add locked frequency range, where only one transmitter is allowed

helpers.lua

@@ -1,11 +1,20 @@
 function ham_radio.validate_frequency(frequency)
+  local transmission_is_allowed = true
   local num_freq = tonumber(frequency)
   local freq = tostring(num_freq)
+  if next(ham_radio.find_transmitters(frequency)) then
+    if num_freq >= ham_radio.settings.locked_frequency.min
+    and num_freq <= ham_radio.settings.locked_frequency.max then
+      -- transmitter is in locked frequency range
+      transmission_is_allowed = false
+    end
+  end
   return freq == frequency
     and num_freq ~= nil
     and num_freq == math.floor(num_freq)
     and num_freq >= ham_radio.settings.frequency.min
     and num_freq <= ham_radio.settings.frequency.max
+    and transmission_is_allowed
 end
 
 function ham_radio.find_transmitters(frequency)

init.lua

@@ -12,11 +12,18 @@ ham_radio = {
     broadcast_color = '#607d8b',
     broadcast_interval = 10, -- seconds
     hud_pos = { x = 0.5, y = 0.8 },
+    -- radio frequency range
     frequency = {
       min = 0,
       max = 9999999
     },
-    beacon_frequency = { -- sub-range of frequency range
+    -- range where only one transmitter is permitted
+    locked_frequency = {
+      min = 100000,
+      max = 9999999
+    },
+    -- sub-range of frequency range
+    beacon_frequency = { 
       min = 1000000,
       max = 9999999
     },