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Author Topic: Puzzle Me This... Motorola Design Feature  (Read 1986 times)
AC5UP
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Posts: 3844




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« on: December 13, 2012, 05:25:42 PM »

So I'm tearing down some of the garage filler to recycle the aluminum. One of the victims of this exercise is an older Motorola Syntor UHF radio with chip date codes of '80 and '81. Big old dude with a seriously stout cast aluminum frame. Gave me a socket size challenge until I realized it was Metric and the back of the SO-239 antenna connector features a large-ish solenoid coil.

Pried the connector loose to find a small glass magnetic reed relay inside the coil and in line with the coax center conductor. First time I've seen anything like this, can't for the life of me figure why the radio would want to switch the antenna circuit through an SPST relay contact. This is not a T/R relay. It's a binary device with two states: antenna connected / antenna disconnected.

Is this protection for the front end in what could be a strong RF environment, like multiple TX's on an emergency vehicle, or some sort of an RF failsafe / shut down feature?

Anyone know?
« Last Edit: December 13, 2012, 05:31:08 PM by AC5UP » Logged

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WB4SPT
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« Reply #1 on: December 14, 2012, 04:53:37 PM »

is it this:  http://www.ebay.com/itm/MOTOROLA-MICOR-TRN4734-ANTENNA-RELAY-UNUSED-NOI-/260950933427?pt=US_Radio_Comm_Coaxial_Cables_Connectors&hash=item3cc1e33fb3

sure it doesn't have two of those reed contacts?

At about those years, I pumped a couple of watts, on channel, directly into Motorola portable radios to see if it caused damage.  It didn't.
« Last Edit: December 14, 2012, 04:57:14 PM by WB4SPT » Logged
AC5UP
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« Reply #2 on: December 14, 2012, 05:19:59 PM »

sure it doesn't have two of those reed contacts?

That looks exactly like the piece I pried out of the chassis and there was only one magnetic reed contact soldered to the center pin of the SO-239. I have another Syntor I'll be tearing down so I can double check.

>>>  If it helps anyone, this is the model I'm working with  <<<

Still can't imagine why the radio would want to open circuit the center conductor to the antenna jack.  Kinda' hard to be a radio with no RX or TX.....
Of course, that never stopped Heathshkit,  Roll Eyes  but this is a Motorola.   Grin
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K1CJS
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« Reply #3 on: December 16, 2012, 09:13:52 AM »

If I'm not mistaken, that was put in there for the simple reason that the unit receiver front end would not be overloaded by another radio unit mounted in the vehicle or with unit that had their antennas mounted close together.
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