This design is a two way combiner made up from two quaterwave transmission lines connected together. The transmission line is therefor one halfwave long - or 1040 mm. 50 ohm N-connectors for the two antennas are placed at each end and in the middle another N-connector for the output.
The two quaterwave sections must transform the antenna impedance of 50 ohm into 100 ohm, so when paralleled in the middle a 50 ohm impedance is again established.
The correct transmissionline impedance is the square root 50*100 or 70.7 ohm.

Measurement of return loss and splitter loss from 50 to 250 MHz:

On 145 MHz the return loss is >20 dB (equal to VSWR 1.2:1). This is acceptable. The splitter loss is 3.0 dB, exactly what theory says when a signal is split into two halves.