2m/70cm Diplexer.

Back in 1997 I wanted simple vertical antennas to communicate with locals on 2m and 70cms. I therefore bought a dual band monopole which I connect via a single coax to my FT736 transceiver in the shack. The 736 has separate antenna sockets for the 2m and 70cms sections so I needed a diplexer of some sort. I wanted something simple to construct and easy to align so I decided to try the design by PA0HVA described by G4LQI in his "Eurotek" column in RadCom, page 69 of September 1994. My 2m TRX unit is connected to a 2-way coax switch so I can select either the monopole or a horizontal beam.

This is a three-port network where the common (centre) port goes to the dual band antenna. Both the 2m and the 70cm sections consist of a series resonant L -C circuit shunting its port and a series resonant quarter-wave coax line connecting it to the common port. For example, in the 2m section the shunt circuit is resonant at 433MHz and the series ¼ wave line resonant at 145MHz. As can be seen in the photo these elements are easily realised by commonly available components just as suggested in the original article. I think the use of a good quality PTFE insulated coax, like RG316, is particularly important so that good short soldered connections can easily be made to the braiding.

I used different types of connector for the 2m and 70cm ports to avoid possible confusion when hurriedly connecting to different rigs in the future! Mine is housed in a standard diecast box approximately 150x80x50mm but dimensions are not at all critical.

Alignment is straight forward:

1. Connect TX (on say 2m) to common port; 50 Ohm dummy load on 2m port; RF millivoltmeter (or other sensitive detector eg. 70cm RX via attenuator) on 70cm port.

2. Trim capacitor in 70cm L - C circuit for minimum 2m out at 70cm port.

3. Repeat this procedure with TX on 2m; trim 2m L - C for minimum 2m at 70cms port.

(I chose to optimise my 2m at 144.750MHz)