When my company began purchasing solid-state AM
transmitters back in the late 1980s, we immediately began to see a considerable
reduction in operating costs. Solid-state transmitters, even the early ones,
were considerably more efficient than their tube-type predecessors. Not only
were the power amplifiers more efficient, they didn’t require kilowatts of
power to heat filaments, and they didn’t have big cooling blowers that likewise
consume a lot of power.
In the summer months, the savings were even greater
because the heat load was reduced and it didn’t take as much air conditioning
to maintain a safe operating temperature (although it might be argued that
tube-type transmitters didn’t require as cool an environment as solid-state
Succeeding generations of solid-state transmitters
have become more efficient, using fewer devices and a smaller number of
combiner stages to get the final power output. Later transmitters also employed
higher voltages and lower currents, which often resulted in lower I2R
losses in the DC wiring and connections.
NX50 offers several MDCL algorithms to choose from.
Perhaps one of the biggest improvements we saw was
the use of Litz wire in RF circuits, particularly in filter components, which
not only reduced losses but also allowed for a much smaller footprint, since
large copper tube coils were no longer required.
Current generation AM solid-state transmitters
boast a range of 83 to 90 percent AC-to-RF efficiencies; impressive and very
A BETTER MOUSETRAP
With the cooperation of the FCC, some transmitter
manufacturers have gotten aboard with Modulation-Dependent Carrier Level (MDCL)
technology, which has been a staple in European MW broadcasting for many years.
In just the last few months, stations have begun to employ this technology and
evaluate its power-saving benefits and effects on the signal.
As soon as the public notice came out, I filed the
required paperwork with the FCC for one of our West Coast 50 kW stations, KCBC,
which already had an MDCL transmitter in place. The Media Bureau issued a
modified license within a few weeks and we were good to go … sort of.
The issue for our digital AM station was that the
firmware in the Nautel NX50 we were using would not permit simultaneous MDCL
and HD Radio operation. We could have simply turned the digital carriers off
for a while to test, but the good folks at Nautel got us a firmware update that
would permit both. Our staff at the station got the update installed and the
MDCL turned on in short order.
in action. Note that the instantaneous output power was 29.0 kW while the
nominal TPO was 52.65 kW, about 2.6 dB of compression
After considering all the available algorithms, we
opted for the Amplitude Modulation Companding (AMC) algorithm with 3 dB of
carrier compression. Our station serves the Bay Area with a talk format from a
site near Modesto. As a result, while most of the service area is within the 5
mV contour, it is also at a considerable distance from the site. My concern was
that receiver AGCs in the distant coverage area would pump up the gain (and
thus the noise) while the carrier was reduced, thus lowering the signal-to-noise
ratio of the demodulated audio. That would be unacceptable.
As it was explained to me, the British AMC
algorithm takes care of that by restoring full carrier power during periods of
low or no modulation, thus holding receiver AGC levels where they should be for
the full-power signal. Further, with the sideband power unchanged, the loudness
and reach of the station should not be changed.
That is exactly what we observed. In short, we have
heard no discernible degradation of the signal, audio or coverage at all,
analog or digital.
What we have observed is a big reduction in our
We activated AMC operation at KCBC the first of
November. When the November billing cycle closed at the end of that month, I
was able to look at the electrical usage for that month and compare it to the
same period for 2010. This showed a 21 percent decrease in electrical usage and
an $800 decrease in the amount billed. That’s significant.
I looked at the December usage in early January and
found similar savings -- right at 21 percent -- for that month as well
(compared to December of 2010). And as I prepared to write this, I checked the
January usage: It showed 23,988 kWh compared to 30,306 kWh for January of last
year, again, right at a 21 percent savings in power (and $882 less in energy
In late December of last year, we replaced the
16-year-old Nautel ND50 at KLTT in Denver with a new NX50, the same model
transmitter that we use at KCBC. Within a few days of installing it, we got the
AMC working and I was personally able to observe the effects of this on the
sound and coverage of the station.
As with KCBC, I could not detect any observable
effects, even in the fringe area on the west side of the continental divide.
What I could detect was a 21 percent decrease in energy usage at the site for
the month of January compared to January of 2011. That is right in line with
the KCBC savings.
Chief Engineer Amanda Alexander with her new MDCL-equipped NX50 and trusty ND50
aux (also MDCL-equipped).
Since those unqualified successes, we have added
two more stations to the MDCL list. Our 50 kW station in Birmingham uses a
2000-vintage Nautel XL60, which by itself does not have MDCL capability. The
good news is that Nautel offered us a firmware upgrade to the AM-IBOC HD Radio
exciter that drives the transmitter with all the MDCL algorithms in it. So we
jumped on that.
We applied for and got FCC authority, upgraded the
exciter and turned on the AMC with 3 dB of carrier compression. As with the
California and Colorado stations, we have observed no degradation in signal,
coverage or audio in either the analog or digital domains. I don’t yet have
power savings numbers for that station, but my guess is that we will see
something in line with what we have observed elsewhere.
We also activated MDCL on a 5 kW AM in Birmingham,
Ala. Presumably the savings will scale. We’ll be watching in the months to
One anomaly that I noticed on the KLTT electric
bill was that while total kWh was down 21 percent, the total cost was down only
It didn’t take me long to figure out why. I noticed
that the demand, which had been running right at 99 kW, dropped to only 90 kW
for January. The reason was that during the early days of January, we had to
operate both transmitters for testing. During this testing, I had limited the
ND50 to 10 kW, but we had the new transmitter running into the dummy load at
various power levels for awhile. It only takes 15 minutes to set the peak
demand for the month, and that’s just what I inadvertently did.
That brought to light an issue. If we have to use
the auxiliary transmitter for more than a very short period of time, we’re
going to face a big jump in the electric bill if we run it at full power. At
KLTT, we had a solution — that aux transmitter, like the XL60 in Birmingham, is
driven by an AM-IBOC HD Radio exciter. We performed the same firmware upgrade
and now the KLTT auxiliary is MDCL/AMC equipped. We are good to run it at full
power if needed, without any peak demand penalty.
KCBC’s aux, also an ND50, does not have an AM-IBOC
exciter that we can upgrade, so the solution there is to simply limit its
maximum output power to 25 kW. If our chief engineer needs to test it at full
power for any reason, he can do it on the generator.
We will undoubtedly continue to learn more about
MDCL. We may experiment with some of the other available algorithms to see what
their advantages and liabilities are. But at this stage, I am prepared to call
this experiment a success. I can’t see ever going back.
W.C. “Cris” Alexander is director of engineering at Crawford
Broadcasting and a past recipient of SBE’s Broadcast Engineer of the Year