Your browser is out-of-date!

Update your browser to view this website correctly. Update my browser now


Introducing FM Class C4

There has been much discussion lately about a proposed new class of FM stations

There has been much discussion lately about a proposed new class of FM stations, the C4. Although this topic is front-and-center again, it is not necessarily a new concept, and has actually been hanging around for several years now.

Contour comparison between KACY class A license and hypothetical C4 license
Click To Enlarge The new fangled FM broadcasting came on the stage around World War II, and following the transition from the early assignments in the 42�50 MHz band up to the spectral real estate we know today, the beginnings of what would morph into the modern class system was well under development.

By the mid�1950s, Areas I and II were defined; however, they were different from what we know today. Back then, Area I southern New England and New York, down to about the District of Columbia and Delaware, and then west to around Hagerstown, Md., and Harrisburg, Pa. Everything elsea was area II.

The commission had already begun to note demand for spectrum at this time. While Illinois really was the Wild West for FM broadcasting, they began to fear there might eventually be a spectrum crunch even in the Midwest. You can still see the echoes of the results of this today in the congestion that really begins ramping up from Hoosier country east.

Of course, being humans, we have an insatiable desire for more and more.

In the year when Marty met his parents as teenagers in Hill Valley, there were two classes of FM stations, that oddly enough were labeled A and B. Protected coverage areas were defined in terms of a familiar metric, the 1 mV/m or 60 dBu contour, but had quite different bases. Class A stations were a maximum of 1 kW ERP at 250 feet above average terrain, while class B stations were defined with an ERP of 20 kW at 500 feet above average. Both of these are well below current contour defined coverage radii, but few cared as AM was still king.

In the 1960s, noise becomes more of an issue, and we find in the early part of the decade the class A stations are now at 3 kW at 300 feet, a definition that would continue until the 1980s. Class B stations become a maximum of 50 kW at 500 feet due to urban sprawl, and Class C now makes an appearance at a maximum of 100 kW at 2,000 feet above average. The old �areas� are now also dispatched in favor of the zones we see today.


73.207 Minimum distance separation between stations; class C4 changes. Nearly two decades later, the famous Docket 80-90 would create the additional classes, minus the C3 and C0, which would come later, giving us the current class landscape. The other tweak that would happen between then and now is the 3 dB ERP increase in the class A stations to 6 kW. In looking at the various ERP values of the classes, we find they are 6, 9, and 12 dB above the class A ERP. Conspicuously absent is the 3 dB increase, which would be filled by the C4 class.

Historically, the class tweaking that has occurred typically falls under one or more of the concepts of service area increases, noise abatement, and increasing the number of available allotments. For example, the class A ERP increase would fall under a service area increase and/or noise abatement, while the subdivision of the B and C classes addresses increasing the number of available allotments. The proposed class C4 would seem to be similar to the class A changes, and fall under both service area increase and noise abatement. The proposed technical parameters of 12 kW at a center of radiation of 100 meters above average terrain means the new class represents a 3 dB increase in the maximum ERP over the current Class A facility.

73.207 Minimum distance separation between stations; class C4 added. Assuming uniform terrain, the reference distance to the 60 dBu service contour for a class A facility is 28.3 kilometers, which is rounded to 28 kilometers. This value assumes an effective radiated power of 6 kW at a center of radiation of 100 meters above average terrain. For a C3, assuming an ERP of 25 kW at 100 meters, the 60 dBu contour radius is 39 km. The C4 radius fits in nicely between the two at a predicted contour distance of 33 kilometers. Depending on locale, this could result in a significant increase in population served.

With an increase in covered radius, we will necessarily also wind up with an increased protected and interference radius. Changes to the spacing tables in Section 73.207, 73.213, and 73.215 will also be required. The spacing tables are predicated on the contour radii determined by the FCC standard contour method assuming uniform terrain, and the standard D/U ratios. In the case of the intermediate frequency spacing, the required distance is based on the sum of the radii of the 36 mV/m (91 dBu) contour for each facility. And so, with a few keystrokes, we can derive the potential 73.207 spacing table for the new C4 class. It should be noted that in the derivation of this table, the assumption is made that the protected field strength for the class C4 is 60 dBu, which is consistent with stations in the class C areas.

As was previously mentioned, the C4 class was proposed back in 2013, but has not been on the front burner of the commission. That is quite understandable, considering the disposition of the remaining translator applications from 2003 being addressed, the LPFM window, AM revitalization and the spectrum incentive auction, all within that period. With Commissioner Pai�s statements at the Radio Show this fall, it seems that the C4 concept may now begin to get some traction.


Contour protection for short-spaced assignments; class C4 changes There are a couple of items worth consideration before we all jump on the class C4 bandwagon.

First, the potential exists that the addition of the C4 class could force translators off the air. It would seem to be somewhat problematic for the commission to come out in favor of translators as an important prong of their revitalization efforts, and then before the filing windows are even completed, to contemplate an action that could take away this boon to AM broadcasters. While this is potentially a rare occurrence, it still must be considered. Tied in with this issue is also the impact that a C4 facility will have on LPFM licensees.

Next, the C4 proposal as originally filed only applies to non-reserved facilities in Zone II. No similar proposal seems to exist for Zones I, and I-A, where the new class would ostensibly be a class B2. Licensees in these regions of the United States would theoretically be subject to the same problems that have spurred the proposal for the C4 class, namely a need for increased coverage in a stopgap class, and increased power to overcome some of the noise floor issues. Additionally, this class increase should be made available to reserved band licensees, although their usage of the class would probably only be in rare intermediate frequency spacing situations.

Contour protection for short-spaced assignments; class C4 added Other authors and the original petitioners themselves indicate that the proposal for rulemaking would have the same mechanics as the reclassifications of facilities from C to C0 that took place a number of years ago. They do�sort of. Where they differ (and this may be the biggest issue especially with the consulting engineering community) is the triggering via Section 73.215 of the FCC�s rules.

Under the C to C0 mechanism described in MM Docket 98-93, class C stations with a 60 dBu contour distance greater than 72 km, but less than 83 km, could ultimately be reclassified as C0 facilities, if triggered by another seeking an upgrade. If triggered, the class C station would have to provide in writing within 30 days their desire to maintain class C status, and within 180 days file a construction permit application, which if granted would result in a three-year construction period. As part of this, the triggering application would be dismissed, and would have to wait until the CP expires, at which point they could re-file.

The subtle point of the C-C0 situation is the Class C station that was triggered would remain authorized under Section 73.207, which would afford them protection to the full reference facilities of the class C0. Under the C4 proposal, the petitioners would impose a 73.215 contour protected status, via a triggering application, on facilities that are authorized with parameters below the maximum for their particular class. This can affect potential relocation options for facilities to maintain coverage consistent with changing populations and demographics.

Furthermore, imposing a 73.215 grant on a licensee appears to make fundamental changes to the nature of that section. Processing under 73.215 is to be specifically requested by the applicant, which is an admission by the applicant that the proposal is a reduction of typical allocation strategies. Additionally, 73.215 affords some less restrictive processing ability, but still necessitates the demonstration of an allocation site meeting the 73.207 spacing requirements. The C4 proposal does not address these allocation issues, nor does it consider issues with grandfathered facilities under Section 73.213.

In the end, the time for serious consideration of class C4 facilities has probably arrived. The proposed technical parameters dovetail in nicely with the existing classes, and no doubt, the extra ERP will enhance the service area of many stations. As always, a balance will have to be struck between the interests of the FM broadcasters, the translator operators, including many AM licensees, and LPFM licensees. As long as that can be achieved, the public interest will seem to be served.

Ruck is the principal engineer of Jeremy Ruck and Associates, Canton, Ill.