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Tait Cross-Band Repeater Build
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.
I didn't find a full write-up of how to do this, and the manual over-complicates it a bit and spreads the information around, so I thought I'd write it up.
Cable
The Tait manual references 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:
| Pin, radio 1 | Pin, radio 2 | Role |
|---|---|---|
| 2 (busy) | 12 (ptt) | when radio 1 receives a signal, assert ptt on radio 2 |
| 7 (tx audio) | 13 (rx audio) | Send audio received by radio 2 to radio 1 for transmission |
| 12 (ptt) | 2 (busy) | when radio 2 receives a signal, assert ptt on radio 1 |
| 13 (rx audio) | 7 (tx audio) | Send audio received by radio 1 to radio 2 for transmission |
| 15 (gnd) | 15 (gnd) | Provide a return path for the signals above |
The D sub 15 pin connector is numbered like this, looking at the back of the radio (and therefore also looking at the solder side of a plug):
8 -------- 1 15 --- 9
Settings
On both radios, you need to set the following configuration:
Programmable I/O form, Digital tab
| Pin | Direction | Label | Action | Active | Debounce | Function |
|---|---|---|---|---|---|---|
| AUX_GPI1 | Input | XBAND_TX | Crossband TX Input or EPTT1 | Low | 10 | PTT input |
| AUX_GPIO5 | Output | Busy | Busy Status (no CTCSS) or Signalling Audio Mute Status (CTCSS) | Low | None | Busy status output |
Programmable I/O form, Audio tab
| Pin | Tap in | Type | Unmute | Tap out | Type | Unmute | |
|---|---|---|---|---|---|---|---|
| Rx | None | A-Bypass In | On PTT | R7 | D-split | Busy Detect (no CTCSS) or Busy Detect + Subaud (CTCSS) | Define which stage of the radio to output received audio |
| EPTT1 | T5 | A-Bypass In | On PTT | None | C-Bypass Out | On PTT | Define which stage of the radio to inject audio when keyed up |
PTT form, External PTT 1 tab
| Field | Sub-field | Setting | Function |
|---|---|---|---|
| Advanced EPTT1 | PTT Transmission Type | Voice | Define that we are transmitting voice not data |
| Advanced EPTT1 | Audio Source | Audio Tap In | Define getting audio from the tap defined above |
CWID
The TM8100 supports CWID, that is transmitting a morse code callsign identifier regularly, the TM8200 does not.
You can set a high morse speed, e.g. 30 wpm, a callsign, and choose to transmit without CTCSS, meaning on a radio with receive CTCSS turned on, people don't get disturbed by the morse ID.
Time-out timer
It is recommended to set a reasonably short transmit time-out timer, to limit the damage if there is a radio in key-down somewhere.
Channels and CTCSS
Define channels as you normally would. On the channel form, you have the option to transmit with a CTCSS tone, and to receive only signals transmitted with a given CTCSS tone. This is highly useful to avoid accidental transmission of noise, so I highly recommend setting up a system like this to require CTCSS for access.
Remember to set your 2m radio channels to use narrow spacing (12.5kHz) and your UHF radio channels to use wide spacing (25kHz).
Timings and latency
There are a bunch of settings you can tweak to reduce the turnaround time, including:
- PTT debounce time (default 10ms) - Lead-out delay