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Re: Power divider for X-yagis



----- Original Message -----
From: "Pieter Ibelings" <sietetrescincoprimo@hotmail.com>
To: <domenico.i8cvs@tin.it>; <pa1jav@AMSAT.Org>; <amsat-bb@AMSAT.Org>
Sent: Friday, May 26, 2006 5:07 PM
Subject: Re: [amsat-bb] Power divider for X-yagis


> Hi All,
>
> Building power combiners out of transmission lines and paralleling
antennas
> has a lot of limitations. If you assume that both antennas are exactly 50
> ohms, then a piece of coax of 35.4 ohm impedance will rotate the impedance
> to 50 ohms. This type of splitter will have exactly 3.01 dB of split loss
on
> each port ONLY if the two antennas are exactly 50 ohms. If for example,
your
> antennas are matched to 1:1.2 VSWR this means that one antenna could be a
> 41.7 ohms and the other at 60 ohms. In this case, one antenna will be at
> -3.9 dB and the other antenna will be at -2.3 dB. At an VSWR of 1:1.4
(still
> considered good) the split between one antenna and the other will differ
by
> almost 3 dB. As you can see, the circularity is going down hill quick. It
is
> much much better to use another type of splitter that offers isolation
> between the antennas.
>
> These numbers above are pretty much the same if you use a pair of 70.1 ohm
> lines to convert each antenna impedance to 100 ohms and then parallel the
> transmission lines. In order to become less sensitive to antenna match,
you
> need to add a 100 ohm resistor between the antenna legs (in the case of
the
> 70.1 ohm line example). That will make the split -3.01 dB even with 1.4
> VSWR.
>
> One last thing to keep in mind is reactive terminations. If your antenna
is
> not matched to look purelly resistive, any reactance (even with
relativelly
> low VSWRs) will make your splitter not be 0 degrees.
>
> Pieter, N4IP


Hi Peter, N4IP

I agree completely with you and infact for my 2 meters X yagi 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 °\            /
                           \        /
                             \    /
                               \/

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 50 ohm coax cable RG 213 or RG 58
depending on TX power.

Section 1-4 is a single run long 3/4 electrical wavelenght made with
1024 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 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 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.

About the problems that arises when the antenna is not purely resistive
in our discussion we should consider the complex impedance R+jX and
R-jX of the antenna and not only a simple 50 ohm impedance but this
refinement was not requested by the PA1JAV question.

Best 73" de

i8CVS Domenico
----
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