Check this video from PV8DX in Brazil

Thank you Paulo!

Fig. 3.

Frequency response plot of the UHF port, while the VHF port is terminated with a 50 Ohms load. The 3 dB point is at 257 MHZ. The insertion loss is 0.20 dB (marker 2) for the 70-cm band (well done!) and the desired attenuation of the VHF channel is more than 50 dB. So, the isolation between the VHF and UHF ports is around 100 dB. For example, if transmitting at UHF and simulatneously receiving at VHF, the unwanted UHF signal at the VHF receiver will get more than 100 dB attenuated! If transmitting at VHF and receiving at UHF, the same applies for the fundamental frequency but consider the power level of the 3rd harmonic which will pass through the UHF port, 50 dB attenuated from the low-pass branch. This is 20dB better than other simpler designs.

Fig. 2.

Frequency response plot of the VHF port, while the UHF port is terminated with a 50 Ohms load. The 3 dB point is at 246.5 MHZ. The insertion loss is 0.71 dB (marker 1) at the 2-m band and the desired attenuation of the UHF channel is close to -50 dB. Please note this measurement is taken without any shielding placed between the filters and without enclosure. Expect the figures to improve further.

One more thing: By saying VHF, it means 50 MHZ and HF too. This diplexer can be used as a 6-m/UHF diplexer or even as an HF/UHF diplexer as well. The attenuation at HF and 6-m is even less than of the 2-m band. Also, operation of the VHF port on 220 MHZ is possible.

ORDERS For a Ready Built & Tested PCB - EUR 41.65

The prices do not include 24% Greek VAT applicable only to individual buyers from European Union. 

The KIT includes a  high-quality ENIG FR4 PCB (58 x 107 mm) that comes with all SMD components factory soldered.

High voltage RF capacitors and high-Q and high current inductors are used everywhere. 

The three N-type connectors are not included. 

The shipping is with registered mail  (your signature is required on delivery). We usually ship the next day and it takes about a week to 10 days for USA or Australia, less than a week for Europe.​

Fig. 1.   The Low-Pass Filter arrangement is shown on the top side and the the High-Pass Filter arrangement at the bottom side. Between them, space for soldering a shield is provided (on the gold-plated strip).

After many hours spent on calculations and simulation, I came up with the design shown here. I have tried to use ready made components to facilitate repeat-ability.  Because other similar designs published use transmission lines or parallel and series LC circuits to add to the isolation figures, I haven't used similar techniques, just two simple filters to see if fabricating them using readily available components can produce a decent result.

Coilcraft today makes nice inductors with self-resonant frequencies well above the target, above 2.5 GHZ and in addition they can withstand much current, in the range of 1 to 4 Amperes. Similarly, SMD RF-grade capacitors to withstand high voltages (> 1500V) are available from Vishay and others. The high voltage ratings of the capacitors used and the high current capability of the inductors used, aid to make the diplexer able to work at higher RF power levels, without damages.

So a PCB has been designed to arrange the placement of all of the  components and some space has been provided to place three N-type female connectors for chassis mounting, if needed. If not needed, their pcb footprints space can be cut out and keep the filters circuitry only. By doing so, one can quickly fabricate an enclosure and complete by just adding copper clad boards to quickly produce a finished item at home. Another possibility is to place the PCB inside a metal enclosure, with the PCB laying on one of the internal sides and just drill the holes for the connectors.

This is done using a low-pass filter and a high-pass filter, with their inputs connected together at the common input or output point. For easier physical implementation, their first element needs to be opposite that the other's, i.e., if the low-pass filter starts with a coil, the high pass filter should start with a capacitor. Reading text from the Anatol Zverev's "Handbook Of Filter Synthesis"  and from the G. Matthaei's "Bible", "Microwave Filters, Impedance-Matching Networks, and Coupling Structures", I realized that their impedance at the common point should not be calculated as being 50 Ohms,  nor any higher value, but infinite. I have done a similar design for the diplexer used in my HF to VHF  Up-converter project, described elsewhere in this website.

For maximum attenuation of the rejection frequency band, while keeping losses at minimum for the wanted frequency band, I have chosen to use a 9-pole design.

If it happens you own a dual V/UHF antenna and you need to connect two different radio transceivers and work them simultaneously transmitting or receiving, or both, or a dual-band transceiver and you need to connect  two different VHF and UHF antennae and again to operate on both bands simultaneously, then you need a diplexer similar to this one. Usually a diplexer is placed to economize from the need of a second expensive RF transmission line cable 

The Diplexer described here has 2 ports and is made using one low-pass and one high-pass filter.

One port (called VHF)  works at 0-240 MHZ and the second port (called UHF) works from 290 to >500 MHZ. It has been designed using the Legendre coefficients with 50 Ohms termination resistance for the VHF and UHF ports and infinity Ohms for the common point.

The so-called "VHF" port provides minimum attenuation from the common point while maximum attenuation from the "UHF" port. Similarly, the "UHF" port has minimum attenuation from the common point, while preserving maximum attenuation from the "VHF" port.