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MFJ-974HB Antenna Matching Unit

 

 

Overview

The MFJ-974HB antenna matching unit includes a balanced T-match matching network covering 6m to 160m that is suitable for balanced and unbalanced loads.  This configuration uses switched inductors plus two ganged variable capacitors on the transmitter side; and another pair of ganged variable capacitors on the antenna side of the network.

The unit also includes an accurate twin-needle power/SWR meter, with optional meter illumination.

For a full product description, see:
https://mfjenterprises.com/products/mfj-974hb

This antenna matching unit uses switched air-spaced inductors for 6 to 80m.   When the 160m band is selected, an additional inductor (L3), wound on a pair of red toroidal cores, is used to supplement the inductance of the air-spaced coil.

 

160m modification

I found that, as supplied, the additional inductor (L3) that is switched in circuit for 160m provided insufficient inductance to properly match my multiband doublet at the low end of the band. 

As supplied, the inductor was wound with 33 turns of Teflon coated wire.  This single layer winding fully filled the available space. 

I decided to re-wind the inductor using thinner, PVC coated wire so that I could get more turns on the toroidal core.  By this means, I increased the number of turns from 33 to 43, with taps at 33 and 38 turns.  Currently, the taps are not needed.

Although this inductor is now wound with thinner, low temperature wire, I have never regarded this MFJ product of being capable of handling high power.  In particular, the switch used for selecting the taps on the air-spaced coil really isn't suitable for high power operation.  I don't believe that my version of L3 is going to be the limiting factor in terms of power handling capability.

The photograph shows the modified inductor. 

              

 

The following photograph shows the modified inductor re-installed in its original location.

 

Settings Crib Sheet

To make band changing easy, users of a manual antenna matching unit need a crib sheet when setting the correct inductance and capacitance values.   I have created my crib sheet in Microsoft Word and some users may find it useful as a template.  You can download my Word file by clicking here.