[amsat-bb] Re: Question about Circulair Polarisation

i8cvs domenico.i8cvs at tin.it
Mon Sep 15 19:46:14 PDT 2008


----- Original Message -----
From: "pe0sat" <pe0sat at vgnet.nl>
To: <amsat-bb at amsat.org>
Sent: Monday, September 15, 2008 9:47 PM
Subject: [amsat-bb] Question about Circulair Polarisation
>
> Hello,
>
> Can I achive circular polarization the following way?

Hi Jan, PE0SAT

Your drawing is not clear to me because I cannot understand from it how
both dipoles are mounted one with respect to the other over the boom.
>
> >From each dipole a length off aircell-7 to a dual power splitter with one
> length being a quarter wave length longer than the other?
>
> Dipool 50ohm ------------    |         |
>                                splitter  --------------- transceiver 50ohm
> Dipool 50ohm --------------- |         |
>
> I made one lenght longer following this calculation:
> lambda/4 * V that gave a lenght of 0.690/4 x 0.83 = 0.143m
> for Aircell-7 coax cable.
>
If both dipoles are mounted crossed + or X i.e. at 90 degrees one to the
other and over the same physical plane then using a 1/4 electrical wave
as a delay line as you correctly calculated a circular polarization is
generated.

> Now switching the coax between the dipoles I will get LHCP or RHCP.

It is correct only in theory because in 70 cm the distance from the vertical
to the horizontal elements over the boom is 2 or 3 cm representing a
considerable fraction of 1/4 wavelenght that must be compensated trimming
the lenght of the delay line but the lenght must increased for one sense of
polarization and decreased by the same amount for the opposite polarization.
This is why the method to space the elements by a 1/4 freespace wavelenght
over the boom is preferred.
In this latter case adding a 1/2 electrical wavelenght to the front dipole
nearest to the satellite reverses the sense of polarization without need of
any additional compensations.
>
> Is there anything wrong in this approach?

If both dipoles are at 90° one to the other and the elements are mounted
over the same plane with a minimum of mechanical distance over the boom
it can be accepted with a certain deviation over the perfect circularity.

> My reason for asking, is the
> fact that now in fixed RHCP position, AO-51 is very weak, weaker then
> before when I used a vertical placed yagi. (see
> http://www.ham.vgnet.nl/?Blog:Sept_2008 for more detail)
>
Are you sure that the delay line is connected to get RHCP ?

What reasoning you did connecting cables to the dipoles to establish
the expected sense of polarization ?

Looking at your website it seems that you are using a Cue Dee antenna but
the definition of the photos is low so that even enlarging the picture it is
impossible to see how the arms of both gamma-match are oriented and
this is very important to determine which sense of polarization you can
achive connecting the delay line to one dipole or to the other one.

In addition the gamma match system is not recommended particularly
for 70 cm and a folded dipole with a 1/2 electrical wavelenght coax balun
4/1 ratio is a more efficient radiator.

> 73's PE0SAT
>
73" de

i8CVS Domenico






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