For an event, I (Tom M0LTE) needed to provide a cross-band FM repeater.

A cross-band repeater is a system which receives signal on one band and re-transmits it on one or more others. This is reversible, i.e. the signal can be received on either band and transmitted by the other radio.

This allows stations which are not line-of-sight with each other to be able to communicate directly.

In my case, I needed a system that would bridge between 2m and 70cm, which is probably the most common combination.

I do have a Wouxun KG-UV950PL, which can do this out of the box, but my gut feel is that is meant for occasional use, since the thermal management of that radio isn't great and cross-band repeat generally means you end up transmitting at a much higher duty cycle than if you were just using a radio as normal.

So I decided to re-purpose some of my commercial radios I have around for the various packet radio projects I do. I chose a Tait TM8100 as the 2m radio and a Tait TM8200 as the 70cm radio.

These radios are explicitly designed to support cross-band repeat, and there are supporting notes in the PDF entitled “MMA-00041-04 - TM8260 Installation and Programming Guide”, available online.

Those notes reference a particular off-the-shelf cable, which I don't have, but you can build one easily:

Using the rear D sub 15 pin connector: