MFJ MFJ-891 - User Manual

This document describes the MFJ SWR/WATT Meter, a series of wattmeters designed to provide accurate measurements of forward and reflected power, as well as Standing Wave Ratio (SWR), for various radio frequency (RF) applications. The device is engineered with several innovative features to enhance performance, readability, and durability, making it suitable for modern transceivers.

Function Description

The primary function of the MFJ SWR/WATT Meter is to measure and display RF power in both forward and reflected directions, along with the calculated SWR. This is crucial for optimizing antenna systems and ensuring efficient power transfer from a transmitter to an antenna. The meter utilizes a precision cross-needle design, allowing for simultaneous display of forward power, reflected power, and SWR, providing a comprehensive view of the RF system's performance at a glance.

The device incorporates a true directional coupler, which is essential for accurate measurements across a range of frequencies. Low bias Schottky diode detectors are used to ensure increased linearity in power readings. For specific models like the MFJ-891, a precision true active PEP (Peak Envelope Power) circuit is included, enabling accurate measurement of peak power for SSB (Single Sideband) and AM (Amplitude Modulation) transmissions.

Usage Features

The MFJ SWR/WATT Meter is designed for ease of use with several features that enhance its operational functionality:

Cross-Needle Meter Display: A large (104mm) cross-needle meter simultaneously displays forward power, reflected power, and SWR. This intuitive display allows users to quickly assess their RF system's status without needing to switch between different measurement modes.
Individual Meter Scales: Each power range has its own dedicated meter scale, which significantly increases reading accuracy by eliminating the need for complex conversions or interpretations.
Range Switch: The device includes a range switch that allows users to select the appropriate power range multiplier (x1, x10, x100) based on the expected power level of the measurement. This ensures that the meter operates within its optimal range for the most accurate readings.
AVG/PEP Switch: For models equipped with this feature (like the MFJ-891), an AVG/PEP switch allows users to select between average RF power measurement and Peak Envelope Power (PEP) measurement. The PEP mode is particularly useful for SSB and AM transmissions, providing a more accurate representation of the actual power output during modulation. In PEP mode, the meter exhibits a slow rise and decay time to capture peak values effectively.
LED Backlighting: The meter features LED backlighting, which provides smooth and even illumination of the display. This enhances readability in low-light conditions. The LED backlight can be turned ON/OFF via a dedicated push button. For the MFJ-891, activating the LED backlight also activates the PEP active mode.
HF/VUHF Band Switch (MFJ-894 only): The MFJ-894 model includes a black, round push button that functions as an HF/VUHF band switch. Users can select between HF (button out) or VUHF (button depressed) bands to match the frequency of the intended measurement.
DC Grounded Antenna Connector: The antenna connector is DC grounded, which helps prevent electrostatic buildup, contributing to safer operation and protecting the device from potential damage.
Rugged Construction: The meter is housed in a rugged metal cabinet with a speckled black finish. The front panel is an injection-molded grey design, which aesthetically complements modern transceivers and ensures durability.
Connectivity: The rear panel provides standard coax connectors for connecting to a transmitter (TX) and an antenna (ANT). For the MFJ-894, a second pair of TX/ANT connectors is available. A 13.8V DC connection is provided for powering the meter's LED illumination and, in the case of the MFJ-891, its active circuitry.
DC Power Cable Included: The device comes with a DC power cable, facilitating immediate setup for features requiring external power.

Maintenance Features

While the manual primarily focuses on operational aspects, some points relate to the care and longevity of the device:

Robust Design: The rugged metal cabinet and injection-molded front panel are designed for durability, protecting the internal components from physical damage during normal use.
Zero Adjust Screws: The analog cross-needle meter includes zero adjust screws, allowing users to calibrate the meter's resting position, ensuring accurate baseline readings over time.
Power Requirements: DC power is primarily required for the meter's LED backlight and the active circuitry in the MFJ-891 model. Most other models can function in AVG mode without DC power, even when switched off, indicating a design that minimizes reliance on external power for basic functionality.
Cautionary Guidelines: The manual provides important cautionary notes to ensure the device's longevity and accurate performance:
- Avoid Excessive Vibration/Shock: Due to the sensitive meter movement, users are advised to avoid excessive vibration or mechanical shock to prevent damage.
- Observe Maximum Power: Users must be mindful of the absolute maximum power that can be applied to the meter, which varies by model, to prevent overload and damage.
- Correct Connections: Always observe the correct connections to the transmitter and antenna as indicated on the rear sockets to prevent reverse connection damage.
- Environmental Protection: The meter should not be exposed to excessive temperatures, high humidity, or strong magnetic fields, as these conditions can negatively impact its performance and lifespan.
- Professional Service: In case of service needs, users are advised to contact their local dealer, with dealer information typically found in the back of the user manual. This encourages professional repair and maintenance, preserving the device's integrity.

The MFJ SWR/WATT Meter series is designed to be a reliable and user-friendly tool for radio enthusiasts and professionals, offering precise RF power and SWR measurements with a focus on clear display and robust construction.