Stronger HD Signals

Scott Fybush
Thu Mar 13 12:57:32 EDT 2014

On 3/9/2014 5:17 PM, Nickolas Noseworthy wrote:

> Do stations have the ability to raise
> the power on their digital transmitters?

Yes, to a certain extent.

The original rules for HD said the HD carrier should run at -20dBC 
(decibels below carrier), or 1% of the analog power. Except for WGBH, 
all of the major FM stations in Boston are "height-derated," operating 
with significantly less than a full 50 kW because their antennas are 
higher than the 150 meters that's standard for a class B FM. The Pru 
stations use roughly 22 kW, so they're putting out only 220 watts on 
their HD carriers. The stations out at FM128 use even less - WJMN, for 
instance, has just 9.2 kW ERP analog, so 92 watts on digital.

The FCC now allows all FM stations to increase to -14 dBC, which is 
approximately 3% of analog power. With a showing of no interference, 
stations can go all the way to -10 dBC, or 10% of analog power. I 
believe WKLB is now using -14 and has experimented with -10.

But there's a significant cost associated with raising HD power - not 
just a higher-powered transmitter but also replacing much of the 
transmission infrastructure (combiners, circulators, etc.) that connects 
the HD transmitter to the antenna. This is especially difficult at a 
shared site like the Pru. And so far, there's no return on investment to 
justify that kind of capital expenditure.

> On another note, some stations seem to have a better digital signal
> then analog signal. From my home in Merrimack NH, my roof antenna
> picks up a barely listenable analog signal of WSNE near Providence.
> But it always catches a full signal of WSNE HD-1, HD-2, and now HD-3.
> WBLM out of Portland is masked by WLLO-LP in Londonderry, but the HD
> signal always kicks in with no interruptions. What makes an HD signal
> travel farther then the analog wave?

It's all about co-channel interference. "93.3-HD" doesn't actually share 
a channel with 93.3 analog. The HD system, while described as 
"in-band-on-channel" (IBOC), is really "in-band-adjacent-channel" 
(IBAC). The digital carriers for WSNE occupy the same frequencies used 
by analog stations on 93.1 and 93.5, and those are fairly open channels 
in southern NH. Same for 102.7 and 103.1, which is where WBLM's HD 
carriers live. Your HD receiver is designed to lock in on those 
sidebands first, if it can find them, and disregard whatever's in the 
analog channel in between. So once it detects HD from WSNE or WBLM, it 
disregards whatever's happening on the analog channel between the HD 
carriers - WLLO-LP (analog-only) in the case of 102.9, or the weak WSNE 
analog (presumably mixing with analog-only 93.3 Belmont/Concord) on 93.3.

Make sense?

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