2m/70cm Diplexer.

Created 3/2/03

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 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.

Inside diplexer

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.

Summary of my Performance Measurements Results.

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

25W (+44dBm) at 144.750MHz
144.0+14.5 29
25W (+44dBm) at 433.0MHz
434.0+7 37

Sorry, I haven't any measurements for insertion loss.
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