I know our MODOT guys cannot talk to one another when they are gang plowing and trying to use the repeaters. They are too close and one must back away from the other before the can talk if using the repeater(s).
(This is part of an over-simplified presentation I use in Firefighter training. Please take it in a good way. We Firefighters sometimes have a difficult time grasping technical concepts, hence the simplistic format. These numbers and frequencies are for explanation and have no real world value.)
Receiver desensing bites folks in the rear all the time. It goes like this:
A transmitter sends signals in strengths from 1 to a brazillion. The farther away from the transmitter you are, the weaker the signal gets like water pressure in a hose. No new news there.
For a receiver, any signal over 10,000 melts solder joints. 300 lets the smoke out of transistors. The FCC 'Safe Harbor Rules' limit signals to a maximum of 125. 100 is
Binford strong. Anything above 5 is clear. Your squelch is set at 2, and mutes the receiver so you don't hear static for strengths of 0 to 2. Again, no news.
Every transmitter, without exception, sends off-frequency signals. It is unavoidable. The farther away / weaker the signal, the less off-frequency energy is received. Picture it as mist coming off an adjustable nozzle set to stream.
Simplex or talkaround: I transmit on 154 MHz. You hear me on 154 MHz. Simple, hence the name. I am hitting you directly with a master stream.
Duplex or repeaters: Mobiles transmit on 155 into the repeater. The repeater simultaneously 'repeats' whatever it hears on 155 back out on 154. A receiver listening to 154 will hear nearby simplex (if the signal is strong enough) or the repeater (again, if it is strong enough). All is well with the world.
Squelch codes: Picture the carnival ride with the sign "You must be this tall to ride." "This tall" equates to the receiver's signal squelch. Squelch codes add "This tall AND a green ticket." Green ticket, you go. Red ticket, blue ticket or no ticket, no go. Squelch codes are like tickets. Again, nothing earth-shattering.
Here is where it gets interesting: Say you and I are in different apparatus driving a hundred feet apart, and a fair distance from the repeater. Our squelch settings mute all signals less than a 2. The repeater is far enough away it comes in at a clear 5. No problem… so far.
I call you simplex on 154 MHz, and your radio receives me at a 70 signal. I transmit my signal with a green ticket. You hear me with no problem. Remember that my radio is also transmitting off frequency noise, and as close as we are it covers 2 MHz either side of 154 at a 10 signal for your radio, but we don't care... so far.
You answer back, but use the repeater; your radio transmits on 155 (with the same amount of off-frequency noise as mine did). My receiver hears your transmitter’s off-frequency 155 MHz noise at a 10 signal, but there is no green ticket so I hear nothing. The repeater kicks on and a 3rd truck 10 miles away hears you just fine through the repeater.
Here is where we get zapped: My receiver still hears your off-frequency noise at a 10. The repeater's signal hits my truck at a 5, but your off-frequency 10 covers up the repeater’s 5 signal. In that barrel battle I hear exactly zilch. I back off two hundred feet. The repeater signal may drop to a 4 but your garbage signal has dropped to a 1. We can talk again.
Bingo! Receiver desense.
Note that desense also takes place in situations where users aren't trying to talk to each other. Example: I, your friendly chief, use my 5-watt portable to call you, the poor schmuck on the wet end of the hose, to pass along the interesting information that the roof is collapsing. By order of the head of the IT and Communications Bureau (a fine fellow with a Cal-Tech Doctorate in SQL statistical database administration), all fireground communications must be on his spiffy repeated system. You are far enough away from the repeater and deep enough inside the house that your radio hears the repeater at a 5.
Nobody will ever explain to your widow that a long-winded taxi driver stuck at the road block was arguing with his dispatcher about overtime. His transmitter’s off-frequency noise was hitting your portable radio at a 10, so you didn't hear the repeater's 5 signal carrying my suggestion that you cut and run for your life.
Analog radios may let you hear something, probably just unintelligible noise, but maybe a word or two, perhaps sounding something like “EVACUATE!” At least enough ‘something’ to pique your interest. Digital radios simply flat fail. There is not enough frog DNA in the world to error correct the mess your receiver is hearing.
In the City’s defense, they will play the dispatch center recording clearly showing that I issued an evacuation order and that the repeater faithfully and crisply rebroadcast it. There will be testimony from a brother firefighter listening from across town that he heard everything perfectly. The blame falls on you, the dumb schmuck at the wet end of the hose, because your didn’t heed my suggestion.
Sounds silly, but over the past five years, this exact scenario has contributed to the death of half a dozen firefighters. Read the published firefighter death and injury reports.
Don't believe it? Grab three portables. and 'go inspect something' several miles away from the closest repeater site. Put two of the portables on repeated TAC3. Put the other on TAC4. Send a greenhorn jogging down the block with one TAC3 radio. Keep the other two. Have him talk to you on TAC3- he will come in just fine. Then, while he is talking on TAC3, key up the TAC4 radio in your other hand. I'll bet dish washing for a week that the TAC3 radio will go silent. Hand the TAC4 portable radio to someone else and find out how far away they have to go before TAC3 comes back. Then try it on different channels, and again with a truck radio transmitting on the different channel. That is useful knowledge.
Beware the evil receiver desense on repeated systems!
PS: With the planned number of MOSWIN sites, this will be a problem there as well. Maybe worse as MOSWIN is P25 digital and even with the latest-greatest error correction digital radios are troubled by strong off-frequency interference that can easily put the BER above the fail point.
