[time-nuts] Antenna failure

Randy Warner Randy at synergy-gps.com
Mon Dec 18 13:24:17 EST 2006


Dave et al,

Be very careful trying to apply +12V to a GPS antenna. Connector type
does not matter. About the only timing antennas I have seen that accept
a 12V input voltage are the timing antennas from Micropulse (now
Andrew). They are typically rated for 5-26V. This makes it easier to use
them on long coax runs, but ONLY if the antenna bias is supplied through
an external bias-T or splitter. I can just about guarantee you that you
will blow up your receiver if you try to run 12V through it.

As far as failure modes go, by far the most common failure I see (at
least on Motorola receivers) is due to a lightning near miss. There is a
little 5.6V zener across the antenna input circuitry that typically
shorts if it sees a large power surge. Occasionally the DC coupling
inductor will get popped too. The rest of the receiver works fine, and
will normally light back up again as soon as the zener is removed or
replaced. When this happens it is quite possible for the antenna and
splitter to survive. They usually have larger surge suppression
components that will survive this type of event. 

If any of you out there have any old Motorola receivers that appear to
run OK but don't pick up SV's, put an ohmmeter across the antenna
connector on the receiver and see what reading you get. If it is down in
the 0-20 ohm range you probably just have a dead sacrificial diode.


Randy 
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-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of David Smith
Sent: Sunday, December 17, 2006 3:26 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Failure mode in GPS receivers

Bruce,

I've owned a Trak Systems 8821 for over a year, and had a problem with
the antenna side.  Yours may be different.

My unit is designed to work with a powered antenna, and it feeds 12V up
the coax to power it.  Apparently this is the "standard" voltage for TNC
antenna connections - the smaller MCX etc. are 5V standard.

In my ignorance, I initially connected one of the regular "mouse" type
antennas to it that requires 5V.  The antenna worked OK for months, and
I only discovered the error when I was fitting another more permanent
antenna.  Strangely, the "mouse" no longer works on 5V, but seems to get
by on 12V.

Because most powered antennas these days seem to be 5V, I modded the
Trak with a 78L05 in the antenna power circuit to match.

So, what you could have done is fried the port on your splitter with the
12V?  Just a guess.

Regards,
David Smith.

P.S. Have you got any software for yours?  I've got a User Manual PDF
for mine if that helps.

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