Microwave Diplexer Purely Based on Direct Synchronous and Asynchronous Coupling

A diplexer realized purely based on direct coupling
is presented. No cross-coupling is involved in the
design process. The microwave diplexer is achieved by
coupling a dual-band bandpass filter onto two individual
channel filters. This design eliminates the need for employing
external junctions in diplexer design, as opposed to
the conventional design approach which requires separate
junctions for energy distribution. A 10-pole (10th order)
diplexer has been successfully designed, simulated, fabricated
and measured. The diplexer is composed of 2 poles
from the dual-band filter, 4 poles from the Tx bandpass
filter, and the remaining 4 poles from the Rx bandpass
filter. The design was implemented using synchronously
and asynchronously tuned microstrip square open-loop
resonators. The simulation and measurement results show
that an isolation of 50 dB is achieved between the diplexer
Tx and Rx bands. The minimum insertion loss is 2.88 dB
for the transmit band, and 2.95 dB for the receive band.