The Transformer Coupled Ammeter (TCA) series of RF Ammeters provides AM broadcasters with an accurate and reliable instrument for measuring common point and antenna base currents. The ammeter uses a toroidal current transformer to obtain a sample voltage proportional to the RF current flowing in a conductor. This voltage sample is connected by a 50 ohm coaxial cable to a temperature compensated rectifier circuit. This circuit converts the RF voltage to a DC voltage which is applied to a linear scale meter, calibrated in RF Amperes, for display of current magnitude. The ammeter provides a DC output voltage to drive a remote indication instrument which may be calibrated against the primary meter and used for remote indication. The ammeter also includes relays to enable remote control of the meter operation.
The RF Ammeter system comprises a toroidal current transformer (TCT) unit, a rectifier/meter unit and an interconnecting coaxial cable. Depending on the RF conductor voltage, the transformer unit may be either the TCT-N
rating) transformer, the TCT-N-HV
rating) transformer or the TCT-N-XHV
rating) transformer. These transformer voltage ratings are the RMS equivalent of the peak modulation voltage. Within each TCT type, several versions are available depending on the RF current magnitude. The TCT provides an RF voltage sample directly proportional to the current. The voltage sample is applied to the rectifier/meter unit by a 50 ohm coaxial cable. The rectifier circuit provides a 50 ohm load resistor for proper termination. This circuit converts the RF voltage sample to a DC voltage for meter display. The rectifier filter circuit is designed to follow the modulation envelope accurately. The meter ballistics average the audio content and thus, the meter indicates the RF carrier current. The meter provides a stable reading independent of modulation, but may deflect slightly under high modulation conditions due to carrier shift. A switch on the unit provides local operation of the meter and selects either the high or low scale for dual scale units. The rectifier/meter unit also provides a DC output voltage plus the audio signal component to drive a remote indicating instrument and relays for remote operation of the meter. When the relays are not energized and the local switch is in the off position, the rectifier circuit input is disconnected from the RF input and grounded for lightning protection.
The RF Ammeter series encompasses both single and dual scale models to provide RF current measurement over the range from 5 Amperes full scale to 160 Amperes full scale. These current ranges follow the FCC rules for linear scale meters. The minimum normal reading must be at least 1/5 full scale, and the maximum reading must not exceed full scale under normal power variations or modulation. Since the TCA series meters do not respond to modulation, it is practical to use the meters up to 95% of full scale.
The standard interconnecting coaxial cable length is six feet. Longer coaxial cable lengths and double shielded coaxial cable are optionally available for any TCA system for high RF field conditions and for RF radiation exposure reduction.
Ammeter Accuracy and Calibration
The RF Ammeter Systems are capable of measuring RF current with excellent accuracy. The accuracy specification of ±2% full scale is guaranteed over a temperature range of -40° C (-40° F) to +65° C (+150°F) and over a frequency range of 500 kHz to 2000 kHz. By using the linearity correction chart on the calibration sheet included with the ammeter, the meter accuracy may be improved to within ±1% full scale.
Every RF Ammeter is calibrated and sealed at the Delta laboratory. Each RF Ammeter is calibrated at a frequency of 1 MHz. The flat frequency response and compensated temperature characteristics of the ammeter assure accurate current measurement. The calibration of the RF Ammeter is warranted for a period of one year under normal use conditions. Excluding damage due to lightning, Delta will recheck the calibration and make repairs, if necessary, without charge during this period. These services are available at a nominal charge at any time thereafter.
The TCA-EX and TCA-EXR series of RF Ammeters include a wide variety of scales, scale combinations, and optional features, all of which are uniquely defined in the model designation system.
TCA-N-EX: The TCA-EX RF Ammeter series has been superseded by the TCA-EXR series. Information on the TCA-EX series is included for legacy equipment applications. The TCA-EX model is a single-scale meter with external output. The system consists of a current transformer, a six foot coaxial cable, and a meter enclosure housing the meter movement, rectifier circuitry, and external output connector. The external output can be used in conjunction with a series 10K Ohm variable resistor connected to a second meter to provide for remote metering. The external output may also be used to drive automatic logging, remote control, or ATS equipment. A two-position toggle switch mounted on the front of the meter box turns the circuit on and off, and when switched off, protects the rectifier circuit against lightning damage. A spring-loaded, momentary contact toggle switch is optionally available to prevent leaving the ammeter in the on position.
TCA-N-EXR and TCA-N/N-EXR: Meters with the “-EXR” suffix provide the external output for remote metering and an internal relay for remote control of the ammeter. A two position toggle switch provides local on/off operation. When the switch is in the off position, the meter is remotely controlled by the operation of the internal relay. When this relay is not energized and the local switch is in the off position, the rectifier circuit input is disconnected from the RF input and grounded for lightning protection. The dual scale models include a second relay for changing scales remotely. These relays are operated by a 24 VDC, 25 mA external power supply. The dual-scale system allows selection of any two of the adjacent (N/2N) scale ranges or selection of specific nonadjacent (N/4N) scale ranges. For example, the TCA-20/40-EXR has a 4 to 20 ampere range on its lower scale, and an 8 to 40 ampere range on its upper scale, and the TCA-5/20-EXR has a 1 to 5 ampere range on its lower scale, and a 4 to 20 ampere range on its upper scale. A three position toggle switch controls the metering function, where the upper position is for the high scale, the lower position is for the low scale, and the center position grounds the rectifier circuit and turns the meter off.
The standard current transformer supplied with the meter is rated for operation with conductor voltages as high as 14 kV peak (10 kVRMS
). The transformer provides a 1.25″ diameter insulated clearance hole for the antenna conductor. A 0.38 to 0.5″ diameter tubular conductor centered in the clearance hole provides optimum voltage capability. The TCA-LS-8
line sections which increase the voltage rating by approximately 50% are available as a factory installed option.
TCA-N-EXR-HV: The suffix “HV” appended to any of the TCA series model numbers specifies that the current transformer supplied with the meter is the high voltage type rated for operation at conductor voltages as high as 28 kV peak (20 kVRMS). The transformer provides a 3.38″ diameter clearance hole for the antenna conductor. A 0.75 to 1.0″ diameter tubular conductor centered in the clearance hole provides maximum voltage capability. Corona rings which increase the voltage rating by approximately 5% are available as a factory installed option.
TCA-N-EXR-XHV: The suffix “XHV” appended to any of the TCA series model numbers specifies that the current transformer supplied with the meter is the extra high voltage type rated for operation at conductor voltages as high as 60 kV peak (42.4 kVRMS). The transformer provides a 6.0″ diameter clearance hole for the antenna conductor. A 2.0 to 2.25″ diameter tubular conductor centered in the clearance hole provides maximum voltage capability.