Free RF Diplexer & LC Crossover Filter Tool
Design lumped-element LC low-pass and high-pass crossover diplexers for shared antenna feedlines (VHF/UHF), port-to-port isolation, and $50\,\Omega$ matching.
📻 Diplexer Frequencies & Topology
📊 Crossover Frequency & LC Network Values
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RF Diplexer Theory & LC Crossover Design
An RF diplexer allows two transceivers or receivers operating on different frequency bands to share a single antenna and coaxial feedline without interfering with each other. It consists of a Low-Pass Filter (LPF) connected in parallel with a High-Pass Filter (HPF) at a common antenna junction.
Geometric Crossover Frequency ($f_c$)
To provide equal attenuation margins for both the upper cutoff of the low band ($f_L$) and the lower cutoff of the high band ($f_H$), the crossover frequency is set to the geometric mean: $$f_c = \sqrt{f_L \cdot f_H}$$ At $f_c$, both filter branches present an input impedance equal to $\sqrt{2} Z_0$, which when placed in parallel restores the exact system impedance ($Z_0 = 50\,\Omega$).
Element Denormalization Formulas
- LPF Series Inductor: $L_{LP} = \frac{g_k \cdot Z_0}{2\pi f_c}$
- LPF Shunt Capacitor: $C_{LP} = \frac{g_k}{2\pi f_c \cdot Z_0}$
- HPF Series Capacitor: $C_{HP} = \frac{1}{2\pi f_c \cdot Z_0 \cdot g_k}$
- HPF Shunt Inductor: $L_{HP} = \frac{Z_0}{2\pi f_c \cdot g_k}$
Frequently Asked Questions
Why must both filters in a diplexer be designed together rather than separately?
If you simply connect two independent 50-ohm filters in parallel at an antenna port, their out-of-band reactive impedances will load each other, destroying passband return loss and causing high SWR. A true complementary diplexer ensures that while one branch is passing signal, the other presents an open-circuit (infinite) reactive impedance.
Can I transmit on both bands simultaneously through a diplexer?
Yes! That is the primary purpose of a diplexer. With sufficient filter order (e.g. 5th order providing >40 dB isolation), you can transmit 50W on the 2m VHF band while simultaneously receiving or transmitting on the 70cm UHF band without desensitizing the receivers.
What type of inductors should be used for VHF/UHF diplexers?
Use high-Q air-core coils wound from heavy silver-plated copper or enameled copper wire (16 to 12 AWG). Avoid powdered iron or ferrite toroids at VHF/UHF because core saturation and hysteresis losses will create intermodulation distortion (PIM) and excessive heating during transmission.