[time-nuts] GPS splitter
jebponsonby at gmail.com
Sat Jun 17 15:29:55 EDT 2017
There is a poorman's way of making a -3dB splitter using lengths of cable of characteristic impedance equal to the system impedance. That is to say using only 50Ω cable in a 50Ω system. It is a variant on the well known square four-port quadrature hybrid. In the standard hybrid with ports A,B,C & D, with a signal entering at A, half the power (-3dB) emerges at B and the other half at C and in principle nothing emerges at D. The outputs at B & C are in RF phase quadrature with C lagging. The four sides of the square are all of length λ/4. The sides BC & DA are of 50Ω cable whilst sides AB and CD are of cable with characteristic impedance 50/√2=35.35Ω. The poorman's version is to replace the low impedance sides (AB & CD) with λ/8 lengths of 50Ω cable and add λ/8 open ended shunt stubs of 50Ω cable at each corner. As a splitter port D should be terminated with a 50Ω load. The OC ends of the stubs should be shielded with caps but in a way that doesn't significantly add end capacitance. This poorman's hybrid is rather narrow-band but adequate to cover the GPS L1 band (1575MHz). Three such hybrids may be joined to form a 4-way splitter which in principle has -6dB from input to each output. There is no DC path from inner to ground.
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