All:
SUBJECT:  Circular Polarized antennas - part 3
 BTW,Jim Grubs passed a posting to me about the 'twisted Yagi' which is 
alled
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 
alues.
 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 math
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 
iameter.
 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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 Here's 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
--- GoldED 2.42.G1219
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