Re: [amsat-bb] Power divider for X-yagis

["Tom Clark, K3IO" <K3IO@xxxxxxxxxxx>]

Domenico -- Let me try a different visualization for what you described:

The "Rat Race" Hybrid (assuming 50 ohms is the desired impedance) makes 
a ring out of 4 pieces of 72 ohm coax; three are 1/4 wave and one is 3/4 
of a wave (i.e. a total of 1.5 wavelengths needed). It's usually easiest 
to coil the long piece and number the ports (in any order) 1, 2, 3 & 4. 
If you put power into port 1, then the power is split equally between 
ports 2 & 4.

Unless the loads at 2&4 are mismatched, no power is present at port 3  
(i.e. the port opposite the input) and it should be terminated to soak 
up any mismatches. The signals at ports 2&4 are either in phase (if the 
coaxes 1-2 and 1-4 are the 1/4 wave pieces) or out of phase if either 
1-2 and 1-4 are the 3/4 wave piece.  You can reverse the polarization 
sense by swapping ports 1&3 with a DPDT relay to swap ports 1&3.

If you are feeding crossed yagis with their feeds at the same place on 
the boom, then you will need one 50 ohm, 1/4 wave coax piece going to 
one element:

               2 = antenna #1
               |
               *
              / \
             /   \  <---- These 3 pieces are all 1/4 wave long
            /     \
      1----<      >---3
                         \         / 
                          \     O    <---- the "O" shows the 3/4 wave piece
                            \   /
                              *
                               |
                              +-------- 4=Antenna #2
                                     ^
                                      |
               an extra quarter wave in the cable to antenna #2


-------------------------------------------------------------------------------------------

There is also a simple coax cable quadrature hybrid that I like. Cut two 
1/4 wave pieces of 50 Ohm coax and four 1/4 wave pieces of 72 Ohm coax. 
Then you can build this quad hybrid:


    A-----------*===========*---------B
                        |                          |
                        |                          |
                        |                          |
     D-----------*===========*---------C

All four sides of the ring are 1/4 wave long. The *====* cables are two 
pieces of 72 ohm coax in parallel (i.e. making a 36 ohm cable), the the 
vertical | cables are 50 ohms. Feed port A, and ports B&C are phased at 
90 degrees and can connect directly to the antenna feed points. If the 
antennas are properly matched, the there is no power at port 4, but it 
is best to use a 50 ohm termination to soak up any reflections.

A DPDT coax relay can swap the A/D input ports to select polarization.

Personally, I like the second scheme because it  is symmetric and leads 
to a neater package and it does not require the extra 1/4 wave phasing 
cable.

----------------------------------------

Notes -- when I mentioned 50 ohm cable, I envision RG58 and RG59 for the 
72 ohm cable. The desired "72 ohm) impedance is really 50*sqrt(2) for 
both designs. And the 1/4 & 3/4 wave is the ELECTRICAL length, which is 
about 2/3 of the physical length ( i.e. ~34.3 cm = 13.5" for 2M).

73 de Tom, K3IO
----
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