Diplexer for 2m and 70cm

There are plenty of recipes for one of these so here is my version. I can't show you a picture since it's been in service for several years now without any issues. But it all fitted into a small diecast case just large enough for the 3 N connectors I used to sit alongside each other.

If you're unsure what a diplexer is then don't read the wikipedia entry:

"A diplexer is a passive device that implements frequency-domain multiplexing. Two ports (e.g., L and H) are multiplexed onto a third port (e.g., S). The signals on ports L and H occupy disjoint frequency bands. Consequently, the signals on L and H can coexist on port S without interfering with each other."

Clear as mud!

You can use a diplexer to combine signals from aerials on two ham bands, say 2m and 70cm, onto one feedline. Here I am using it to combine a 2m and a 70cm aerial onto one feedline which is connected to a dual band radio. A picture is worth a lot of words here:

You can use a diplexer to connect two radios on different bands to one run of coax. However, the diplexer must have good isolation when connecting two radio's together. You don't want the transmitted signal from one radio appearing at the aerial connection of the radio on the other band. For example, if you were transmitting 50W on 2m would you be happy if 0.5W was making it's way to the 70cm radio? I wouldn't connect a handheld directly to a radio and press transmit on the handheld. Even 0.05W, or 50mW, would make me uncomfortable. 50mW represents 30dB of isolation and even this diplexer does not achieve that. I consider this diplexer, and all other 3 and 5 element designs, to be for diplexing aerials only.

So isolation is important. How much you need depends on the application. My goal was to build something that needed no tuning once assembled. So no trimcaps.

I used some diplexer software from Tonne Software, www.tonnesoftware.com, to generate the filter values and settled on a 5th order design. After fiddling with the crossover frequency and ripple I had a filter that was close to the values of transmitting capacitors I had to hand.

From this point on the challenge was to wind the inductors. Which really wasn't that hard with my test jig described here.

73's
Richard