Common Sense Yagi
The use of the first director to make the impedance matchable by the Gamma match is one of the fallacies in modern day antenna design in my estimation. The first director of a multi-element yagi should be left forward of the driven element by approximately 0.12 wavelength, and the reflector behind the driven element by approximately 0.15 wavelength. Then, concentrate on the driven element configuration to effect a good match to the transmission line. The rule of thumb used for driven element is to have it about 3 to 5% shorter than a free space wavelength. In reality, it doesn’t make any difference how long or how short the driven element is as it can be lengthened or shortened once gain and front to back ratio have been acquired. It can be the final tweak for point of resonance of the yagi. Basically the DE is simply an active launcher of energy to the rest of the structure. This is borne out by the original papers on the “Yagi Antenna” by Uda and Yagi.
It is my opinion that probably the most preferable driven element for use on a yagi is the folded dipole; 1) it has a high terminal impedance, which will drop as soon as a reflector and DE are set in place, 2) since current is constant in a series circuit there will be greater current distribution through out the entire driven element than just the first 1/8 of a wavelength either side of center in a single element dipole. By the time current reaches the end of a dipole where the impedance is on the order of 2500 ohms to infinity – there is little to no current. The only reason to have a second 1/8 wavelength in series with the first on either side of a half wave dipole is to satisfy the inductance requirement of a resonate circuit. A dipole driven element on a yagi makes the terminal impedance on the order of 20 ohms and that is why we have to use there first director and a Gamma match, Beta match, hairpin match to get the antenna matched to a 50 or 75 ohm transmission line.
In the design of a four element yagi, using a folded dipole for the driven element, with spacing of REF to DE of 0.15 wavelength, DE to D1 of 0.12 wavelength and stretch the D2 out to 0.2 from D1. The boom length is now approximately 0.5 wavelength boom supporting four elements. The diameter of the under side of the folded dipole can be sized to get any impedance transformation required or is convenient to match a transmission line of choice. The gain will be a true 8 dBi or 6 dBd and a F/B ratio of >20 dB. The antenna will have the center of its main lobe, bore sight with the boom, the antenna will be quieter and it will hear much better than most of the commercial offerings of today with their silly little inefficient Gamma matches. A compromise of the folded dipole driven element is the “T” match. The “T” arms are matching a resonate dipole out at the ~300 ohm points of the DE and being transformed to 50 ohms via a coaxial balun a little lossy yes, but still beats the heck out of the inadequacies of a Gamma matched half wave dipole. Try it see what you think, 73, Dave…
Dave W6OAL –
Olde Antenna Laboratory 41541 Dublin Drive Parker, CO 80138