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Teardown of a Mitsubishi ST251 Mobile Satellite Terminal

Last Update: August 11, 2018


While in the process of looking for UHF SATCOM dishes on eBay, I stumbled across several of these portable satellite phones.

The manual for the ST200 series has a copyright date of 2000 - these units were probably designed in the late 1990's and are now obsolete. I picked up this unit for about 100 USD, but I found evidence that a full kit of an identical unit was auctioned off by Goodwill for a whole eight bucks.
Given the low cost, some of the internal parts might be interesting to the radio hobbyist.


The basic system consists of a base unit with a fold up/out patch(?) antenna, and a handset from where the user can place calls.

The antenna is not removable, but the Radio Frequency Unit (RFU), and Transceiver Unit (TU), can all be removed as discreet sub-units with just a few screws. The RFU and TU are encased in metal-coated plastic, presumably for shielding purposes. The battery back (NiMH) is held in with a few clips and is intended to be swapped out readily.

Photos and Comments

The base has this rather primitive-looking power circuit which appears to manage switch over and battery charging when using the battery with the external power adapter.

The handset really isn't anything to write home about; it has a keypad, LCD, speaker, and a microphone on the end of a coiled cable. The reed switch in the center is for detecting when the phone is off the hook.

The Transceiver Unit was way more complex than I was expecting.
Check out the rework in the center and how some of the surface mount diodes are angled. Production problems?
The DB-25 connectors at the top are a serial port and a connection for a remote control unit, if memory serves. The connector on the far left is for the handset.
The big ol' chip in the center appears to be custom silicon. If fact a lot of parts in this system are from Mitsubishi. Vertical integration, ahoy!
The two D-sub connectors at the bottom (note the wicked alignment spikes) connect to the RFU (left) and power (right). The connector on the right is a typical DA-15, while the connector on the left is much weirder.

Check this out.
Completely custom connector, as far as I can tell. It's unclear why Mitsubishi would make a custom part when various signal+coax de facto standard D-sub connector variants exist.
This particular connector crams 14 signals and two RF connections into the space of a DA shell. I wouldn't be surprised if the two coax connections were I and Q signals (or transmit/receive?), but honestly I have no idea.

This is the inside of the RFU.
This is only one side of the board, but check out all that microstrip work, and that huge transistor (FA01324 / RKS5GN0024-001) at the top. Antenna connection is at the bottom left.

Future Work

The antenna uses an SMB(?) jack, which makes it trivial to use with another radio - if you need an L Band antenna on the cheap, you could do worse than this.

The modularization of internal components makes me wonder if/hope the RFU can be reused with zero or minimal modifications. Need to do some bench work with a logic analyzer and oscope to figure out the pinout signaling, however.

The Transceiver Unit is probably mostly useless, but it would be interesting to dump the flash chip (upper right) for interesting info. Crypto keys? Hints for identifying the internal processor core?