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--- member-projects:tait-cross-band-repeater [2025/08/02 17:28] – m0lte
+++ member-projects:tait-cross-band-repeater [2025/08/02 17:55] (current) – [Timings and latency] m0lte
@@ Line 77: / Line 77: @@
 There are a bunch of settings you can tweak to reduce the turnaround time, including:
-- PTT debounce time (default 10ms)
+  * PTT debounce time (default 10ms)
-- Lead-out delay
+  * 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.