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Old Dec 16, 2009, 09:59 PM
IBCrazy is offline
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Building the antenna - Calculating your quarter wavelength

Building the antenna - Calculating your quarter wavelength

The first step in building any antenna is to calculate your effective wavelength. Many of us know the old formula: 2.98 * frequency in MHz = Wavelength in meters. This is the wavelength of a signal in free space, however we are interested in the wavelength in our transmitters. Thus we must include a “velocity factor”, which is usually around 95% of the actual calculated quarter wavelength in free space.

Everything can be determined based on a quarter wave antenna. I highly suggest using the exact frequency you plan to transmit. Why? Varying from the frequency causes antenna mismatch resulting in reduced VSWR! This is why the store bought antennas aren’t always the best match. They are made for a small range of frequencies.

Below are the formulas to calculate your active quarter wave assuming a 95% velocity factor:

Length in Inches (for 22AWG wire) = 2808/frequency in MHz

Or for you metric folks:

Length in Centimeters = 7125/ frequency in MHz

Or for 12-14 AWG:

Length in inches = 2750/f in MHz

Or on Centimeters = 6985/f in MHz

Alternately, you could go to any of these sites:
http://www.crompton.com/wa3dsp/hamradio/antcalc.html
http://www.qsl.net/kd4sai/antencal.html
http://bfn.org/~bn589/antenna.html
Thus for my 910 MHz system, my quarter wave is 3.086 inches or 7.84 centimeters.

Note: In all of the antennas designed this number will be the length of the ACTIVE part of the antenna. The length inside the coaxial line or inside the coupler is not considered active. It is active once the conductor leaves the shielding.
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Last edited by IBCrazy; Apr 15, 2011 at 08:47 PM.
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