Thread: 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.

Why do you want to build an antenna? Should it be better then existing antenna in some way? What is the existing antenna performance? According to attached picture I guess you want something omnidirectional that can be placed on a car roof? If you need improved gain, have you calculated a realistic goal in dB?
Find out what you want your antenna to do and in what frequency range it need to do it, for your local area.

From your specification, 3G, I assume you have less antenna experience as 3G not gives any information about the most important antenna parameter, actual frequency range.
It is different 3G frequency allocations over whole world so it is hard to guess what you need. It is possible to build a multi-frequency antenna that is covering all 3G frequencies from 700 - 2200 MHz, but that is not a simple thing to build even as experienced antenna designer and with required tools.
This log-periodic type of multi-frequency antenna is the most simple antenna that comes in my mind, which is rather simple to build even without measurement tools. It can be a good starter object were you can calculate your own antenna. How to mount it on a car roof will you also have to find out your self.

Originally Posted by E Kafeman

Why do you want to build an antenna? Should it be better then existing antenna in some way? What is the existing antenna performance? According to attached picture I guess you want something omnidirectional that can be placed on a car roof? If you need improved gain, have you calculated a realistic goal in dB?
Find out what you want your antenna to do and in what frequency range it need to do it, for your local area.

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hi the antenna shon in the figure is not with modem .. i want to make smiler one .. to increase signal strength .

The proposed log-periodic antenna is perfect then.

Hello,

Here is RFsupplier YouTube to show how to fit the pigtail cable with SMB cables.They are the same way to assembly the crc9 pigtail cable.

The DIY SMB cables YouTube: http://www.youtube.com/rfsupplier#p/u/4/4WjIHNrVcg8

PS:if someone who would like to know how to assembly cables by pictures ,warmly welcome.

KiKi