diff --git a/files/www/cgi-bin/scan b/files/www/cgi-bin/scan
index 21ce1d0c..9657a55f 100755
--- a/files/www/cgi-bin/scan
+++ b/files/www/cgi-bin/scan
@@ -47,6 +47,7 @@ end
 local wifiiface = aredn.hardware.get_iface_name("wifi")
 local phy = iwinfo.nl80211.phyname(wifiiface)
 local rfband = aredn.hardware.get_rfband()
+local nf = iwinfo.nl80211.noise(wifiiface) or -95
 
 if not nixio.fs.stat("/tmp/web") then
     nixio.fs.mkdir("/tmp/web")
@@ -189,7 +190,7 @@ html.print("<button type=button onClick='window.location=\"status\"' title='Retu
 
 -- display scan
 html.print("<table class=sortable border=1 cellpadding=5>")
-html.print("<tr><th>Sig</th><th>Chan</th><th>Enc</th><th>SSID</th><th>Hostname</th><th>MAC/BSSID</th><th>802.11 Mode</th></tr>")
+html.print("<tr><th>SNR</th><th>Signal</th><th>Chan</th><th>Enc</th><th>SSID</th><th>Hostname</th><th>MAC/BSSID</th><th>802.11 Mode</th></tr>")
 
 -- load arp cache
 local arpcache = {}
@@ -248,7 +249,7 @@ do
     else
         html.print("<tr>")
     end
-    html.print("<td>" .. scan.signal .. "</td><td>" .. chan .. "</td><td>" .. scan.key .. "</td><td>" .. scan.ssid .. "</td><td align=center>" .. hostname .. "</td><td>" .. scan.mac:upper() .. "</td><td>" .. scan.mode .. "</td>")
+    html.print("<td>" .. (scan.signal - nf) .. "</td><td>" .. scan.signal .. "</td><td>" .. chan .. "</td><td>" .. scan.key .. "</td><td>" .. scan.ssid .. "</td><td align=center>" .. hostname .. "</td><td>" .. scan.mac:upper() .. "</td><td>" .. scan.mode .. "</td>")
     html.print("</tr>")
 end
 
@@ -260,7 +261,7 @@ local cheight = 400;
 html.print([[
     <center style="font-family: Arial">
     <div style="padding:8px">Spectral View</div>
-    <div style="position:relative;display:inline-block"><div style="position:absolute;left:-34px;top:-200px;transform:rotate(-90deg)">Signal</div></div>
+    <div style="position:relative;display:inline-block"><div style="position:absolute;left:-30px;top:-200px;transform:rotate(-90deg)">SNR</div></div>
     <canvas id="spectral" width="]] .. cwidth .. [[" height="]] .. cheight .. [[" style="border:1px solid grey;background-color:white"></canvas>
     <div style="padding:4px">Channel</div>
     </center>
@@ -273,7 +274,7 @@ function u8tos8(b)
     return b >= 128 and b - 256 or b
 end
 
-local nf = iwinfo.nl80211.noise(wifiiface) or -95
+local cnf = nf - 17.5 -- sub-carrier noise floor
 local start_freq
 local end_freq
 if rfband == "900" then
@@ -305,7 +306,7 @@ end
 bw = tonumber(bw) or 10
 
 local xscale = cwidth / (end_freq - start_freq)
-local min_sig = nf
+local min_sig = cnf
 local max_sig = -60
 local i = 1
 html.write("const p = [")
@@ -320,31 +321,29 @@ do
             local rssi = u8tos8(samples:byte(i + 6))
             local noise = u8tos8(samples:byte(i + 7))
             local datasqsum = 0
-            local dlen = l - 18
             local v = {}
-            for dptr = 0, dlen - 1
+            for dptr = 1,56
             do
-                local data = nixio.bit.lshift(samples:byte(dptr + i + 20), max_exp)
-                if data == 0 then
-                    data = 1
-                end
-                datasqsum = datasqsum + data * data
+                local data = nixio.bit.lshift(samples:byte(dptr + i + 19), max_exp)
+                data = data * data
+                datasqsum = datasqsum + data
                 v[dptr] = data
             end
-            datasqsum = math.log10(datasqsum) * 10
-            for dptr = 0, dlen - 1
+            for dptr = 1,56
             do
-                local data = v[dptr]
-                local sig = noise + rssi + 20 * math.log10(data) - datasqsum
-                local fr = freq + bw * (dptr / dlen - 0.5) - start_freq
-                if sig >= -125 then
-                    if sig < min_sig then
-                        min_sig = sig
+                local datasq = v[dptr]
+                if datasq ~= 0 then
+                    local sig = noise + rssi + 10 * math.log10(datasq / datasqsum)
+                    local fr = freq + bw * (dptr / 56 - 0.5) - start_freq
+                    if sig >= -125 then
+                        if sig < min_sig then
+                            min_sig = sig
+                        end
+                        if sig > max_sig then
+                            max_sig = sig
+                        end
+                        html.write(math.floor(fr * xscale) .. "," .. math.floor(-sig * 4) .. ",")
                     end
-                    if sig > max_sig then
-                        max_sig = sig
-                    end
-                    html.write(math.floor(fr * xscale) .. "," .. math.floor(-sig * 4) .. ",")
                 end
             end
         end
@@ -360,7 +359,7 @@ html.print([[
     for (let i = 0; i < p.length; i += 2) {
         const sig = p[i+1];
         const idx = 4 * (p[i] + ]] .. cwidth .. [[ * Math.floor(yscale * (ytran - sig)));
-        if (sig < ]] .. (-4 * nf) .. [[) {
+        if (sig < ]] .. (-4 * cnf) .. [[) {
             d[idx] = 0;
             d[idx+1] = 0;
             d[idx+2] = 255;
@@ -395,19 +394,18 @@ html.print([[
     ctx.textAlign = "left";
     ctx.strokeStyle = "lightblue";
     ctx.beginPath();
-    for (let signal = -30; signal >= -120; signal -= 10) {
-        const y = Math.floor(yscale * (ytran + signal * 4));
-        ctx.moveTo(30, y);
+    for (let snr = 60; snr >= 0; snr -= 10) {
+        const y = Math.floor(yscale * (ytran + ]] .. (cnf * 4) .. [[ + snr * 4));
+        ctx.moveTo(20, y);
         ctx.lineTo(]] .. cwidth .. [[, y);
-        ctx.fillText("" + signal, 2, y);
+        ctx.fillText("" + snr, 2, y);
     }
     ctx.stroke();
     ctx.strokeStyle = "blue";
     ctx.beginPath();
-    const y = Math.floor(yscale * (]] .. (nf * 4) .. [[ + ytran));
+    const y = Math.floor(yscale * (]] .. (cnf * 4) .. [[ + ytran));
     ctx.moveTo(30, y);
     ctx.lineTo(]] .. cwidth .. [[, y);
-    ctx.fillText("NF", 4, y);
     ctx.stroke();
     </script></form>
 ]])