There is a rather simple fact of life for FM stations that is totally different than AM stations. For an AM station, the TOWER is the ANTENNA.
For FM, the tower is just there to suspend the attennas. The "radiating device" usually hangs on the side of the tower.
I guess it would be possible for two stations to have their antenna on opposite sides of the tower and actually be at exactly the same height above the average level of the terrain, but as a practical matter it seems like the antennas are always mounted one above the other.
In the case of the station that shows only a very slight change in eleveation.... (and hasn't been completed) without taking the time to pull up the FCC records and look, my guess is that they are applying to replace one antenna with another antenna of a different design.... maybe more or less bays, thus the "center of the antenna" may end up slightly higher or lower than the previous antenna.
If you will search for an FM antenna manufacturers catalog and view it, you will see FM antennas can be quite varied. They can be like a magnifying glass for the transmitter power. A simply one-element or one-bay antenna can take the output of a 1,000 watt transmitter and deliver an EFFECTIVE RADIATED POWER of may 400 or 500 watts. Two bays, one stacked abover the other takes the 1,000 watts and has and E.R.P. of maybe 1,000 watts. Stack 7 or 9 bays one above the other and that same 1,000 watt watt transmitter may radiate 3,500 to 4,000 watts.
So why would anyone ever use a one, two or three bay antenna? It's like a wide-angle camera lens vs. a tele-photo lens. If you are in hilly country, the 1, 2 or 3 bay antenna may do a better job of poking down into the valleys between the hills and mountains. In a city with tall buildings, the waves reach down into the canyons formed by the buildings and people driving the streets or walking the sidewalks can receive your signal better. On the other hand, if you are in the flat parts of Indiana, Illininois or Kansas where the land is flatter that a pancake, go for broke. Put up a multi-bay antenna and aim for the horizon.
The engineers who know how to make these decisions are paid to know when and where to use what.
The other factor that may decide what antenna you use is the existing, available tower. If the tower is lightweight and/or there are already several other antennas mounted on that tower, the tower owner may tell you that the only way you will be allowed to rent space on the trower is to keep your weight down below a certain value... which translates into a reduced number of antenna bays.
Why is someone waiting so long to put an antenna in place that received a grant a year ago? Any one of a number of reasons. Possibilities:
Budget problems. waiting to save up enough money to buy and intall the new antenna.
Negotiating. Another station needs for you to change your power or location so they can more their station. You are working with them to see if you can make a deal.
Negotiating. Maybe you are trying to sell the station and you want the proposed buyer to borrow the money and pay for the install after the sale.
If it is rented tower space: dispute with the tower owner. Maybe a misunderstanding about location of a new shelter for the transmitter at the bottom of the tower.
The mechanics of radio when it comes to hardware and technical issues can be a dirty, grimy business that results in scars and abrasions.
Maybe someone in your own area will have facts instead of all my theories.
