AM Stations Have New Options
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Nov
21
Airchecker
AM Stations Have New Options
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By Cris Alexander

I love AM radio. It’s been a lifelong thing for me, starting when I was a kid and AM was all we had. Then in my teen years, I got a ham license and was fascinated by amplifiers and antennas, and positively intrigued by the arrays of towers in the fields around my hometown. Once I got inside one of the stations and had a peek at the glowing tubes behind the glass in the transmitter, I was inexorably on my way to a career in broadcast engineering.

Cris Alexander

Somewhat ironically, my first jobs in radio were at FM stations. They sure didn’t want to give a kid a job on the all-important AM stations, but the FMs, which few people had receivers for anyway, were a good training ground where mistakes could be made with little consequence.

It was really close to 10 years before I did anything at an AM station, having spent those years working mostly in television, but I found I still had a love for AM — and that continues to this day.

With AM somehow part of my genome, I am especially saddened by the state of affairs at many AM stations these days. AM is the victim of progress, among other things — progress in technology and progress in the form of population growth.

I won’t take the time to discuss either of these issues and their various facets in these pages. Instead, I will focus on the options that AM stations and licensees have in today’s challenging environment.

TOO VALUABLE FOR AM

In recent months, I have had discussions with several individuals about AM siting issues. Stations many times lose their land leases or have to sell their land for economic reasons. Landlords and station owners find that the dirt under the AM tower or towers is worth far more for another purpose than as an AM site.

Many times, this news comes with little warning, and stations don’t have a lot of time to find another site. The other side of this double-edged sword is that it isn’t easy to build a tower anymore, even out in the middle of nowhere (I have recent and excruciating experience with this!).

Tighter ASR regulations, in addition to NEPA (National Environmental Protection Act) and NPA (Nationwide Programmatic Agreement) compliance, can add years to the tower approval process. Add to that the state and local environmental, zoning and land use regulations that many venues have in place, and you may find that it will take three or four years just to get all the approvals needed to build — if you can get them at all.

None of that regulatory compliance is cheap. The cost can easily exceed the cost of the tower or towers. The sad economic reality can well be that it’s just not worth it. The earnings potential of the AM station over five or 10 years may not come close to paying for development of the new site. All of that pushes AM station owners to look at other options, one of which may be shutting the station off and turning in the license.

COLLOCATION

“Collocation” is a word that has gained popularity with local regulatory bodies in recent years. I have found that some local planning bodies have the word written into policy or even codified into statute. If an applicant comes to them wanting to build an antenna support structure of any kind, their first question is whether it can be collocated on an existing site. The bar is often set fairly high for this, making collocation a much more attractive route than new construction.

Of course, these rules and policies were written mostly to address the cellular proliferation of the past 20+ years. AM (or any broadcast) use was not even a factor; but a tower is a tower, so AMs get lumped in with the rest and have the same burden of proof as to why they can’t simply hang their little antenna on the side of the 60-foot LTE monopole behind the Wal-Mart.

That being said, it’s a pretty rare thing for an AM station to be the only broadcast outlet in a town, especially in urbanized areas, and that opens up the possibility of some kind of collocation.

Fig. 1: These cabinets contain diplex filters, ATU and prematch components to allow two 15 kW AM stations to share the tower.

AM WITH AM

The easiest kind of collocation to do is with another AM station. If the tower is tall enough to present a reasonable impedance and the two stations are sufficiently far apart in frequency (>120 kHz), diplexing two AM stations together is a fairly simple matter of using pass/reject filters on each frequency. Fig. 1 shows cabinets enclosing the needed components.

Even if a tower might otherwise be considered too short for the frequency of the station to be collocated, there are things that can be done to make it work. Reactance can be resonated with shunt components to raise the impedance, and broadbanding networks can sometimes be used to produce a better VSWR bandwidth.

Until February 2016, stations didn’t often have this option. The FCC’s minimum antenna efficiency standards required in most cases for an antenna to produce at least 282 mV/m per kilowatt at 1 km. Fifty-five electrical degrees was about as short as you could go and still meet the standard.

In the FCC’s initial AM Revitalization effort, the minimum antenna efficiency standard was reduced to 215 mV/m per kilowatt at 1 km. Curve A in §73.190, Figure 8 (see reference [1] at the end of the article) only goes down to about 18 electrical degrees (0.05 wavelength), and that corresponds to about 214 mV/m, so presumably a 19-degree antenna would meet the minimum antenna efficiency standard. That really gives stations some options. The lower efficiency could be made up for with transmitter power (and electricity usage).

A station on 600 kHz could, for example, diplex with a station on 1550 kHz that uses a 90-degree (158-foot) tower and still easily meet the minimum antenna efficiency standard.

Of course, we’re talking about non-directional daytime operation here. At night, the vertical plane radiation pattern comes into play, known as the “function of theta.” Short towers are notorious “cloud burners,” radiating a lot of energy well above the horizon. A full-time non-directional AM station that moves from a quarter-wave tower to one that’s 30 or 40 degrees tall will have to reduce power at night to keep from raising the night limits of all the other stations on frequency, particularly those within a few hundred miles.

Can directional stations diplex together? Certainly, if the tower lines and spacing are right for putting the lobes and nulls in the right places. Years ago, I had a 5 kW 1290 kHz station in Portland that diplexed into all three towers of a 50 kW 1520 kHz station. The tower line and spacing were just right and it worked. That kind of thing is rare, however.

How about a non-directional AM diplexing on one tower of another station’s directional array? That’s fairly easy to do, although pass/reject filters and detuning components will be required at the unused towers for the relocated station. It’s also possible to use as a directional antenna just a few towers of another station’s array that has more than that, again provided that the tower line and spacing are right, and again with the understanding that pass/reject filters and detuning components will be required on all the unused towers.



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