Alpha 4510 / 4520 Wattmeters

QRP TO QRO.  Our Wattmeters can measure from 200 milliwatts to 5 kilowatts, from 160 meters to 10 meters with no slugs!  They display both SWR and POWER at the same time!

 Note: This text is common to the entire line of Alpha 4500 Series Wattmeters.

The Alpha 4500 Series High Frequency RF Power Meter is a laboratory grade device capable of measuring and displaying Transmit Forward power, Reflected power, Delivered power, and SWR at continuously variable power levels.The 4510 and 4520 models are 3 kilowatt and 5 kilowatt models with a Serial interface and come with a fully functioning front panel including a digital display and a multi purpose analog meter movement. The 4510 meter can range from 0 to 3 kilowatts using nine ranges from 200 milliwatts (-7dBW) to 3 kilowatts (+34.8dBW) full scale, while the 4520 meter can range from 0 to 5 kilowatts using the same nine ranges from 10 watts to 5 kilowatts full scale. Experience the ease of use with our 4500 series wattmeters. Effortlessly measure Power and SWR over a broad frequency range with no need to purchase, install, or twiddle slugs to get a reading. And, our wattmeters are very accurate over a broad range of power levels, frequency bands, and temperature.

The wattmeter is highly automated and includes circuitry to protect the unit against typical operating anomalies. The wattmeter automatically determines the direction of power flow, allowing the rear panel coax connectors to be connected interchangeably between RF source (transmitter) and load (antenna). The coax connectors normally supplied are UHF style (e.g., they mate with PL-259 plugs) but they are field replaceable if a different style of connector is preferred (BNC or type N) or if the connectors become damaged.

The wattmeter is capable of simultaneously displaying the Transmitter output power level in the seven segment LED display, plus either Reflected power, SWR, or Forward power in the analog meter display. Transmitter output power can be displayed as either Forward power or Delivered power (Forward Power minus Reflected Power).

The two models of wattmeter report all measurements as streaming data through the Serial port located on the back of the unit, thus enabling real time data logging of station performance. The wattmeter is designed to work across the frequency segment 1.8 MHz to 30 MHz. The RF output levels reported by the wattmeter are temperature and frequency compensated to improve accuracy. Transmitter output power (either as Forward power or as Delivered power) is continuously displayed using the seven segment LED readout, and Forward, Reflected, and SWR are individually selected and displayed using the analog meter.

The wattmeter is designed to switch automatically between nine different power ranges, permitting optimum analog meter deflection and display accuracy. Alternatively, at the touch of a switch, any one of nine specific power ranges can be individually selected from the front panel and the analog meter display “locked” into that power range. Returning the meter to the automatic mode simply requires pressing one switch.

As a built in safeguard, if the applied power exceeds the full-scale reading when the unit is set to one selected power output level, no damage will occur to the analog meter.

The wattmeter is supplied with a universal-input (100-240 V AC, 50-60 Hz) power supply that provides the unit a 12-Volt DC source. It is internally protected against reverse polarity and DC input transients that do not exceed 16 Volts. Upon removal of the DC power source, the analog meter movement is automatically shorted. The unit also features increased meter damping that allows rapid meter response during normal operation. These features also reduce the likelihood of meter damage during transit.

Read the qst 4510 review
Gordon Hardman wrote an article for the November 2010 newsletter:

Calibrating Wattmeters at RFC

Gordon Hardman

 Accurately measuring RF power is a tricky business, to put it mildly. Doing so accurately in a meter, such as the AP4510, that can be sold at a reasonable price and used in a typical radio shack for years is even trickier. One of the keys to the dependability of the AP4510 and others in the family is the careful calibration that each meter undergoes during production. As we were breaking down and moving the production setup for the RF Concepts wattmeters to our new location last week, I was reminded of this and thought it would be worth sharing a little of it with you.

Power, defined as the rate of doing work, is not a fundamental quantity, such as energy, that can be referred to a single fundamental standard. The best that can usually be done is to use some proxy, such as temperature, to compare one power reading against another. In many practical meters for use in radio power measurements, it is really voltage that is being measured. Since we know the impedance of the system in which the voltage measurement is being made (often 50 ohms), the power can be inferred from the voltage measurement.

At the heart of the 4510 series of meters is a rugged directional coupler constructed out of strip line in a four layer printed circuit board. This coupler is designed to maintain its’ high directionality (ability to discriminate between forward and reflected power) over time. The coupled arms are terminated in high stability resistors, and the voltage across these resistors is what is measured by the instrument. The RF voltage is turned into DC using wide dynamic range logarithmic detectors. In addition, a sample of the RF voltage on the “through” arm of the coupler is used to estimate the frequency of the power being applied to it. These are all digitized by the RF microprocessor (there is a second “display” micro as well). One final parameter that is sensed by the micro is the temperature of the strip line PC board in the vicinity of the coupler.

In a more simple wattmeter, there would be generally only one adjustment to calibrate the power sensor at a particular power and frequency. This would be the case, for instance, in the venerable Bird 43 wattmeter. If you want to calibrate, say, the “2-30 MHz 2.5kW” sensor, you would have to pick the power and frequency at which you want to do this. If done with care, the power at this calibration point could be “dead on”. At other powers and frequencies, you would have to rely on the basic design of the instrument to yield a reading that is as accurate as the designer could make it. There is no ability to compensate for errors due to non-linearities in the detector, errors due to different coupling factors at different frequencies, and finally, no compensation for errors due to temperature changes. The 4510 compensates for all of these, not just by hardware design, but by applying mathematical adjustments to the measurements to yield readings of improved accuracy.

The 4510 series is specified over a wide frequency range (1.8-30 MHz), a large range of power (300mW to 3,000W) and over the -25 to +65C temperature range. Each meter is calibrated to determine a set of coefficients which form the basis of the mathematical corrections that are applied to the readings taken by the micro. This could be a formidable set of data if it was in the form of a table at every frequency, power, and temperature combination. Fortunately, this is not necessary. It was determined that a set of so-called fourth-order-polynomials is sufficient to turn a set of raw readings of voltage, temperature and frequency into a highly accurate measurement of forward power, reflected power and SWR. About twenty coefficients are sufficient to describe these polynomials. The coefficients are determined in two stages during manufacturing.

The first step is frequency calibration. After being assembled and undergoing a basic functional test, each RF board is connected to a computer automated test (CAT) setup. A computer controlled signal generator applies power to one port of the board, and an accurate power meter with a computer interface is connected to the other. The computer communicates with the micro on the RF board over its’ serial interface. The CAT computer steps through from 1800 kHz to 29700 kHz in 100 kHz steps, and records the forward and reflected power readings. This data is stored in a file.

The second step is frequency compensation. In this test, a group of six boards are placed in a temperature chamber, and again the CAT computer controls everything. The temperature of the boards is first taken down to -25C. Once their temperature (as reported via their own individual on-board sensors) has stabilized, then the readings from all the boards are recorded in files. Two different power levels are recorded here, since one of the major effects we are measuring is the drift of the log detectors intercept point with temperature, and a minimum of two measurements are needed to determine this. The temperature is then increased in steps up to a high of +65C, and measurements are recorded along the way.

The third step is to “crunch” all this data and distil it down to the twenty coefficients needed by each board when it is part of a meter. This calibration data is stored in yet another file on one of the CAT computers. In order to keep all these files from getting mixed up, we use a unique electronic serial number (ESN) which we get by using a specific temperature sensor which has a unique code embedded in it. All files for a particular board have this ESN as part of their filename.

The fourth and final step is to upload the coefficients into the board. Again, the CAT computer makes sure that the ESN of the target board matches the filename of the coefficient file, and passes the data over the serial port to the RF board. There it is stored in non-volatile EEPROM, and the calibration is complete.

We have been using this process for many years, and found it to be quite reliable. The equipment is being cleaned, re-calibrated and installed at our new location, and so wattmeter production will continue there to the same high standards as at the old location.

Gordon

Documentation

Alpha 4510 / 4520 Wattmeter Manual
4510 / 4505 SoftMeter 3KW
4520 / 4515 SoftMeter 5 KW