NPR Labs - IBOC Power Allowance Calculator For All U.S. FM Stations

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IBOC Power Allowance Calculator For All U.S. FM Stations

(Based on the FCC's Order in MM Docket 99-325, released 29 January, 2010)

FCC Engineering Data is supplied by Cavell, Mertz & Associates, Inc.

Open the Power Allowance Calculator

This online calculator from NPR Labs provides an estimate of allowable HD Radio (IBOC DAB) transmission power for all U.S. FM stations, as adopted by the FCC Media.  The calculator:

  • Identifies the most critical first-adjacent stations related to a digital power increase,
  • Lists the allowable powers separately for a candidate’s upper and lower sidebands (in support of asymmetrical transmission), and
  • Identifies other stations that must protect the target station, if they transmit IBOC.

Please read the notes below before preceeding to the discussion on this page about the proposed methodology and how to use the calculator.

NOTE 1: IBOC power limits for stations with FM facilities in excess of their class [based on the contour distance standards of 47CFR73.211(b)] are expected to be treated separately under the FCC final ruling. We have attempted to identify most of these stations, which are flagged and not processed by the power calculator.

NOTE 2:This calculator is intended for use only as a guide and users should perform a detailed contour study before considering an IBOC power increase.

On November 5, 2009, National Public Radio and iBiquity Digital Corporation submitted a proposal for resolution of a then-pending request to allow FM stations to increase the power of their digital signal.  The increase was based on discussions between NPR and iBiquity, in consultation with a broad range of commercial and noncommercial station licensees, which proposed a 4-fold (6 dB) increase in allowable digital power above the original level of 1% (‑20 dBc) relative to the analog host FM carrier power. The FCC adopted this "blanket" increase as part of its 2010 Order.

Increases above the blanket ‑14 dBc digital power were determined by the results of NPR’s Advanced IBOC Coverage & Compatibility Study, 2009, (available on the NPR Labs web site) which determined the threshold ratios that may cause harmful interference to first-adjacent neighbors.

IBOC proponents having first-adjacent neighbors with D/U ratios above prescribed minimums may operate at increased power on both digital sidebands (if symmetrical, or separately for each sideband, if asymmetrical) as indicated in this calculator.

This increase is applicable to both reserved band and non-reserved band stations of all classes, except for grandfathered super-power Class B stations, which are goverened by a different method.

The digital power levels for these super-power facilities should be limited to the higher of:

  • ‑20 dB relative to their analog carrier as is permitted by the current rules, or
  • At least 10 dB below the maximum analog power authorized for this class of station as adjusted for height, absent any grandfathered super power.

[In the initial month's release, IBOC power for super-power stations are not calculated according to the special method. Users should determine these powers manually. Super-power Class B stations will be correctly determined by the next month's release.]

The potential for interference to closely-spaced first-adjacent neighbors exists at power levels above ‑20 dBc, according to NPR’s study.  Consequently, the FCC adopted an “interference remediation” provision was suggested by NPR to protect existing FM services that may receive harmful interference as a result of the blanket IBOC power allowance. (See the FCC's Order for details on the remediation process.)

Calculator Methodology
As discussed above, an increase in IBOC transmission power beyond 6 dB may be possible depending on conditions that limit harmful interference.  Stations that seek to increase digital power are required to file an application with the FCC setting out compliance with the following criteria and formula:
Between the maximum IBOC power of -10 dBc and a blanket minimum power of -14 dBc, the allowable digital power (for symmetrical sidebands) for the digital station, toward any point on the 60 dBu contour of any first-adjacent analog FM station, is:

Allowable IBOC power =
[2.27 * (60 - (IBOC station F(50,10) dBu)) - 33.6]

Note: The above formula results in combined symmetrical sideband power levels, whereas the NPR Labs Calculator provides power for individual sidebands at 3 dB below what is calculated by the formula. The FCC is currently considering approving asymmetrical sideband transmission.

Two Examples of the Methodology
The following shows how this formula is applied in two cases.

The first illustration considers WOUB-FM, channel 217B, Athens, Ohio, as an IBOC power-increase candidate, shown in the center of the map.
The significant first-adjacent neighbors surround WOUB-FM, with their protected F(50,50) 60 dBu contours shown in blue (on channel 216) and green (of channel 218).
The red contour line is the F(50,10) 54 dBu field strength of WOUB-FM, which is in contact, or near-contact, with several of the protected contours.

Please click on the map to view it in a separate window

Please click on the map to view it in a separate window


WOUB-FM’s critical contour value of 54 dBu results in a calculated IBOC power, as follows:

[2.27 * (60 – 54) – 33.6]  =  ‑20 dBc

However, since the blanket increase allows a digital power of ‑14 dBc on both sidebands, WOUB-FM is permitted to operate at up to  ‑14 dBc.
The smaller calculated value is determined by NPR’s study of IBOC compatibility.
Thus, some caution is indicated in increasing digital power to the full blanket allowance of ‑14 dBc.


The second illustration considers WFUS, Channel 278C0, Gulfport, Florida as an IBOC power increase candidate. The following tThree first-adjacent channel stations are significant for this determination:
WRUF‑FM, channel 279C1, Gainesville FL,
WQOL, channel 279C2, Vero Beach, FL and
WVYB, channel 277A, Hollyhill, FL.

The F(50,50) 60 dBu contours of the lower channel stations are shown in blue, and the upper channel station is shown in green.

Please click on the map to view it in a separate window

Please click on the map to view it in a separate window

The F(50,10) 52 dBu contour of WFUS, in red, is in contact with the protected contour of WQOL, resulting in an allowable IBOC power increase on the upper digital sideband to approximately:

[2.27 * (60 – 52) – 33.6] = -15.4 dBc

It is apparent that the red contour does not reach the protected contour of WRUF-FM, leaving an additional allowance for digital power if a directional antenna were employed for the IBOC transmission.
The F(50,10) 43.5 dBu contour of WFUS, in orange, is in contact with the protected contour of WVYB, resulting in an allowable IBOC power increase on the lower digital sideband to:

[2.27 * (60 – 43.5) – 33.6] = +3.9 dBc

Thus, the maximum allowable IBOC power for the upper sideband of WFUS is ‑10 dBc.


Considerations When Using The Methodology
The values listed by the IBOC Power Allowance Calculator are based on the direct line between the candidate station and each first-adjacent neighbor.  In a few cases, variations in the distances to protected service contour and the candidate interference contour at other bearings may slightly reduce the allowable power.

Licensed facilities are used for all stations, however, Construction Permits should also be considered for protection.

The station data used in the study is updated monthly, but changes in facilities may occur at any time.  This calculator should be used only as a guide and users are advised to perform a detailed contour study before completing plans for an IBOC power increase. NPR Labs can help perform such detailed studies.

This plan does not consider potential effects on the host FM stereo or SCA subcarrier operations at elevated power.  We recommend that engineers refer to Section 8 in the Advanced IBOC Coverage & Compatibility Study regarding host compatibility in determining the appropriate IBOC power increase for their station.

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