Convert the MPD portable to BNC antenna mount

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® MPD VHF portable

Antenna mount conversion to BNC.

ALWAYS USE EYE PROTECTION AROUND POWER and HAND TOOLS!

Unless you have, and are comfortable around, machine tools I recommend you have a machine shop drill and tap the rear casting. If you attempt to "make do" with a handheld power drill the resulting hole will probably be off center, off axis, and oblong. The final result of using handheld power tools could result in your radio being destroyed.

The MPD can be successfully converted from it's proprietary screw in antenna mount to BNC. You will need the following:

1. Drill bit - 5/16 inch.

2. Tap ("plug" type) - 3/8 inch x 32 thread (not usually available at the corner Hardware Store).

3. Panel mount BNC - (there are two types, a long one and short one. Use the short one).

4. File - fine tooth flat (Mill file).

5. A small piece (3x3 inches) of 600 to 800 grit emory cloth (sandpaper).

6. Small (jewelers) crosspoint (Phillips) screwdriver.

7. Regular size "flat" screwdriver.

Remove/unscrew the original antenna and set it aside. Remove the belt clip. Remove (unscrew) the round auxillary RF connector on the side of the radio at the microphone (accessory) connector using a flat screwdriver (looks like the end of a small piece of co-ax cable). Open the radio by loosening the 4 crosspoint (Phillips) screws on the rear of the radio. Gently separate the radio halves by hand. Remove the RF section from the rear half of the radio by unscrewing all the silver headed crosspoint screws (keep track of which screw goes where - as they are different lengths).

Chuck the 5/16 inch drill bit in a drill press. Securely anchor the radio rear shell in an appropriate vise for the table of the drill press, use small strips of wood to protect the casting. CENTER the drill to the existing antenna mounting hole. Turn on the drill press and S-L-O-W-L-Y feed the drill down through the existing hole so the entire hole is reamed out to the new dimension. Remove the radio rear shell from the drill press. You may see a little of the weather seal (black "O-Ring" that surrounds the casting) in the enlarged hole after this operation - no harm done, it will seal against the BNC when it is screwed in place.

Securely anchor the radio shell in a bench vise (use wood to protect the casting). Tap new threads in the newly enlarged hole all the way through using liberal amounts of appropriate tap lubricant. Clean the rear shell of the radio to remove all metal chips and left over lubricant. Wash the rear shell with HOT water and soap. Rinse the rear shell well with HOT water, shake the excess water off, and set the casting aside on a towel to dry.

While the rear shell is drying, place the BNC connector in a bench vise with wood strips to protect it and the tip facing up. Use the fine flat file to file down the center pin to so the depth from the shoulder to tip of the BNC MATCHES the shoulder to tip depth of the original antenna. De-burr (round off) the tip very well by using the sandpaper all around the end feeling for any sharp edges.

Verify that the rear shell is clean and dry. Install the RF section into the shell replacing all the silver crosspoint head screws into their proper places (it does make a difference where the screws go), do not overtighten them - just snug them down. Fit the front and rear halves of the radio together making sure the two multi-pin interconnects mesh properly. Secure the halves together "snugging" the four crosspoint screws in the corners. Re-install the auxillary RF connector in the side of the radio at the microphone (accessory) connector, using the flat screwdriver to bottom the connector in place. Install the BNC connector into the hole in the top of the radio and hand tighten it down.

Test your installation by transmitting into a dummyload through a Watt meter. The Watt meter should show the same power level that the radio was programed for. If it shows less, the tip of the BNC is too short - try modifying another BNC making the tip a bit longer and install it.

Install a BNC whip on the radio and try to access a repeater which you were just barely able to "raise" with the old (original) antenna. Listen to the received signal, it should be much stronger. Make contact through that repeater with a friend that knows what your signal used to sound like through that machine and get a "signal report".

Congratulations! Your MPD radio now has a standard antenna mount upon which you can put your favorite HT antenna (or put a jumper to feed a brick amp).

For us perfectionists...

Melting a small drop of solder into the end of the BNC before installation won't hurt (makes it so the end is a gentle, smooth surface for the 'spring' inside the radio to mate against. Make the solder bead SMALL - just fill the end of the pin...
Using Loctite® to make sure the BNC stays screwed in doesn't hurt either. If you plan to use Loctite®, also use the lock washer that came with the BNC and cut/file the center pin that much longer so the case penetration measurement stays the same. This will ensure that the outside of the BNC has a good ground continuity to the case of the radio. The Loctite® will restore the weather resistance of the antenna area that the lock washer took away. I recommend you use the Blue Loctite®.

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