MFJ MFJ-1025 User Manual

MFJ-1025 Instruction Manual

Noise and Interference Canceler / Signal Enhancer

MFJ-1025

Noise and Interference Canceler / Signal Enhancer

Instruction Manual

INTRODUCTION

The MFJ-1025 is designed to cancel noise or interference, or boost signal-to-noise ratio, before the noise affects sensitive receiver circuits. Unlike conventional noise blankers, the MFJ-1025 can be effective on all types of noise, including unwanted signals. The MFJ-1025 provides user adjustment of phase and amplitude of two antenna inputs. Both phase and level is adjustable allowing signals to be combined. The resulting signal product is the vector addition or subtraction of the two antenna inputs.

The circuitry in the MFJ-1025 has exceptional phase amplitude flatness, making adjustments easy and very repeatable. Gain shift with full phase rotation is less than 2 dB, and typically less than 1 dB.

This method of signal enhancement has several advantages over conventional noise blankers. The advantages are:

•Interference can be much stronger than the signal and be completely removed without affecting the desired signal.

•The MFJ-1025 is effective with all types of interference and on all modes.

•Signals can be added instead of subtracted with the simple push of a button.

Effective and proper operation occurs only if the noise (for nulling noise) or the desired signal (for signal enhancement) is present on both the auxullary (AUX) and main antennas.

The MFJ-1025 contains interface circuitry necessary for operation with most modern HF transceivers. It has built in RF sensing, as well as provisions for external keying from an external control line.

THEORY OF OPERATION

It is commonly perceived that noise interacts with an antenna differently than a "desired" signal. Since noise is an electromagnetic wave, it influences the antenna exactly the same as desired signals. Noise also follows the same rules of propagation as the signals from our transmitters. Noise does not have to be local in origin, it can also arrive by ionospheric propagation from distant sources. This effect can allow noise from sources thousands of miles away to accumulate at the receiving location and "mask" weak signals.

A phased or directional antenna system cancels or nulls unwanted signals, noise, or interference. The MFJ-1025 turns any two antennas into a phased antenna system. The MFJ-1025 has four primary advantages over typical phased antenna systems:

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MFJ-1025 Instruction Manual

Noise and Interference Canceler / Signal Enhancer

1.The user can electrically "rotate" the array even though the antennas remain physically stationary.

2.The user can adjust both direction and wave angle of the null or peak.

3.The user can change from nulling to peaking a signal with the push of a button.

4.Mutual coupling effects can be ignored as phase or level are adjusted.

In all signal or noise canceling situations, the noise and main antennas must "hear" the same signals. It is preferable that the antennas be reasonably close together and share the same polarization. This allows both antennas to clearly "hear" the same undesired signals, and the phasing circuit to completely null or subtract interference.

If the primary problem is local noise, it is preferable the noise antenna "hear" the noise much louder than desired signals. In this case, the noise antenna should be located as close to the noise source as possible.

When peaking or adding signals, best performance occurs when signals have equal and the best possible signal to noise ratios on both antennas. In this case, it is preferable to locate the antennas as far from any noise sources as possible.

CIRCUIT DESCRIPTION

The MFJ-1025 contains a noise amplifier, bridge-type phasing circuit, active combiner, and transmitter change over relay circuit in one package. The noise amplifier consists of Q8. Q8 drives the bridge-type phasing control.

The PHASE control (R16) allows signals from the auxiliary antenna to be rotated in phase nearly 180 degrees (ø >145 degrees) over the entire HF spectrum. The PHASE switch (SW3) allows the selection of a normal or inverted phase by selecting the signal from Q7's source or drain. The total phase shift range is the sum of the PHASE control amount and the amount selected by the PHASE switch.

Q5 and Q6 form the active combiner. Q4 is the line driver that supplies output to the receiver.

Q1, Q2 and Q3 are part of RF sensing circuitry. They are included to automatically bypass the MFJ1025 during transmissions.

REAR PANEL CONNECTIONS

From left to right (rear view) the connections are:

MAIN ANTENNA (antenna input): This SO-239 has a direct connection to the RADIO connector when power is removed, if the external RELAY jack is grounded, or transmitter power is applied.

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MFJ-1025 Instruction Manual

Noise and Interference Canceler / Signal Enhancer

RADIO (transmitter input): This SO-239 has a direct connection to the MAIN SO-239 connector when power is removed, if the external RELAY jack is grounded, or transmitter power is applied. This connector always connects to the receiver input.

T/R CONTROL: This RCA phono connector can be used to switch the MFJ-1025 to a standby mode. It switches when pulled low. Never apply voltage to this jack.

AUX ANTENNA (antenna): This SO-239 and the adjacent RCA phono jack to it's immediate right are inputs for the high gain receive amplifier system.

POWER: This 2.1 mm power connector requires a nominal supply voltage of twelve volts at approximately 150 mA. The center pin is positive.

WARNING: Do not reverse the power connections or damage will result. Remember, the center pin is positive, and the outer shell is negative.

INSTALLING THE MFJ-1025

Please read the following suggestions and examples.

The best location for this unit is at the operating position next to or above the transceiver or receiver. The MFJ-1025 controls must be adjusted during normal operation while listening to the receiver or watching the receiver's S meter.

For most installations, connect the MFJ-1025 as follows:

1.Connect the MFJ-1025 "TRANSMITTER" connector to the transceiver or receiver's antenna terminal with a short coaxial jumper cable.

2.Connect the lead that used to go to the transceiver or receiver's antenna terminal to the MFJ-1025 connector labeled "MAIN ANTENNA".

3.Connect the noise antenna to the MFJ-1025 connector labeled "AUXILIARY ANTENNA".

POWER SUPPLY

This unit requires 10 to 15 volts dc from a negative ground supply. It consumes less than 150 mA current. The power jack is a 2.1 mm coaxial power receptacle. The outer connection is grounded and the center pin is the positive terminal.

While we have made every effort to decouple this line for unwanted noise or RF, it is still possible for unwanted signals to pass from the power or relay control lines to the receiving system. We recommend testing for unwanted noise ingress by disconnecting all antennas from the unit, and listening to the noise. You should hear only a smooth low level hiss with the unit turned on and both gain controls at maximum.

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