Building and testing an eBay diplexer kit for 2m/70cm

The recent purchase of a UV-Pro has me in need of a satellite antenna for the "Easy-Sats" and International Space Station.

Arrow makes the perfect antenna but I thought I could homebrew one at a much lower price.

The main component that differentiates the Arrow from other dual-band Yagis is the diplexer, not duplexer, as it is incorrectly called by Arrow (and others).

A search online located several suitable but expensive diplexers for the necessary bands - and one inexpensive kit ($13) from a US seller on eBay - Steven AI4NV.

The kit arrived today and took all of 10 minutes to assemble.

There are three SMD capacitors and three coils to wind; two on torroidal cores and one air-core. An aluminum tube is included to assist in winding the air-core inductor.

After winding the coils and soldering all 9 parts to the board, it was time to tune the diplexer.

The instructions that come with the kit mention that a dummy load should be placed on the port not being tested, implying that the port being tested is left open. In reality, a dummy load should be placed on both ports.

The idea is to tune each output for minimum SWR with that output's frequency being injected. This will result in minimal insertion loss and maximum rejection of each band from the opposite port.

It is important to make sure that the dummy loads used are adequate up to 450 MHz.

To tune for minimal SWR, I used a nanoVNA as a 1-port device. Any antenna analyzer can serve this purpose if it will work up to the 70cm band.

Tuning for minimum SWR on each band was easy. I thought there would be interaction among the bands but this was not the case. I first tuned for minimum SWR on 146 MHz ( I had to slightly compress the windings on one of the inductors).

Then I tuned for minimal SWR on 436 MHz by slightly opening the windings on the small air-wound inductor. Going back to the 2m band, I found that it was just where I left it. Perfect.

At this point I was curious as to how well the diplexer rejected the unwanted  band on each opposite port and how much it attenuated the desired signal. And how broad-banded the diplexer was over the entirety of each band.

Again, the nanoVNA is the perfect device - this time as a 2-port device to look at S21 values.

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I was happy with the results. Now, onto building the Yagi itself - in an upcoming post.

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