MFJ MFJ-66 User Manual

MFJ-66 Dip Meter Adapter

Instruction Manual

MFJ-66 Dip Meter Adapter

Thank you for purchasing the MFJ-66 Dip Meter Adapter. The MFJ-66 Dip Meter Adapter works with your MFJ-209/249/259 SWR Analyzer.

The MFJ-66 Dip Meter Adapter is a kit consisting of 2 coupling coils and a UHF to RCA female adapter. The larger probe coil covers 1.8 through 50 MHz, and offers maximum sensitivity in the 10 to 20 MHz range. The smaller probe coil covers 20 through 175 MHz, and offers maximum sensitivity in the 100 to 150 MHz range.

Dip Meter Theory of Operation

The MFJ-66 Dip Meter adapter is very versatile. When properly used, it can make accurate measurements of many different RF circuits. The following description will help you get maximum accuracy and versatility from the MFJ-66.

The MFJ-66 is designed to adapt a SWR Analyzer to work as a dip meter. Your SWR analyzer contains an internal bandswitched oscillator circuit. The oscillator is buffered to increase the signal level and prevent the load from affecting the oscillators frequency. The high level RF output covers all Ham bands below 170 MHz and is available on the ANTENNA jack.

The SWR Analyzer also contains a meter used to measure the unbalance in an impedence bridge circuit. As the load on the ANTENNA connector approaches 50 ohms resistive, the meter reads a lower amount (toward 1:1).

By connecting a small coil through the adapter to the ANTENNA connector, an SWR Analyzer can be used to test external circuits for resonance. The magnetic field surrounding the coupling coil provides the required connection or coupling to the circuit under test. The meter on the SWR Analyzer is then used to measure the amount of RF signal absorbed by the circuit under test. The meter reads lower as resonance is approached.

Any resonant circuit will absorb RF from the coupling coil as long as the circuit is tuned to the same frequency as the oscillator in the SWR Analyzer. As the "Q" of the circuit under test increases, the dip will become sharper and deeper. High "Q" circuits absorb more RF energy in a narrow range of frequencies. If the "Q" of the circuit under test is low or coupling between the coupling coil and the inductor in the circuit under test is inadequate, the dip shown on the meter will be small or perhaps not even visible.

Unlike the tuning coils of a conventional grid dip meter, the MFJ-66 coupling coil is not a part of a resonant tank circuit. This adapter depends on the "Q" of the external circuit to improve the

MFJ-66 Dip Meter Adapter

Instruction Manual

circuit coupling. If the external circuit has a very low "Q", the coupling will have to be increased by placing the inductor of the external circuit very close and in line with the axis of coupling coil. This has the advantage that stray coupling is reduced and frequency pulling of the oscillator is eliminated. Frequency readings can thus be made with more precision.

To insure accurate readings always keep the coupling as loose as possible while still getting a readable dip.

Maximum coupling is obtained when the coupling coil is either placed inside a larger coil under test, placed against a coil of equal size or placed over a coil of small size. When using the smaller coil to couple with a very small coil, such as a molded inductor, the outer plastic sleeve must be removed or cut to allow the molded inductor to be inserted into the small coupling coil. Otherwise coupling may be insufficient to create a dip.

Once a dip is found the coupling coil and the inductor of the circuit under test should be separated until the dip is barely evident. The frequency should be read at this point for maximum accuracy.

The coupling coil can be removed and the coupling coil jack can be used to directly feed a one or two turn link coil on a toroidal inductor. The coupling can be easily varied when testing resonant circuits containing toroids by adding or removing turns from the coupling link. Never try to couple with the standard method of using a double link. The air wound external link acts like a shorted turn on the toroid and lowers the inductance. This "shorted turn affect" will cause inaccurate measurments for many types of toroids.