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Re: Power divider for X-yagis



----- Original Message -----
From: "Robert Bruninga" <bruninga@usna.edu>
To: <amsat-bb@AMSAT.Org>; <pa1jav@AMSAT.Org>;
<sietetrescincoprimo@hotmail.com>; <domenico.i8cvs@tin.it>
Sent: Saturday, May 27, 2006 4:09 PM
Subject: Re: [amsat-bb] Power divider for X-yagis


> >>> "i8cvs" <domenico.i8cvs@tin.it> 05/26/06 2:53 PM >>>
> >... I have used for several years a very simple and effective
> >hibrid ring made of coax cable as per the following drawing.
> >
> >                           3
> >                             /°\
> >                           /     \
> >                         /         \
> >                        /            \ 4       50 ohm
> >                  2  °|              |°----/\/\/\/\----|
> >                        |              |
> >                        |              |
> >                    1 °\            /
> >                           \        /
> >                             \    /
> >                               \/
>
> Would this drawing be an easier visualization?
>
>         2----------------3
>         |                      |
>         |                      |
>         |                      |
>         |                      |       50 ohm
>        1 *------------- 4----/\/\/\/\----|
>

Hi Robert

Yes, your drawing visualize correctly my original one but please amend
some mistake that I did  typing my previous text. The following text with
some additionals is correct.

> For 145 MHz and using coax cable with velocity factor 0.66
>
> Section 1-2 , Section 2-3 and Section 3-4 are each long 1/4 electrical
> wavelenght made with 34 cm of  75 ohm coax cable RG 11 or RG 9
> depending on TX power.
>
> Section 1-4 is a single run long 3/4 electrical wavelenght made with
> 102,4 cm of  75 ohm coax cable RG 11 or RG 59 depending on TX
> power
>
> The impedance at port 2 is 50 ohm and if a 50 ohm feedline is connected
> to a TX/RX than the power appearing at ports 1 and 3  is divided by two
> (3.01 dB) and both signals appearing in 1 and 3 are in phase.
>
> The impedances at port 1 and 3 are 50 ohm and both ports are in phase so
> that each dipole of the X yagi must be connected to 1 and 3 using two runs
> of 50 ohm coax cable having the same lenght and taking in to account if a
> delay line is used or not to get circular polarization.
>
> Port 1 and port 3 are isolated between them by at least 30 dB so that
> through the ring one dipole cannot see the other and the circularity do
> not suffer too much in case of small difference in SWR between them
> but in case of fault of a dipole the 50 ohm termination must be capable to
> dissipate 1/2 of the full TX power.
>
> This hibrid ring can be built on a brass plate with 8 holes having the
> same diameter of the braid of the coax cable used for it.
> The 8 braids of the coax sections and the 3 flanges of the N connectors
> must be tin soldered to the brass plate.
>
> To save room all sections of coax cables can be shaped in any form and the
> full job using material available in any junk box take about half day.

                                   - additionals -

If you use 75 ohm coax cable type RG 302/U wich is teflon insulated it
will be easier to solder the braids to the brass plate without damaging the
dielectric and since the above cable is only about 4.5 mm outside diameter
the hibrid ring became very small while it is capable to handle about 1 kW

Since the velocity factor of RG 302/U is 0.695 than Section 1-2
Section 2-3 and Section 3-4 are each 36 cm long and Section 1-4
is a single run 107,8 cm long at 145 MHz

The advantage of this hibrid is that all uncorrelated currents due to
mismatch and different SWR between ports 1 and 3 will flow in to
the 50 ohm termination dissipating in it.

In general the characteristic impedance of the coax cable to be used
for the hibrid is 1.41 time the impedance that must be seen at all
ports so that for 50 ohm ports a coax cable of 75 ohm is satisfactory.

The above hybrid ring designed for 400 MHz was even used in the early
years for a receiver at the Radiotelescope "Croce del Nord" of Medicina
Bologna and the above one designed for 145 MHz was originally described
in italian by OM radioastronomer Goliardo Tomassetti i4BER for Radio
Rivista Magazine 2/1973

If of interest I will try to describe in more details how it works from
the theoretical point of view.

> Best 73" de
>
> i8CVS Domenico
----
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