Original article from: Wayne Sarosi 
 Reposted by Rob Dennis,Moderator HAM_TECH,for use of the FIDO HAM_TECH echo.
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The Quad series:
 For this series Quads will be discussed in the general sense.
 This series,construction,dimension formulas,matching,mounting,and such will
be covered. Quads are powerful antennas.
 A three element quad will give a yagi a fit in head to head comparison.
 On two meters,a ten element quad is hard to beat.
 Quads can be set up vertical or horizontal.
 They can be tuned. They can be part of an array.
 For HF use this series will discuss 2 and 3 element arrays.
 On VHF/UHF arrays will cover 3 to 10 elements up through 70cm.
 Just what is a quad? The term is used loosely to discribe a beam antenna 
ade
up of two or more full wave elements, where the elements a parallel to each
other and the direction of the radiation pattern is perpendicular (end fire)
in nature.
 There are many terms we are familar with such as Cubical Quad,Quad 
rray,Loop
Array,Moon Raker,PDL-2,Field Gun,etc.
 All are quads and yes,some are CB antennas listed here (last three).
 Let me explain why I included these antennas.
 These antennas are good examples of quad arrays that work and work well.
 Although designed for 11m,it wouldn't take much in the form of calculations
to make a 10m,12m or 15m antenna out of them.
 Quad means four,which discribes the four quarter wave sides of each element.
 I refered to a loop array which is sometimes attached to a quad but be aware
that there is a circular loop array which packs a bit more punch than a quad.
 This I'll explain later.
 The quad elements can come in various shapes from square to diamond.
 The area that the element encloses will play an very important part in the
impedance of the antenna. Quad elements can be combined with yagi elements to
form a quagi.
 Usually the driven element and reflector are quad while the directors are
yagi.
 Polarization is simple to spot on a quad element. It is as such:
              -------------       -------------
             |             |     |             |
             |             |     |             |
             |             |     |             |<---- Feed
             |             |     |             |      (Vertical)
             |             |     |             |
              -------------       -------------
                    ^
                    |
                   Feed
                 (Horizontal)
 Note 1: Either feed placed on the opposite side will work the same.
 Note 2: The closer the area of the enclosed quad element approaches a 
ircle,
        the closer the impedance approaches 50 ohms.
 The quad element can be tuned using a tuning bar and dock.
                        ---------------
                       |               |
                       |               |
                       |==|= *         |<--- Feed
                       |               |
                       |               |
                        ---------------
                        * dock and tuning bar.
              |
              |
              |
              |_________|__
               dock     | <---Tuning bar
               _________|__
              |         |
              |
              |
              |   Close up of dock and tuning bar
 What this does is provide a way to adjust the length to the element
electrically by sliding the tuning bar along the dock.
 With the tuning bar close to the element side,the length is short or higher
in frequency. With the tuning bar at the end of the dock the length is longer
or lower in frequency.  This tuning method will help with SWR adjustments.
 A typical dock length is 8 in at 10m. The ratio holds at lower frequencies.
 In the VHF and UHF ranges this method of tuning may prove impractical.
 Instead,a shorting arm is used.
  Also note that in VHF/UHF frequencies,Circular loops are used especially at
UHF frequencies.
 The shorting bar is exactly what is sounds like. It cuts a corner making the
length of the circumference shorter. This works for elements longer than what
was desired in the design.
              |
              |
              |
              |\
              |  \
            a |    \ c
              |______\________
                  b               c < a+b
VHF/UHF adjustable active element an elements:
 Above we discussed the use of a dock and the shorting bar for making
adjustments in the circumference. Both these methods are limited in their
adjustments. The following method allows a greater range of adjustment.
 This method also has some limitations because it is designed to work with
tubular elements and not wire elements.
 Thus,it's greatest area of use is with VHF/UHF antennas. Within the confines
of this modification there are two different applications:
 One for circular elements and one for square elements.
 Let's look at the latter first. It is also the easy one to build and adjust.
 The Square adjustable element is built using two main componets and two 
mall
hose clamps.
 The adjustable points are located adjascent to the the feed point:
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                       |                    | * Origin: VE3SJN....Moderator....HAM_TECH (1:163/506.4)