2m / 70cm diplexer

With this diplexer, you can connect a 2m (144-146 MHz) and 70cm (430-440 MHz) transmitter to a single dualband antenna, or a dualband transmitter to two seperate antennas.
This diplxer is for a maximum power of 25 Watt (both 2m and 70cm, even at the same time). You can probably go as far as 40-50 Watt, but keep your transmissions a little shorter. If you want to use more power, use air trimmers. For power rates below 30 Watt, you can also use .8 mm silver plated copper wire for the inductors.
What you need is...

L1Inductor, 1 turn, 6mm inner diameter, 1mm silver plated copper wire
L2Inductor, same as L1
L3Inductor, 3 turns, 6mm inner diameter, 1mm silver plated copper wire
L4Inductor, 4 turns, 6mm inner diameter, 1mm silver plated copper wire
L5Inductor, same as L3
C1 - C3Variabele capacitator (foil trimmer for < 50W, or air trimmer above), 3-22 pF
C4 - C5Variabele capacitator (foil trimmer for < 50W, or air trimmer above), 4-40 pF

If you want to use this diplexer for exactly what it is ment for, you also need a tin box (10 x 5 x 2 cm or something like that) and some VHF/UHF chassis connectors, like BNC or N. But if you want to build this inside some antenna switch panel you -maybe- don't need this (well, that's to say because you really need the tin box either way offcourse).

Now, look closely at the picture. The inductors, both at 2m and at 70cm side, have to placed at an angle of 90 degrees to eachother. The inductors for the 2m-band can be made out of 1 piece. It's not mandatory, but it will prevent any loss in the soldering points. Mine (see picture) is made of one piece of silver plated copper wire from the 2m-connector to C1.

Connect a dummy load to the combined 2/70-connector and a transmitter along with a SWR-meter to the 2m-connector. Turn trimmer C4 and C5 until you reach a SWR of 1.1 or less.
Do the same at the 70cm-connector with a 70cm transmitter and turn trimmers C1, C2 and C3. However, C1 and C3 have to be set in the same position!
Repeat this exercise a couple of times, because the circuits will interfere with eachother.
If you can't get both bands to a SWR < 5, the settings just can't cope with eachother. Mess up the settings of the trimmers completely, and start again with the band you couldn't get straight.
IMPORTANT: start with LOW power, let's say 5 Watts or better 1 Watt. If you cannot transmit at these low power rates, buy more capacitators at the first place because they will melt!

Have fun, 73's de Jeroen PA3JT