SUBJECT:  Circular Polarized antennas - part 3

    BTW, Jim Grubs passed a posting to me about the 'twisted Yagi'
which is called a concentric yagi.  Now that I have the name right
and a lead on the articles in CQ magazine, I can fill in the rest.
Thanks Jim!

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    HELIX:  Well, I promised some figures for all of you concerning
the helix antenna as it is probably the most popular CP antenna.  After
researching the subject of helix antennas, I found the design parameters
are all basically the same with the only difference being the variables
used to describe the different values.  The ARRL Antenna Handbook has
a very detailed construction dialog.  There are plenty of pictures
and easy to use formulas.  A couple of technical antenna theory books 
at work and from college tend to bury themselves in mathmatical theory
removed from practical application that we require for working models.

    Theory is good, don't get me wrong, but theory at college level for 
amateurs that have not been to college, is not good for contruction.
It assumes too much on the average amateur in the area of math.

    For now, I will give the basic design parameters for the helix 
antenna and then conclude with the some information on the cavity backed 
spiral.


    In the ARRL Antenna Handbook, 16th ed, 19-22, provides about the best
description of a helix and how to build one.

    The Helix has two componets; a ground plane and the helix.  It is fed
with a coax.  The diameter of the ground plane is 0.8 to 1.1 wave lengths
in diameter.  This section is connected electrically to the coax braid.

    Since ascii does not provide all those neat little greek letters I'll
have to improvise here.

    C is the circumference of a coiled turn.
    S is the spacing between one coil and the next.
    G is the diameter of the ground plane.
    g is the gap between G and the begining of the coils.
    AR is the axial ratio
    D is the diameter of a coil turn.
    n is the number of turns
    L is the length of conductor in one turn.
    w is the wave length.
    Z is the impedance in ohms
    Gain is the gain of the antenna (now that was a smart statement)
    HPBW is the half power bandwidth


    C = 0.75w to 1.33w where w = 984/fMHz in ft or 300/fMHz for meters

    S = 0.2126*C to 0.2867*C

    G = 0.8w to 1.1w

    g = 0.12w to 0.13w at 45 degrees to the ground plane.
        Note: This is similar to the distance between a DE and D1 
              in a yagi.

    AR = 2n+1/2n  which should be as near to unity as possible.  The 
         closer you can get this ratio to one the closer you will 
         have a pure circular pattern.

    D = C/3.1415927 => 0.2387w to 0.35w

    n => use AR until you get a balance between a good ratio and a 
         practical length you can handle.
         AR = 2n+1/2n => n=6 then 13/12 = 1.083333, n=3 then 7/6 = 1.1666,
         n=10 then 21/20 = 1.05.  As you can see there isn't a vast 
         improvement between 6 and 10 turns.  n=15 then 31/30 = 1.03333.
         Not much more here either.

    L = sqrt[(piD)^2 + S^2]

    Z = 140*C ohms which is 0.75w*140 to 1.33w*140 ohms

    HPBW = 52/(C*sqrt[n*S]) degrees

    Gain = 11.8 + 10log((C^2) *n*S))

    First nulls beamwidth = 115/(C*sqrt[n*S]) degrees

    Pitch angle between the coils should be 12 to 16 degrees or
    arctan(S/C)


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    Heres some stats from an antenna we use at the EMLab.  The antenna
is a broadband cavity backed spiral antenna made by EM systems, model
A2200.  It's good from 1 to 18 GHz with an SWR of 1.8:1 over the entire
band.
              Diameter = 6.85-in
              gain = 0 dB minimum 5.5 dB max
              Polarization = LHC or RHC
              axial ratio = 3 dB
              3dB Beam width = 80 degrees
              MIL-E-16400 class 2 equip.


-WS