Hello Topbanders,
I've been reading several threads on the web recently concerning the
use of 90 degree hybrids as splitters or combiners. A hybrid has four
ports. Port 1 is the input port and phase reference. Ports 2 and 4 are
at plus and minus 45 degrees relative to port 1. Port 3 is the
isolation port, energy reflected back into ports 2 and 4 is dumped into
a termination on port 4. A diagram of the hybrid coupler can be found in
Figure 2.72 in ON4UN?s ?Low-Band DXing.? A twisted-pair 90 degree
hybrid directional coupler wound on a ideal ferrite or ideal powdered
iron toroid theoretically has a voltage phase shift of 90 degrees
between its output ports over an very wide frequency range. Realizable
hybrids can have substantial operating bandwidth in preserving phase.
Typically the relative phase between ports will have much wider
bandwidth
than the relative power split. The power ratio between the output ports
is
determined by operating frequency and termination impedance.
The properties of a hybrid are determined by the following equations
where Zo is the design impedance of the hybrid.
Zo = sqr(L/C)
Equal (-3dB) power split between -45 degree and +45 degree ports will
occur at a frequency fo when
2 pi fo L = ½ pi fo C = Zo
Coupling energy between ports 1 (input) and 2 (+45degree) is
P1/P2 = 1+ (2 pi fo /2 pi f)squared
For ports 1 and 4 (-45 degree) :
P1/P4 = 1 + (2 pi f/2 pi fo)squared
You can used the hybrid as a unequal power splitter by making the
design frequency different from the operating frequency. This has many
applications since in practice the 90 degree phase shift remains
relatively constant over octaves. The coupling energy of -3dB equal
split between the output ports 2 and 4 occurs only within a few percent
of the design frequency. Two or more hybrids can be cascaded to make
the system more broadband. Port 3 of the first hybrid is connected to
port 2 of the second hybrid. Also, port 4 of the first is connected to
port 1 of the second. This two stage hybrid system will provide an
equal 3dB split (0.7dB of ripple) over an frequency range of
approximately of 2fo when properly terminated.
I've used hybrids for many applications, such as feeding simple 2
element endfire vertical and beverage arrays, binomial current fed
"six-pack" (broadside-endfire) vertical and 3 element endfire beverge
arrays, directional couplers, etc with great success. To feed each
element of a current fed array, feedlines of 1/4 wave or odd multiples
thereof should be used to ensure proper current balance.
There are other ways to feed phased antenna systems to achieve to same
goal, but I've found that hybrids behaved well even when the loads are
slightly mismatched. These devices are more forgiving than many think
since output ports are isolated.
73, Peter WW2Y
--
FAQ on WWW: http://www.contesting.com/FAQ/topband
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|