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A Brief History of the 0.5 mV/m Protected Contour

A historical blanket fourfold power increase doubled the de facto protected contour for one class of station

One of the Federal Communications Commission’s proposals in the portions of the AM Revitalization rulemaking still pending is to change the daytime groundwave “normally protected” service area of Class B stations.

The present protected value is the 0.5 mV/m (or 500 μV/m) contour. It’s protected from co-channel interference on a 20:1 basis, or 26 dB. Historically, the first-adjacent channel protection was a 1:1 ratio, or 0 dB, but this was modified in the MM Docket 87-267 proceeding to be 2:1 (D/U), or 6 dB.

Fig. 1: The across-the-board power increase from 250 watts to 1 kW resulted in a doubling of the signal strength with no change in the area within the “protected” contour.

This relatively recent modification was purported to be an improvement because it assumed wideband receiver characteristics. It was, in fact, a chimera, since essentially all AM receivers have bandwidth limited to 5 kHz (or more often even less). Return to 0 dB adjacent-channel protection is also proposed by the commission in the AM Revitalization proceeding for this reason.

The maintenance of the 0.5 mV/m protected contour for co-channel stations is not even well justified by past commission action. The rise in both urban and rural noise level was already underway in the 1930s, when the standard was established, and those conditions have gotten far worse, but aside from that problem, the FCC has historically adopted a higher 1 mV/m protected contour standard in two important situations.

That’s explicit in the former §73.37(b) rule:

An application for a new daytime station or a change in the daytime facilities of an existing station may be granted notwithstanding overlap of the proposed 0.5 mV/m contour and the 0.025 mV/m contour of another co-channel station where the applicant station is or would be the first AM broadcast facility of a community of any size wholly or partly within an urbanized area, or when the facilities proposed would provide a first primary service to at least 25% of the interference free area within the proposed 0.5 mV/m contour: provided that (b)(2) No overlap would occur between the 1 mVm contour of the proposed facilities and the 0.05 mV/m contour of any co-channel station.

This rule was deleted in the MM Docket 87-267 proceeding, but there are dozens if not hundreds of stations licensed on the basis of groundwave overlap up to their 1 mV/m contour.

A much subtler — and undoubtedly even more widespread — use of 1 mV/m as the protected contour is still in effect! That’s the provisions of the current §73.37(b) and (c) rule, which is the result of the increase in maximum power from 250 watts to 1 kW for Class C (formerly Class IV) stations. It’s subtle because the rule itself doesn’t actually talk about the level of the protected contour and the allowable interference level.

Although this came about in stages — daytime first in 1958, and only for some, then daytime for essentially all class IVs, and eventually nighttime as well — it’s clear evidence that the commission recognized that Class IVs simply did not have enough signal strength to overcome urban and rural noise, and that co-channel interference simply wasn’t that significant. (See Report & Order, Docket No. 12064, 17 RR 1541 et al.)

And it was modification of the protected service area to the 1 mV/m contour in disguise — the rule just states to use (for interference received by a new station) “… the assumption that … all existing Class IV stations operate with 250 watts and non-directional antennas.” 6 dB is 6 dB, no matter how it is framed — a 4 to 1 power difference, or a 2 to 1 signal strength difference!

And the result of the Class IV across-the-board power limit increase to 1 kW, illustrated in Fig. 1, is simple: the normally protected contour of Class IV (now Class C) stations became the 1.0 mV/m contour and no station’s protected service area was reduced, but the signal strength in that service area was doubled.

The author is president/senior electrical engineer of Hatfield & Dawson Consulting Engineers.