the basin to one side and the water rushes to the lower end. If you tilt the basin the other way, the water rushes to the other side. When you tilt the basin back and forth, most of the time the water just moves back and forth with little other effect.
This is like the electric field of an EM wave that pushes the electrons in the antenna to one side. At a low frequency of tilting, the water just moves back and forth, settling at each end before the tilt reverses. At a high frequency, the water barely has a chance to move before the tilt reverses. But, if you tilt back and forth at just the right frequency, the sloshing of the water builds up and builds up until it splashes completely out of the basin.
This is the resonant condition of the basin of water. You can imagine that longer basins have a lower frequency of resonance since it takes longer for the sloshing water to go from one end to the other. It's the same with an antenna: longer antennas have lower frequency resonance because it takes longer for the EM wave to bounce back and forth between the ends.
The following is a view of the charges in an antenna that shows the sloshing of electrons at resonance the red shaded area; blue shows the magnitude of the velocity of the charges.
It is the other way. For every length of an antenna rod there is an optimized frequency for which the energy loses to the radio wave has a maximum in relation to the energy loses in the wave generator "circuit". Since for all frequencies the velocity of propagation in vacuum is the same there seems to be a simple relation between the rod length and the wavelength.
But in general it is possible to drive every rod length as well as every conducting body with a wave generator of any frequency. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
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