Glen, How a Quagi or Yagi works is dependent on element spacing which can give narrow band higher gain performance at the expense of F/B ratio and very low driven element impedance. An un-normalized impedance Yagi can have an impedance of less than 20 Ohms. The distance of the first director has the most control over reducing the antenna driven element impedance.
A folded dipole can be used for an initial impedance of greater than 200 Ohms, or an inductor (hair pin match) can be used to raise the dipole impedance. The distance of the first director has the most control over the antenna impedance.
The initial impedance of a loop is 100 Ohms (Quagi), and the Dipole is 75 Ohms (Yagi) hence there are some cases that the Quagi will provide a closer to 50 Ohm un-normalized impedance than a Yagi. All require some means of transformation of balanced to unbalanced when using a coax feed line.
A quad type Yagi has slightly more gain per number of elements and would be the best choice for gain VS boom length.
Art, KC6UQH
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Glen Zook Sent: Sunday, March 22, 2009 10:30 AM To: [email protected]; Bert Audenaert Subject: [amsat-bb] Re: yagi design
For a "bullet proof" and "cookbook" antenna design you cannot beat the quagi. The do not require any special feedline matching or anything else. Basically, build it like the instructions and they work the first time. Go to
http://commfaculty.fullerton.edu/woverbeck/quagi.htm
for instructions on building one.
Glen, K9STH
Website: http://k9sth.com
--- On Sun, 3/22/09, Bert Audenaert [email protected] wrote:
Hereby my first mail on this BB. I want to start with satellite communication. I already build an eggbeater antenna, but togheter with my TS2000 i receive only very poor signals. To work more comfotable, i want to build two yagi antennas (one for 2m and one for 70cm) with an azimuth/elevation rotor system.
For the 2m version, I took the following design of DK7ZB as a start: http://www.qsl.net/dk7zb/start1.htm -> 144MHz yasis -> 6 El-28-Ohm (2m60). Because I want to tune this antenna to the upper side of the 2m band (I had 145.800MHz as center frequency in mind), I entered the data of this yagi in MMANA GAL, a free NEC program (http://mmhamsoft.amateur-radio.ca/mmana/). After Calculation, i got totally diferent results as on the DK7ZB website.
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