Original article from: Wayne Sarosi 
 Reposted by Rob Dennis,Moderator HAM_TECH,for use of the FIDO HAM_TECH echo.
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SUBJECT: Circular Polarized Antennas,part 2
 One antenna that uses circular polarization is the helical antenna.
 It comes in two flavors,LHCP and RHCP for left and right hand circular
polarized. As stated on the previous posting,RHCP is a clockwise rotation as
the wave receeds from the point for transmission. The RHCP helical antenna is
phsyically configured this way with the turns of the helix rotating clockwise
away from the mounting point.
 The LHCP helix is phsyically configured counter-clockwise away from the
mounting point. The helix is an end-fed antenna. For satellite and EME work,
both LHCP and RHCP helix antennas are required for proper signal reception as
the polarization changes when the satellite passes overhead or the returning
signal from an EME transmission is received.
 In the next posting,there will be some other CP antennas for you to look at
that are used at the Kennedy Space Center. For a refernce of Helical 
ntennas,
read the ARRL Antenna Handbook,pages 19-22 thru 19-33 in the 16th edition.
 To repost the figures here would require several listings,but an overview
will be coming in the series.
 In general,the helix is very predicable in gain and pattern.
 Introduced in the 40s,the helical antenna resembles nothing more than a 
arge
air-wound coil with characteristics showing its electrical pattern.
 There are other types of antennas with circular polarization such as the
cavity backed sprial which comes in the same flavors,LHCP and RHCP.
 These type of antennas are commercial grade and seldom used in the amateur
world. One strange antenna is the 'twisted yagi' of which I am still trying 
o
find some info on. I don't know if the name is correct,but I have heard it
refered to that way.
 The twisted yagi is nothing more than a yagi with the elements offset in a
continous rotating fashion. Each element is positioned about 30 degrees from
the one before. They can get rather long for the designed frequency.
 I'll see what I can dig up on this type.
 For true CP design,the choice of antennas are limited. Elipical
patterns are most used for satellite and EME use.
-WS
--- GoldED 2.42.G1219
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