[amsat-bb] Re: Arecibo and circular polarization using cheap yagis

[i8cvs]

----- Original Message -----
From: "Edward Cole" <kl7uw at acsalaska.net>
To: "Rick - WA4NVM" <wa4nvm at comcast.net>; "i8cvs" <domenico.i8cvs at tin.it>;
"Douglas Quagliana" <dquagliana at aol.com>; "amsat" <amsat-bb at amsat.org>
Sent: Friday, April 16, 2010 8:20 PM
Subject: Re: [amsat-bb] Re: Arecibo and circular polarization using cheap
yagis

My M2 436CP42 looks just like Dom's drawing but
it has a 1/2 wavelength longer phasing line
between the rear and front driven elements.  They
say this is the setup for RHCP?  It would seem
the M2 system would have the front antenna
180-degrees out of phase?  So would that not
produce LHCP?  It is actually easier to visualize
with both elements at the same point on the boom
and one fed with a 1/4 wave longer feed line.

73, Ed - KL7UW, WD2XSH/45


Hi Ed, KL7UW

What the M2 say is correct.

In a crossed yagi with vertical and horizontal elements spaced 1/4
wavelenght  ( 90°) over the boom if you look from the rear of the
antenna toward the direction of radiation the phase relations are as
follows:

Assume that the power is splitted in two equal parts and the horizontal
and vertical dipoles are feed in phase with equal lenghts of coax cable.

The inner conductor (+) of one coax cable is connected to the right side
of the horizontal rear dipole.
The inner conductor (+) of the other coax cable is connected to the upper
side of the front vertical dipole.

With the above coax cable connections  and +90° of phase shift over the
boom the antenna radiates RHCP

If you now add a 1/2 electrical wavelenght (180°) to the coax cable
supplying the front dipole then the polarization reverses in LHCP because
90° of mechanical shift over the boom minus 180° for the 1/2 wavelength
of coax cable make a difference of - 90°

This is exactly what your M2 436CP42 does when switching between
RHCP to LHCP

I agree with you that it is actually easier to visualize with both elements
at the same point on the boom and one fed with a 1/4 wave longer feed
line.

73" de

i8CVS Domenico