Louisville’s system is a multi-site system. The three towers are strategically places across the city in an effort to provide the best possible coverage. One site does the transmitting while all three sites receive. A voter determines which of the three sites is receiving the signal best. All sites are linked via microwave or T1 lines. To answer your question, if all sites were to transmit at once the audio would be unintelligible.
There’s really no need in switching frequencies. Nearly everyone has to deal with skip at one point or another. There are simply not enough frequencies available for everyone to have their own, realistically. So the only way to get everyone on the air is to assign the same frequency to multiple agencies. These frequencies are coordinated so that they are just far enough away as to not interfere with each other under normal conditions. However, temperature inversions are just a normal part of the atmosphere’s process and there’s no way around this phenomena. When these do occur we all have to put up with skip. Luckily they don’t occur that often. Just remember that your county isn’t the only county suffering from the skip.
Are you familiar with CTCSS tones? You may be familiar with them but I will mention them just in case someone else reading this isn’t familiar with how they work. They were designed to reduce the interference such as skip. Each agency will sent a different CTCSS tone. The radios will only let the audio through if the tones match. Here in Dickson County, law enforcement uses the frequency of 460.05 MHz. It just so happens that La Vergne Police (60 miles to our east) uses the same frequency. However, the law enforcement radios in Dickson County are set to decode a tone of 107.2 Hz. So unless the signal on 460.05 MHz is sending a tone of 107.2 Hz, nothing is heard on the Dickson County radios. Since La Vergne Police use a tone of 131.8 Hz, their signals do not come through on the Dickson County radios. I can set my scanner do decode the tone of 107.2 which allows me to hear only Dickson County. If I do not decode any tone at all with my scanner then I will hear both Dickson County and La Vergne as well as any other agency out there that happened to be on that frequency.
Can you find out what CTCSS tone is being used in your county? I'm guessing it is 114.8 since that is the tone used on the 460.175 repeater in your county. Maybe the interference problems are as simple as tones from the other agency coincidentally matching. This seems unlikely but possible. Switching a CTCSS tone is much easier than switching an entire frequency. Do all of the county's radios actually hear the skip?
The following is a list of other nearby agencies that use the frequency of 460.15 MHz:
Hardeman County / Bolivar, TN
Maury County, TN
Obion County, TN (Union City) –as you mentioned.
Trousdale County / Hartsville, TN
Blytheville, AR
Florence, AL