The nation's three large cell tower owners -- SBA Communications, American Tower and Crown Castle -- have been enjoying a pretty incredible run in the stock market in recent years.

Their collective shares are up 93% over the past three years, and have outperformed the S&P by 46%, as the analysts at Wall Street firm New Street Research point out.

"We never would have anticipated the stock action we've witnessed over the past year, at least with this speed and intensity," agreed the analysts at Wall Street research firm MoffettNathanson in a recent report. "American Tower at $209! Crown Castle at $134! SBA at $233! Wow. Since bottoming in the wake of the 2016 presidential election, the Towers (with the notable exception of Crown Castle) have meaningfully outperformed the S&P 500."

Figure 1: Source: MoffettNathanson

Companies including SBA and Crown Castle are the ones that own many of the cell towers that AT&T and Verizon are hanging their 5G antennas from. As a result, tower companies can charge rental fees for their "vertical real estate" at rates that would make landlords in other industries blush.

But the Wall Street analysts at MoffettNathanson, New Street Research and elsewhere are warning that the good times in the tower sector are coming to a close, and that the realities of 5G -- as well as shifts in the tax code, Wall Street investment funds and other financial realities -- likely will drag on the sector for the foreseeable future.

"2019 may be the peak year of activity," wrote the New Street analysts in a recent report on the tower sector. "The deployments of several spectrum bands drove a sharp increase in amendments for tower companies in 2018, and we expect them to be even higher in 2019. AT&T is driving most of the activity as they deploy AWS-3, WCS, and FirstNet spectrum, while T-Mobile's 600MHz and, to a lesser extent, Sprint's 2.5GHz deployments have also helped. We expect amendments to peak in 2019 and to decline sharply in 2020. Thereafter, we don't foresee a major amendment cycle until spectrum is deployed for 5G. We expect macro site colocations to remain relatively stable over the forecast period, as carriers shift densification efforts to small cells."

The analysts at MoffettNathanson largely agreed, writing that tower stocks today "do not offer much upside."

In a lengthy and detailed report on the financials of the tower companies, the analysts at MoffettNathanson argued that SBA, American Tower and Crown Castle have benefited from financial trends including real estate-oriented funds recently increasing ownership in tower stocks. The firm said the recent decrease in corporate tax rates also likely raised interest in tower stocks. Other issues affecting tower stocks include changes in interest rates and the cost of capital.

However, uncertainties could affect tower companies in the days, months and years to come -- and that ambiguity could drive investor activity. Specifically:

— Mike Dano, Editorial Director, 5G & Mobile Strategies, Light Reading | @mikeddano