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Best Practices for AM Directional Systems

DAs are marvels of engineering but can seem daunting. Here are useful tips!

Fig. 1: A 1960s phasor with commonpoint controls added.

Our previous article “Find Your Modulation Sweet Spot,” published in the Oct. 9, 2019, issue of Radio World, is still available online, just Google that headline. It sets the stage for a two-part discussion of AM directionals, beginning here.

As you surely know, fewer engineers are qualified to work on AM directional antenna systems today. Younger ones who maintain these marvels of engineering may not be as well versed as their predecessors.

If you are a member of the new breed, my advice is to be careful not to do the wrong thing when tackling a problem. Don’t make adjustments without analyzing a situation first.   

PROBLEMS

If you find antenna monitor phase and ratio readings are off more than a few degrees or a tower radio is off more than 5%, don’t start adjusting the phasor to compensate. Keep your cool. Write down all the phasor dial readings for future reference. Do a complete physical inspection, including eyeballing the inside of the phasor cabinet and antenna coupling networks. Look for broken connections everywhere and on lines running to the towers. Go out and measure the monitor points. You may find they are in spec and that your real problem is in the antenna monitor or other part of the sample system. 

Jumping to the wrong conclusion could result in chasing your tail to fix an antenna problem that you don’t have. 

TOO MANY KNOBS

An engineer maintaining a two-tower AM directional antenna station called me for advice recently. He is one of the engineers I am mentoring. 

A station’s commonpoint impedance (RF input to the phasor) was difficult to adjust because the variable commonpoint resistance coil was at one end of its travel. All had been well a few months before. Today’s solid-state transmitters want to see exactly 50 ohms with near zero reactance, as you know, so setting the commonpoint impedance correctly is very important. 

As it turns out, the phasor has phase and ratio controls for both towers. In this case, it was too many knobs.   

The ratio controls could be tweaked on either tower to keep the station in specs. This led to the commonpoint problem when he used both to get the correct antenna monitor phase and ratio numbers. The phasor’s input matching network was looking at a phasor “buss” impedance that was not according to the original plan. 

PHASOR DESIGNS

It’s important to understand how phasing systems are designed and built. A transmitter feeds RF power into a phasor cabinet, where there is a  three-coil impedance matching network. It feeds RF energy to a point called the “buss.” This is where power is rationed out to phase and ratio controls for each tower. The buss is rarely 50 ohms, and it is normal to have the impedance change a bit as an engineer tweaks phase and ratio controls to maintain FCC licensed specifications. The input matching network normally has enough operating range to compensate for these adjustments.    

Not all phasors have a front panel adjusted ratio control for the reference tower. That is for good reason. The reference tower normally gets more power than the other tower or towers. There is normally no need to adjust power to the reference tower after the station is initially tuned and licensed. It is the standard/reference that the other towers or towers are compared/referenced to. 

I told the engineer to carefully adjust the reference tower ratio control to full, or close to full on, while keeping the other tower ratio and phase correct. Then, don’t touch the reference tower radio control again. Mark it as “don’t adjust.” That solved his commonpoint problem. He then had good resistance and reactance control because the buss impedance was as expected in the design. Also, running both ratio controls down to their lower ends could cause some phasor components to run hot. 

DESIGN EVOLUTION

Fig. 2: Commonpoint controls were added to this phasor.

Phasor systems did not typically have front-panel adjustable input matching networks years ago when tube transmitters were the norm. That changed when solid-state transmitters came along. Now input resistance and reactance controls are required to keep reflected power low and transmitters happy. 

Fig. 1 shows a 1967 vintage Gates three-tower phasor. It did not originally have an input matching network that is adjustable from the front panel. I added those knobs to help control the input impedance and transmitter power when switching from 5 kW non-directional day to 5.4 kW directional night. One control is for resistance, which is tweaked to keep the input at the licensed 50 ohms. The other is commonpoint reactance. Fig. 2 is the dial I normally adjusted to get the transmitter to make just the right amount of power at night. 

In this case, the day non-directional antenna coupling unit has clips on fixed coils for adjustments. Once set, it was good to go. The maintenance procedure is to get the transmitter running at the correct power level in the day mode, then switch to night and adjust the night commonpoint reactance control to get the correct directional antenna power. It is that simple.

You’ll also note there are FCC-required operating parameters listed on the front of the phasor and transmitter. They are right where needed most.     

Yes, that is a Gates BC-5H Transmitter next to the phasor. It has been in full service, running 5,000 watts day and 5,400 watts night since 1973. That’s 46 years! This transmitter is on its third high-voltage transformer, third set of AC contactors, fifth set of high-voltage rectifiers and the solid-state audio driver section has been rebuilt four times on site without sending it to a factory for repair. The transmitter lives on, but will likely be replaced by my engineering successor when the next serious failure occurs. Fortunately there is a Collins 5 kW AM to back it up. Both are excellent tube designs. 

ANTENNA EFFICIENCY

Can you assume that all is well when the monitor points are below FCC limits? Not necessarily. You might have serious impedance mismatches and power divider mismatches, as described above. RF power could be turning to heat. 

You can get a readout on antenna efficiency by going to the original proof of performance documentation and making six or more measurements in the major lobe or lobes. The readings should agree, although there are seasonal variations. RF travels better over frozen ground so winter signals are inherently a bit higher. It is not a big deal in the first couple miles from the transmitter. Ground loss changes become more apparent the further out you go, especially at  20 miles and beyond.

OOS, OOM

Just because the phasor doesn’t have active components, that doesn’t mean it should be ignored. Rodents get in sometimes and need to be dealt with. Loose hardware is common on RF contactors because they are usually operated twice a day with plenty of vibration in the process. RF contacts wear and should be replaced before they fail completely. 

HEAT

Get one of those infrared temperature meters and go through the phasor, then antenna coupling networks looking for hot spots. Use it around electrical load centers too. You might be amazed to find hot electrical contracts and wires that are about to fail. Best to take care of the problem before it causes an off-air emergency at a bad time.      

PAPERWORK

Fig. 3: Keep good records.

Keep good visit-to-visit documentation on an AM directional (Fig. 3). It is a history of how the system has been behaving. You don’t just log that everything is OK. AM directionals have many parameters to keep track of. That includes phase and ratio readings on each non-reference tower, dial settings on the phasor, transmitter PA readings, commonpoint current and monitor point readings. along with date and time. You’ll likely see seasonal changes on monitor point measurements.

Fig. 4 shows a phasor adjustment crank. Note that normal counter dial reading is labeled nearby for easy reference. This is one more way to prevent an “oops” from becoming a major problem.

You should keep accurate phasor schematic documentation on hand at the transmitter site. Designs and “as built” are often a bit different. I recommend you pencil any changes on the original schematic. Also, mark down the number of active turns on each coil in the system. As you know, silver-plated straps with clips are used to short out unused turns on fixed coils. They are set once and then normally not touched again. It won’t take but an hour to do the documentation and will save a lot of headaches in the future. Lightning can blow up a coil or capacitor beyond recognition. Having parts values and settings on a schematic diagram can be a life saver.

Fig. 4: Mark your normal dial settings for easy reference.

A nearby station was visited by vandals one night. Somehow they got into the antenna coupling networks and pulled clips off the fixed coils. No other damage, just mischief. It took the engineer only an hour to put the coil clips back in the right spots, plus do a little tweaking, to get the antenna system working properly again. Imagine trying to start from scratch to get the system operational without that knowledge. Ouch! 

By the way, spare parts and equipment manuals belong where the equipment is, not back at the studio. That includes books and programming information for remote control systems. 

The best is yet to come. Stay tuned for an upcoming issue, where you’ll find a real-life story about a 197-foot tower that came down in the parking lot at a directional AM station.

Comment on this or any article. Write to radioworld@futurenet.com.

Mark Persons, WØMH, built four new AM directional systems, from the ground up, using only schematic diagrams and parts. He is an SBE Certified Professional Broadcast Engineer and was named SBE Engineer of the Year in 2018. Mark is now retired after more than 40 years in business. His website is www.mwpersons.com.

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