[time-nuts] eBay GPS antenna test results.

Dana Whitlow k8yumdoober at gmail.com
Fri Feb 9 19:37:16 EST 2018

I would surmise that the antenna has (or rather had) a protection network
to protect against voltage spikes or surges.  Your description of events
seems consistent with an triggered SCR crowbar circuit, and the 2nd
attempt at the higher current limit shorted the SCR.  Note that this may
not have been a regular SCR, but perhaps a NPN/PNP pair hooked up
to emulate an SCR.   If you can find evidence of such a circuit, it may
be replaceable or simply removable, leaving the antenna fully functional.

Assuming this works, do keep the supply voltage down to something
reasonable like 5 or 6 volts.   Excess voltage will result in unneeded
heat generation in whatever regulator the antenna uses, shortening
its life.


On Fri, Feb 9, 2018 at 5:00 PM, Dr. David Kirkby <
drkirkby at kirkbymicrowave.co.uk> wrote:

> On 9 February 2018 at 21:43, John Green <wpxs472 at gmail.com> wrote:
> > To those who doubted that the antenna was actually a 3.3 to 18 volt
> design,
> > it seems you were correct. Today, I hooked it up to a variable power
> supply
> > and slowly raised the DC voltage fed to the antenna. It began to pull
> > current at about 2.9 volts and at 3.3 volts, took about 40 mA. I
> continued
> > to slowly raise the voltage. At about 7.5 volts, the current suddenly
> > dropped to 10 mA. At just below 12 volts, it suddenly increased to 80 mA
> > and the supply went into current limit. I increased the current limit to
> > 130 mA and repeated the exercise. Everything went as above until I
> reached
> > 12 volts and the current went to 130 mA and the supply went into current
> > limit. Lowering the voltage didn't lower the current. I disconnected it,
> > waited a minute, and tried again. Yep, shorted. It would have worked well
> > with the T bolt, but would have blown anyway if I tried to use it with my
> > 12 volt supply and bias T. I guess I will get inside it somehow to see if
> > it can be repaired. My first attempt ended in failure. I guess I need a
> > bigger screwdriver with which to pry the top off. I am going to contact
> the
> > seller and tell them it was not as advertised. I kind of doubt that will
> > get me anything, but it won't hurt to try. There is a saying about
> > experience being a cruel teacher. You get the results first, and the
> lesson
> > after. Oh well.
> >
> You should not open it up, but open an eBay case for item not as described.
> If it said it would do 3-18 V, but does not, then its not as described, and
> you should get your money back. The chances are the seller will not want to
> arrange collection, so you will probably get to keep it anyway. But you
> should get a refund before opening it up.
> Dave
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