First, a word of warning. If the information in this post doesn't make sense to you, you probably shouldn't be modifying your antenna analyzer. These things cost a bit of money and can be easily damaged by such a modification if you are not careful.
The MFJ-259B and similar instruments are very useful for trimming or adjusting antennas to resonance. The MFJ-259B is great for Amateur Radio purposes, but it doesn't go low enough to cover the Medium Wave frequencies (like the AM Broadcast band) or even lower for LOWFER people.
I found that several people had posted on the internet about how to modify the MFJ-259B to cover these lower frequencies. One such set of instructions is a PDF from the link below. I liked this one because it has pictures, showing very clearly which trace to cut on the circuit board.
http://njdtechnologies.com/wp-content/uploads/2012/08/Modifying-an-MFJ259B.pdf
I didn't go to extremes and add several ranges, as shown in the PDF. I just wanted to cover the entire AM band. This meant that I needed to add two inductors. One just won't do it. The parts I used are listed below. These values of inductance worked for me. You may find you need slightly different values. Note that there is quite a bit of overlap between the two ranges.
120 uH - This gets me 840 KHz to 1.8 MHz
470 uH - This gets me 490 KHz to 1.0 MHz
A SP3T toggle switch
When soldered together and mounted, the Up position on the toggle switch is shorted and gives me the original range of 1.8 to 4.0 MHz on the rotary band switch. The Center and Down positions on the toggle switch give me the two lower ranges (leaving the rotary band switch at 1.8 to 4.0). The other rotary band switch positions are not affected by the toggle switch and added inductors.
So, now I can use my MFJ-259B to trim antennas, tune matching circuits, etc. on the AM band.
BTW, I did a quick and dirty check of a single frequency in the AM band, to make sure that the oscillator actually was close to the frequency that was being displayed. With a Sony digital PLL AM radio on that frequency, I slowly tuned the MFJ-259B. As I could hear the zero beat of the oscillator on the radio, the frequency reading was dead on. I may check it more extensively later with some calibrated equipment, but this quick check tells me that the modified unit is working just fine.
Joe
A means to measure the feedpoint impedance of an antenna system is a great way to assess and adjust an antenna for optimum performance. At resonance the reactive component of the impedance is 0 ohms and the resistive component is a good indicator of the effectiveness of the ground scheme as well as a means to check for changes over time. I use a two channel scope with a current transformer for this purpose and the modified analyzer as detailed in the linked article should serve the same purpose well. Thanks for the link and please let us know of your experiences with this.
I sometimes wonder if the voltage peaking method used to adjust antenna tuning with some transmitters actually works as intended. In my experience, voltage peaks can be observed when way off resonance. Knowing the actual impedance is a reliable method of tuning.
Neil
Do you have a copy of that pdf that website no longer there.