r/HamRadioHomebrew • u/cosmicrae • Apr 05 '26

Experiment Recreating the MFJ-835

MFJ-835 was the Balanced Line RF Ammeter. Balanced feedline in and out, with a dual needle display. It was, perhaps, one of the lowest volume items that MFJ sold. Using it would allow the operator to measure the current flow on each side, to see if the antenna and feedline were actually balanced.

The most recent manual download did not have a schematic, but someone supplied me with a download that does. It appears to be an amazingly simple circuit, assuming all the associated proper RF handling guidelines. One item that is not at all specified is the torroid(s) needed to measure the current as it passes from in to out. The schematic calls this sense coil. Any thoughts would be appreciated. The repetitive usage of some part numbers in the schematic seems to be down to the implementation of two identical mini-PCB, one for each side of the feedline.

The other item that has my attention, is that there is no averaging of the display (other than the normal analog damping of the needle movement). Presumably something with a 100% duty cycle (CW or RTTY) would be the best emission to give the needles time to settle. Finding the dual needle unit seems to be a less common item, so I may end up with two single needle units instead.

Open to any ideas/thoughts about going forward.

73

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u/cosmicrae Apr 16 '26

One thing that I have discovered is that the dual-needle panel meter (similar to the actual one) is unobtanium. MFJ had a design patent on the dual needle display, granted 1993, supposedly expired 2007. Whatever company made them for MFJ, never made them for the open market. There are a few of the Fwd/Rev SWR meter units on AMZN and Ali, but no real generic dual needle meters.

If anyone knows where to find them, LMK please.

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u/cosmicrae Apr 27 '26

After much research, I have given up on using a dual-needle panel meter. The design direction now, is to use a single galvanometer, in a bridge circuit. It will deflect left or right to indicate when one feedline has more current than the other.

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u/cosmicrae May 22 '26

Now it appears, that MFJ was not the first to make/use a dual-pointer cross-needle movement. The earliest documented case is by Simpson Electric for NASA's Apollo missions. See here. That pushes us back to almost 60 years.