I acquired this piece of test equipment with a view to compactness as well as the ability to test open wire antennas for resonance at high impedances up to 800 Ohms. It is more compact than the MFJ-259 antenna analyzer. While it does allow the high impedance test, the paper calibration sheet on the back of the unit (see photos) does not allow sufficient calibration accuracy to do meaningful tests unless you compare your indication to a known carbon resistor. Worse yet, the frequency is so unstable that you cannot depend on the dial calibration, without resorting to an external frequency counter. The oscillator pulls off frequency near resonance, making any attempts to mark the dial with calibration points futile. Yet another aggravation is the placement of the B model band edges near a ham band. You may have to switch the bandswitch to another range to complete the tests on a single HF band. The earlier models had the ham bands nearer the center of the dial, so the calibration still was bad, but you did not have to change bands to complete SWR tests at a single ham band. The MFJ-204 could possibly be used to adjust a transmatch, employing an antenna switch (be sure to use one with sufficient isolation to prevent damage), as long as you use the receiver to adjust the frequency of test. But the MFJ-212 noise bridge has the necessary switching and none of the instability problems for the same price for this function.
I found the MJF-204 a complete disappointment. If you spend the same money, you can almost get the cheaper Autek antenna analyzer. I had one on loan and it worked well. The eham reviews note considerable quality problems with the Autek units; I did not encounter this problem, but be sure that you check it over carefully when you first buy it. I got my MFJ-259 broken and repaired it. The W8JI website gives complete calibration procedure for the older HF only units. I would never settle for less than the MFJ-259. I love that one. Object lesson: do not do things on the cheap.
[Pictures coming soon]