Not much amateur radio test equipment exists that extends below 1.8 MHz. Measuring antenna
characteristics using one of the MFJ SWR analyzers is very useful. I detail below how to
modify the MFJ-269B model for use at 500-KHz.
The MFJ-269B has an internal variable frequency oscillator that ranges from 1.8-170 MHZ and
420-513 MHz. By adding inductance to the 1.8-4.0 MHz coil the VFO can be shifted to tune at
a lower range of frequencies. The modification is for 450-950 KHz.
By cutting the trace from the band switch to the 1.8-4.0 coil, a toroid coil can be added to
lower the frequency of oscillation. This can be directly measured by the internal frequency
meter.
The coil is made from two small FT 37-77 toroids (Amidon) with eleven turns of 24 ga. magnetic
wire wound through both toroids. A SPDT mini-toggle switch selects either to add the toroid
in series with the 1.8-4.0 MFJ coil or to bypass it, restoring operation to 1.8-4.0 MHz.
- Fig. 1 & 2 - the toggle switch with toroid coil
- Fig. 3 - Schematic of oscillator circuit
- Fig. 4 - Diagram showing pc board modifications
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