Modulation Dependent Carrier Level

Coming to an AM transmitter near you. The FCC announced that starting immediately, stations can employ Modulation Dependent Carrier Level or MDCL technology on AM transmitters.  According to Public Notice DA 11-1535 (.pdf):

Use of MDCL technologies requires a waiver of Section 73.1560(a) of the Commission’s Rules, which sets upper and lower limits for an AM station’s operating power. We hereby establish procedures for AM broadcasters to seek a rule waiver in order to use energy-saving MDCL technologies.

Several transmitter manufactures offer some some version of MDCL in their newer models with the ability to update some older models.  Harris Corporation offers something called “Amplitude Modulation Companding” (AMC) and “Adaptive Carrier Control” (ACC). While Nautel includes an option called “Dynamic Carrier Control” (DCC) on all NX series transmitters with the ability to upgrade some older transmitters.  Continental offers Controlled Carrier level Modulation (CCM) on later model shortwave transmitters by installing SSM modulator, which can be retrofitted.

Nautel’s Dynamic Carrier Control (.ppt) (.pdf available here) reduces the carrier level during moderate modulation periods.  The effect of this is to increase the perceived loudness at the receiver.  During higher modulation periods, the carrier is increased to prevent distortion.  The net effect is between 3 – 6 dB carrier reduction.  During quite periods, the carrier is returned to full power to reduce noise.

Nautel AM Dynamic Carrier Control wave forms
Nautel Presentation on Dynamic AM carrier control

The potential savings is from 20-40%, which for a 10 or 50 KW station, would represent a significant reduction in the power bill.  For a 50 KW station running an older transmitter, the savings would fall in the $37,000 to $56,000 per year range. In some cases, smaller stations may be able to get rid of a demand meter, which would also represent a significant savings.  The threshold for demand meters around here is 5,000 KWh per month.

The FCC further notes that:

The reduction in AM signal power at certain modulation levels inevitably exacts some penalty upon audio quality. Depending on the content of the audio program, MDCL algorithms may introduce some audio distortion or may decrease the signal-to-noise ratio in the receiver. In addition, MDCL algorithms may erode coverage slightly at the fringes of the AM station’s protected service area. Both the long experience of transmitter manufacturers and broadcasters abroad, and the initial reports from experimental operations in Alaska however, indicate that such adverse effects are generally imperceptible.

Would be especially true for higher powered stations that stand to save the most money.

The malfunctioning STL antenna

Right after Tropical Storm Irene, it was noted that the STL signal strength at the WHUD transmitter site was low. Normally it was 300+ µV, now reading around 100 µV, which is a problem. Upon further investigation, it was revealed that the STL transmitter on the intermediate hop had higher than normal reflected power.

Time to call the tower crew.

The STL transmit antenna for WHUD’s STL (WPOU464) hop is a Scala Paraflector (PR-950), mounted at the 280 foot level on this tower:

Scala PR-950 on a guyed tower
Scala PR-950 on a guyed tower

The fact that it happened after a major storm and the transmitter was showing higher than normal reflected power indicates a problem with either the antenna or the jumper between the 7/8″ Cablewave coax and the N connector on the antenna.  A measurement with a spectrum analyzer shows very high return loss:

WHUD STL antenna return loss
WHUD STL antenna return loss

This shows distance to fault 413 feet, with a return loss of -7.4 dB.  That distance is either near or at the antenna and -7.4 dB indicates a lot of reflected power.  We had the tower climber take apart the jumper connections and terminate the jumper with a known good 50 ohm load.  The return loss did not change.  We then had him swap out jumpers and reconnect to the antenna.  That did the trick:

WHUD STL antenna with new jumper
WHUD STL antenna with new jumper

Much better, most of the power is now being radiated by the antenna, the VSWR is 1.02:1.  The impedance bump at 51 feet is a sharp bend in the coax where it is attached to an ice bridge.  Reconnecting the transmission line to the transmitter and turning it on confirms that all is normal again.  The problem with the jumper was found in one of the connectors, it was full of water.

Water contaminated Andrew flexwell connector
Water contaminated Andrew flexwell connector

I cut away the boot, water had entered the connector from the back because waterproofing and tape was not applied all the way to the coax.  This was installed in 1998 when the station moved from Peekskill to their current location in the town of Fishkill.  The fact that it happened now in the nice weather when Mt. Beacon is still accessible and not in the middle of winter means the radio gods are smiling on us.