[amsat-bb] Re: yagi design

[Art McBride]

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: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Glen Zook
Sent: Sunday, March 22, 2009 10:30 AM
To: AMSAT-BB at amsat.org; 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 <audbert at gmail.com> 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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