Chicago has examples of "Roof Top" AMs. WNTD 950 AM day site is located on top of a multi story warehouse building. Been there for about 50 years. Previous to that it was WAAF/WGRT on top of a 12 story building in the Chicago Union Stockyards. From the warehouse site it's a 1kW signal. Roof has radials spread on top of it.Are we talking about AM stations?
The Chicago "full" FMs are already on the Hancock and the whateveritiscallednow Sears tower. Just as the NYC stations are mostly on the ESB.
Remember that an effective AM tower is at least a quarter wave. At the bottom of the dial, that is around 400 feet and at the top around 150 feet (I am giving rounded numbers just as examples). Less than quarter wave starts to reduce coverage significantly.
There are some alternative "tiny antennas" but they do not create signals like a quarter wave or greater tower. And you can use an inverted L or "T" antenna using wire between two masts, but that can be prohibitavely directional as well as being less efficient.
And any antenna needs some kind of ground system. While roof antennas can use counterpoise systems or even the frame of a steel building, those are not efficient.
In other words, rooftop AM antennas of any kind are not going to give the coverage of a conventional tower with a ground system. And I have not touched on multi-tower directional AM stations.
(An exception would be a full tower on top of one of those huge Amazon-like warehouses with a complete ground system laid out on the roof. But I doubt most warehose oweners woud want the hassle for what would be limited budget AM station payments.)
WCPT day site and WSBC are located on a one story building diplexing off the same tower originally designed for WEDC which used to share the 1240 frequency with WSBC. From this site WCPT is 5.8kW day and WSBC is 1kW. Roof appears to be cooper clad too small for radials. In Chicago the original 820 was WAIT with a 5kW site in Elmhurst, IL, sold off about 35 years ago as one of the first AM's to sell it's transmitting site for real estate development. When 820 AM came back on the air in 1993, it was using a wire cage antenna supported by the former WXRT FM tower on Chicago's northwest side. When News Web bought AM 820 and tuned it into WCPT, they used the same cage wire antenna until moving to the WEDC site to diplex with WSBC.
That said, it's not practical for what was once referred to as a Class 1A AM running 50kW to use a roof top tower. The gold standard for Class A AM's were 1/2 to 5/8 wave towers for lower angle radiation and better ground wave conductivity and full radial ground systems. The original WMAQ now WSCR, WGN and WBBM all had/have a tower around 750 Ft which is approximately 1/2 wave length in height. WLS has the only full 5/8 wave vertical on the air in the Chicago area.
WSCR and WBBM moving to the WYLL site in Desplaines, IL will be using verticals designed for AM 1160 which are not even 1/4 wave length in height for ether station which years ago would have nor even been considered for a Class 1A 50kW station.
I would wxpect WGN to want better than what Audacy is doing with their AM properties but, it's 2026 and anything goes! The most ideal solution for WGN would be to diplex with WLS providing Vertical Bridge doesn't sell the site off. It's the tallest AM tower left standing once the WSCR and WGN legacy towers are dropped for real estate development. The next choice would be to diplex with WIND in NW Indiana. They could use anyone of the 4 towers which were designed back in the late 70's all with capacitance hats. Only issue with both of these sites is they are 25 miles further south than the more desirable site which NextStar is willing to sell for profit in Elk Grove, IL.
So, there are choices for WGN. All will be compromises from what their current situation is. They're pulling the trigger. They don't have to sell the Elk Grove site but NextStar wants the profit for bottom line. Maybe buy an FM but not really sure NextStar wants to be in the radio broadcast business as WGN is the only radio station it owns.


