Re: Make antenna for 3g usb data modem
Start with a simple quaterwave monopole antenna. If you know the frequency of operation (approx. center of the band), you can find the length using the following equation.
λ = c / f
, where λ = wavelength in meters
c = speed of light in freespace (~air) ~= 3 x 10^8 m/s
f = frequency of signal (Hz)
For example, if we wanted to make a quaterwave monopole for 250 MHz, we would do the following:
λ = 3e8 / 250e6
λ = 1.2 meters
so λ/4 (one quarter of a wavelength) = 0.3 m, 30 cm or ~11.8 inches
The antenna element would connect directly to the center conductor of the coax, and the shield should be connected to a ground plane. If you have a panel-mount coax jack, simply screw it into a metal sheet, and stick the antenna out orthogonal (at right-angles) to it.
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