diff --git a/drivers/net/wireless/ath/ath9k/common-spectral.c b/drivers/net/wireless/ath/ath9k/common-spectral.c
index 5cee231cca1f99510e21410e362a1086cae19672..8752634d007d8eb6c6e8811474774002e2236bc2 100644
--- a/drivers/net/wireless/ath/ath9k/common-spectral.c
+++ b/drivers/net/wireless/ath/ath9k/common-spectral.c
@@ -160,8 +160,10 @@ int ath_cmn_process_fft(struct ath_spec_scan_priv *spec_priv, struct ieee80211_h
 		upper_mag = spectral_max_magnitude(mag_info->upper_bins);
 		fft_sample_40.lower_max_magnitude = __cpu_to_be16(lower_mag);
 		fft_sample_40.upper_max_magnitude = __cpu_to_be16(upper_mag);
-		lower_max_index = spectral_max_index(mag_info->lower_bins);
-		upper_max_index = spectral_max_index(mag_info->upper_bins);
+		lower_max_index = spectral_max_index(mag_info->lower_bins,
+								num_bins);
+		upper_max_index = spectral_max_index(mag_info->upper_bins,
+								num_bins);
 		fft_sample_40.lower_max_index = lower_max_index;
 		fft_sample_40.upper_max_index = upper_max_index;
 		lower_bitmap_w = spectral_bitmap_weight(mag_info->lower_bins);
@@ -189,7 +191,8 @@ int ath_cmn_process_fft(struct ath_spec_scan_priv *spec_priv, struct ieee80211_h
 		mag_info = ((struct ath_ht20_mag_info *)radar_info) - 1;
 		magnitude = spectral_max_magnitude(mag_info->all_bins);
 		fft_sample_20.max_magnitude = __cpu_to_be16(magnitude);
-		max_index = spectral_max_index(mag_info->all_bins);
+		max_index = spectral_max_index(mag_info->all_bins,
+							num_bins);
 		fft_sample_20.max_index = max_index;
 		bitmap_w = spectral_bitmap_weight(mag_info->all_bins);
 		fft_sample_20.bitmap_weight = bitmap_w;
diff --git a/drivers/net/wireless/ath/ath9k/common-spectral.h b/drivers/net/wireless/ath/ath9k/common-spectral.h
index 82d9dd29652cd2912239ff51246bdc7f24e6b8a8..072ff76c358108692b806b75354516a173603c7d 100644
--- a/drivers/net/wireless/ath/ath9k/common-spectral.h
+++ b/drivers/net/wireless/ath/ath9k/common-spectral.h
@@ -111,17 +111,32 @@ static inline u16 spectral_max_magnitude(u8 *bins)
 }
 
 /* return the max magnitude from the all/upper/lower bins */
-static inline u8 spectral_max_index(u8 *bins)
+static inline u8 spectral_max_index(u8 *bins, int num_bins)
 {
 	s8 m = (bins[2] & 0xfc) >> 2;
-
-	/* TODO: this still doesn't always report the right values ... */
-	if (m > 32)
+	u8 zero_idx = num_bins / 2;
+
+	/* It's a 5 bit signed int, remove its sign and use one's
+	 * complement interpretation to add the sign back to the 8
+	 * bit int
+	 */
+	if (m & 0x20) {
+		m &= ~0x20;
 		m |= 0xe0;
-	else
-		m &= ~0xe0;
+	}
+
+	/* Bring the zero point to the beginning
+	 * instead of the middle so that we can use
+	 * it for array lookup and that we don't deal
+	 * with negative values later
+	 */
+	m += zero_idx;
+
+	/* Sanity check to make sure index is within bounds */
+	if (m < 0 || m > num_bins - 1)
+		m = 0;
 
-	return m + 29;
+	return m;
 }
 
 /* return the bitmap weight from the all/upper/lower bins */