[time-nuts] GPS antenna selection — lightning

Ian Stirling is at opus131.com
Fri Aug 5 15:43:09 EDT 2016

On 08/05/2016 01:45 PM, Bob Camp wrote:

> A ten foot long antenna cable is no more or less an issue indoors than a ten foot serial cable
> to a laptop or a ten foot test lead running off of a DVM. They all will pick up a spike if the field
> is strong enough. If you are in a high risk location, then yes you will need to go to extremes
> for all of those cables. In some cases the only real answer is an external faraday cage around
> the entire structure (plus a lot of other stuff).

   My outside GPS aerial came with the NTBW50AA that's still available
from RDR Electronics. It is on a plastic pole tied to the corner of my
deck and is about 12 feet above the ground. It works with the Lucent
RFTG-u pair as well. The 70 ohm cable is about 40 feet long and both
GPSDOs lock quickly from cold in my cellar lab/shack.

   My first venture with GPS was with my Trimble Flightmate Pro that
I bought in October 1993. I knew GPS was supposed to be a precise time
signal. I had built and programmed a Science of Cambridge SC/MP
(INS8060) based computer that decoded the MSF Rugby (no longer there)
60 kHz signal in 1979, still using it in 1994. Comparing it to the
received GPS time on the Trimble, I was dismayed. My records show
that on 1994 June 23 1700 UTC, GPS, or the Trimble, was a whopping
3 seconds slow,vbehind. 1994 July 03 1538 UTC, zero, seemed to be
synchronous. Same day, 1545, 1/2 a second slow. 1552, 2 seconds slow.
1556 1/2 a second slow. I suspect this was due to Selective Availability
that was not turned off until President Clinton ordered it off
in May 2000.

   I have a Navsync CW12-TIM that I bought in 2009. Its diminutive
antenna sticks magnetically to a steel filing cabinet in my office.
I get a good signal and lock there.

     In August 2003, I put my hand out of the back door to test the rain.
A lightning bolt split a tree 60 feet away - my wife called me Thor
for many years. The doorbell rang, the garage door control board was
fried and needed replacing, a router in my upstairs office was
blackened, and in 2003, every television and computer monitor in the
house was a CRT - every one of them had to go through several degaussing

Best wishes,

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