Thread: Mini-HowTo Build your own TX antenna for FPV View Single Post
 Dec 16, 2009, 09:59 PM Engineer for Christ Amherst, VA Joined Jun 2006 11,092 Posts 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. Last edited by IBCrazy; Apr 15, 2011 at 08:47 PM.