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--- member-projects:tait-cross-band-repeater [2025/08/02 16:12] – m0lte
+++ member-projects:tait-cross-band-repeater [2025/08/02 17:55] (current) – [Timings and latency] m0lte
@@ Line 13: / Line 13: @@
 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:
+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:
@@ Line 26: / Line 30: @@
 | 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):
+-------- 1
+--- 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
+  * others... need to look up
+===== Antenna System =====
+When you have two radios you need to provide either two antennas, one suitable antenna for each band, individually connected, or you can use a single dual band antenna to transmit and receive on simultaneously.
+To do so, you need a device called a diplexer. (To confuse matters, Diamond, a common manufacturer of such devices, calls them duplexers.)
+A suitable device for 2m and 70cm is the Diamond MX72. There are a few variants, each with different letter suffixes, these denote the connector types.
+==== Safety ====
+There is 60dB of isolation between the ports. Some simple maths reveals the strength of the transmitted signal on the receiving port, for example for 25W:
+W = 44dBm
+dBm (transmitted power level) - 60dB (isolation) = -16dBm
+  -16dBm = 0.0000251189W = 0.0251189 mW
+Pretty low for a transmitted signal, but pretty high at a receiver input.
+The risks are:
+  * densensitisation (receiver is temporarily deafened by a nearby transmitter)
+  * damage (receiver is permanently damaged by much higher signal voltage than the circuitry can withstand)
+The Tait TM8100 specification manual states "no degradation after 5 minutes exposure to on-channel signals at + 27 dBm (2.2 V)" - we are 43dBm clear of this so we are a very long way from damaging the receiver.
+However -16dBm is a pretty strong signal (-93dBm is considered 5/9 at VHF and above, so this is "77dB over 5/9") but the Tait filtering and receivers seem excellent and don't appear to be desensitised in practice.
+===== EMF Calculations =====
+Don't forget to do your EMF calcuations, these are not optional under the UK licence, even for temporary use. See:
+  * Information: https://rsgb.org/main/technical/emc/emf-exposure/
+  * Calculator: https://rsgb.org/emfcalculator
+===== Physical layout =====
+I recommend rigidly attaching the two radios to each other and to a baseboard - this can be cheaply achieved using "all round band" - the 12mm stuff has perfect hole spacing for the Tait side threads.