Digital Phasor for AM Directionals

AM radio transmission technologies have changed greatly over the years. Transistors have replaced vacuum tubes; Heliax cable has replaced open transmission line.
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AM radio transmission technologies have changed greatly over the years. Transistors have replaced vacuum tubes; Heliax cable has replaced open transmission line.

Relay control ladders have been replaced by microprocessors. Another component of the AM transmission chain that could be replaced with modern technology is the phasor.

The traditional phasor

Used only by directional AM stations, the traditional phasor is in many ways like certain reptiles, more akin to the prehistoric past than to the present.

(click thumbnail)Fig. 1 is a chart. You can scan the print supplied or you can use "phasor flow chart.doc" in queue
A phasor serves two basic functions. The first is to divide transmitter output power between two or more towers. The second is to delay the current to the towers by a small amount. The towers, when fed by these individual currents, create the directional pattern.

The traditional phasor uses reactive components, capacitors and inductors, to divide the power and create the delay. The traditional phasor has many drawbacks:

1. It uses large, expensive components. Components usually are derated up to 50 percent of current and voltage ratings. Expensive vacuum capacitors often are employed.

2. Higher power levels require larger and more expensive components.

3. Additional patterns require more components. Each pattern requires its own phasor, and often its own ATU network.

4. Tuning is difficult due to many variable components. Component interaction adds to the difficulty.

5. Distortions may be caused by non-constant bandwidth impedance. The common point impedance varies from carrier frequency to sideband frequencies due to reactive components, causing poor audio performance.

6. Problems with pattern bandwidth. The directional pattern varies from carrier frequency to sideband frequencies. The directional array may not adequately protect adjacent channel stations and audio performance is affected.

7. Susceptible to failure due to high power levels and lightning.

8. Subject to arcing. Requires periodic maintenance to keep components free from dust, which can facilitate arcing.

9. Prone to drift due to environmental factors, i.e. wet ground, dry ground.

10. Must be custom designed and custom built.

11. Is physically large.

The Digital Phasor concept is fundamentally simple: Perform the same functions as the traditional phasor, but with modern systems.

The Digital Phasor comprises three elements: a virtual power divider, a digital RF envelope delay and a phasor control unit.

Virtual Power Divider – A traditional phasor typically uses inductors to divide common point current. The Digital Phasor creates power division virtually; that is, no common point exists and no circuitry divides the common point current.

Instead, banks of solid-state power amplifier modules generate tower current. The number of power amplifier banks equals the number of towers in the array.

Fig. 1 shows one possible configuration. This example uses n transmitters, where n is the number of towers in the array. A modulator drives each bank with the output power controlled by a phasor control unit. The absolute output power of each PA bank is measured; current ratios are calculated and controlled by the PCU.

Ultimately, the towers see the same currents and current ratios it would with a traditional phasor.

Digital RF Envelope Delay – A traditional phasor uses a network of passive, reactive components to create carrier phase shift, what I call "envelope delay." Variable inductors, through a system of mechanical linkages, allow for manual adjustment of the delay.

The phase shift, a time delay measured in nanoseconds, is created at a high power level. To me this seems archaic; it would be better if we could create envelope delay at a lower signal level.

The Digital Phasor creates RF envelope delay by digitally delaying the carrier signal and program audio at a low power level prior to modulation. A DSP or digital/analog hybrid is used to create the delays in the digital domain; the number of delay stages is equal to the number of towers. The PCU controls the amount of delay of each stage.

RF envelope delay is calculated using the following equation:

T(1 = 1/360f

Where:

T(1 = one degree of phase in seconds and

f = carrier frequency in cycles per second

The amount of delay necessary is really quite small. A transmitter operating at 540 kHz requires a phase delay of about 5 nanoseconds per degree of phase.

Phasor Control Unit (PCU) – One difference between the traditional phasor and the Digital Phasor is that the latter has a "brain." Both the virtual power divider and the RF envelope delay stages are controlled by the PCU; the antenna monitor is linked to the PCU providing feedback of antenna phase and current.

Absolute power samples are taken from each PA bank and fed to the PCU where virtual common point current is calculated.

The PCU adjusts the amount of RF envelope delay and PA power against an internal table of system parameters. This gives the Digital Phasor the ability to keep DA power ratios and phase parameters within tight specifications. When the array drifts out of tolerance, the PCU adjusts envelope delay and power output to bring it back in.

Each PA bank will have a maximum power output equal to the highest power level required by the related tower. PA banks could be located in a cabinet or building at the base of each tower, eliminating the high-power transmission line. Individual PA banks, envelope delays and the PCU, could communicate with each other via wired or wireless data networks.

Standard user interfaces such as monitor, keyboard and modem would be considered part of the PCU. The Digital Phasor could be a "black box" that modifies an existing AM transmitter or it could be part of a new transmitter topology.

Benefits

Theoretical advantages of the Digital Phasor over traditional phasors include:

1. Better impedance bandwidth

2. Better pattern bandwidth

3. Better audio quality

4. Mass producible

5. Components operate at low power levels

6. More stable

7. Self-adjusting with feedback link to antenna monitor

8. Multiple patterns possible with marginal increase in cost

9. Active power division. Adjustments can be made automatically

10. Requires less space

11. Fewer variable components. Easier to tune

12. Fewer mechanical parts

13. Less prone to arcing

14. High-power RF transmission line is optional. PA banks could be located at each tower

15. Diagnostics can be built into the PCU

16. System can be upgraded periodically with software revisions

Several patents, foreign and domestic, are pending for the Digital Phasor system.

Tower mutual coupling

The most common question I’ve been asked regarding the Digital Phasor is how tower mutual coupling would affect the performance of the device.

Mutual tower coupling is an interesting phenomenon whereby current from an adjacent tower is induced and then absorbed or reradiated. In some array designs, a tower may absorb more power than it radiates; such a tower is described as having negative impedance.

The bottom line: we don’t know exactly what the driving point impedance of a tower will be until everything is turned on and tuned up.

If this all sounds complicated and spooky, it is. The question we need to be asking is, "Will the tower in question be positive or negative impedance?" This question exists whether the phasor is reactive or digital.

If the tower is positive impedance, net power is radiated and all is well. In a negative tower, more net power is absorbed and either feeds a dummy load or is fed back into the phasor. Neither of these scenarios is ideal. Burning up absorbed power in a dummy load lowers both array and system efficiency.

Feeding absorbed power back to the phasor isn’t that great of an idea either. The returning power is delayed by an amount determined by the distance from the phasor to the array.

This "dirty power" is nearly certain to be out-of-phase in reference to the phasor current. When added back at the phasor summing-node, it will reduce phasor power by an amount determined by the phase and amplitude of the signal. Another effect will be a "comb filter" effect, where time-delay and recursion cause signal notch filtering.

A better approach, assuming that the transmitters are solid-state, would be to convert the RF current at the negative tower to direct current and feed it into the transmitter DC power supply. Array efficiency would still suffer, but system efficiency would be improved.

There also exist arrays that are "unstable" meaning that one or more towers drift from positive to negative (or vice-versa) impedance over time. A Digital Phasor system could theoretically "catch" the instability just as it happens and automatically correct the system by slightly adjusting antenna current and phase.

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