Random Radio Thoughts for March

March 11, 2012
By

Cris Alexander, CPBE, AMD, DRB Crawford Broadcasting Company

More MDCL in Colorado (and elsewhere)

Now that we’ve had a couple of months to evaluate the Modulation Dependent Carrier Level (MDCL) operation on KLTT, I am prepared to call it an unqualified success.

From last month’s newsletter you may recall that we activated MDCL on the new KLTT Nautel NX50 transmitter in early January, shortly after we installed it. Reviewing our usage, we noted a 21% reduction in the number of kilowatt-hours, mirroring the power savings we noted on our central California station, KCBC. That is encouraging, and that consistency further cements the value of MDCL operation.

Both those stations are running the AMC algorithm, which reduces the carrier during periods of high modulation, restoring it when things get quiet. The sideband power remains unchanged. This reduces the overall peak envelope power of the transmitter, but it very effectively masks the reduced signal-to-noise ratio that any MDCL operation produces.

I have, since we turned MDCL on, made several trips to the edge of the KLTT coverage area and critically listened to the station. I couldn’t tell any difference. Up in Grand County, KLTT is usually listenable with something close to 0.5 mV/m. On my January and February trips to the Grand Lake area, I had good analog and digital coverage. We have made similar observations in the San Francisco area on KCBC.

Just for fun (and to reduce stresses in the transmitter and antenna system), we fired up MDCL on one of our 5 kW stations in Alabama earlier this year. The first electric billing cycle showed only a 5% decrease in power consumption. At first that surprised me, but then I considered that the fixed loads (tower lights, HVAC, rack power, security lighting, etc.) represent a much larger portion of the total site power consumption at a 5 kW station than they do at a 50 kW site, so that 5% is probably about right.

I filed the paperwork with the FCC to operate KLZ using MDCL and expect a grant shortly. I suspect that we’ll find the same kind of power savings there, probably even less since that site is shared at night with another station (KLVZ).

Consulting engineer Ben Dawson made a good point in a letter to Radio World recently: Aside from power savings, the AMC MDCL scheme also reduces stresses in the transmitter and antenna system. The peak RF voltage is the vector sum of the carrier plus the peak sideband power. In a normal (non-AMC) system, that amounts to something in the neighborhood of 150% of the carrier power (100% for the carrier plus 25% for each of the sidebands, more in the sidebands if asymmetrical modulation is used).

Since the carrier power is reduced during modulation in AMC-equipped systems, the peak RF voltage can be cut by 40% or more. In an AMC system, the total peak modulated power, assuming 3 dB of carrier compression, would be 50% for the carrier plus 25% for each of the sidebands. In a 50 kW system, that would result in 50 kW of peak RF power (again, more if asymmetrical modulation is employed).

This reduces the voltages across capacitors, insulators, RF contactors, spark gaps, transmission lines and everything else in the system, which can have a very positive effect on the longevity of those components and even their immunity to lightning damage.

We have for years had an issue at the KLTT transmitter site where when lightning hits one of the high-power towers in the daytime array, the low-power tower (which normally receives about 5 kW) is hit with the full 50 gallons for an RF cycle or two until the transmitter figures out there is a load problem and mutes the output. That has produced burnouts in that low-power tower’s 7/8-inch transmission line on several occasions. With the peak power reduced considerably, our exposure in this kind of situation is also significantly reduced.

 

Wind

No, I’m not talking about all the campaign rhetoric in this election cycle. I’m talking about the high winds that we have sustained around the Front Range for days on end over the last month or so. Usually those kinds of winds wait for March, but not this year. My guess is that you’re as sick of it as I am.

There were a few outages here and there for broadcasters as a result of the sustained high winds. One outage was at the Ruby Hill tower site where the mounting arm hardware on a microwave dish vibrated loose to the point that the dish ended up pointing straight down at the ground! That dish has been up there for years, and it’s never had an issue before in all the previous wind events, so that hardware was tight at one point. The vibration caused by the wind evidently caused those nuts to back off enough to let the dish move. It makes me wonder what else is loose on that and other towers in the area!

If you have news to share with the Rocky Mountain radio engineering community, drop me an email at crisa@crawfordbroadcasting.com.

 

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