[time-nuts] GPS Timing Antenna Failure - Long
kb8tq at n1k.org
Sat May 12 17:16:10 EDT 2018
As mentioned a number of times, quadrafilar antennas were only popular for a very short
while back in the 1980’s. Once people started using GPS for “stuff” they rapidly lost out in
the antenna race. They were made popular by an early NIST paper. Later on NIST effectively
said “oops !!” in reference to that paper.
So yes, any modern GPS antenna is likely to be a patch antenna. Trimble and Novatel both
have “exotic” antennas, but they still are fundamentally a patch.
Why all of this? Multi-path. You want to *reject* signals close to the horizon since they are
the ones most likely to be distorted by reflections. Indeed choke rings and the various other
exotic approaches are all aimed at multiparty rejection by reducing gain at (or below) the horizon.
> On May 12, 2018, at 4:07 PM, Larry McDavid <lmcdavid at lmceng.com> wrote:
> I recently had an unexpected failure of a white-conical-dome HP/Symmetricom 58532A GPS antenna that had been in-place about 5 feet above the roof of my two-story home in Southern California for about ten years. I have two similar GPS antennas located about ten feet apart on this roof, one fed with about 50 feet of Andrews Heliax and the other with LMR400; the other antenna continued to work ok. The antennas feed 4x and 8x amplified GPS Source (brand name) antenna splitters. I noticed the failure when several GPSDO units and a GPS Clock failed to sync with the GNSS. I confirmed the failure was not the antenna splitter and I replaced the failed GPS antenna one of the same type, after which all returned to normal.
> I removed the conical radome from the failed antenna and was surprised to find the antenna element was actually a patch, not the quadrafilar I expected under that conical dome. Subsequently I opened the radomes of three other similar GPS timing antennas made by various manufacturers and found that all use patch antennas. I had believed these timing antennas used a quadrafilar design to benefit from higher low-angle gain.
> So, it appears the conical radome shape is really only to prevent snow accumulation. Well... from my experience here on the flatlands of Anaheim near Disneyland, that seems to be completely effective as I've surely had no snow buildup! :) But, I had surely expected the conical radome covered a quadrafilar antenna. Am I alone in expecting a quadrafilar antenna?
> Further troubleshooting of this failed antenna revealed many discrete components on the underside of the round board holding the patch antenna. The circuit uses a three-stage gain amplifier with three Toko bandpass filters, numerous bypass capacitors and stripline inductors. Probing the circuit with a sig gen and spectrum analyzer showed that all three gain stages were working about as expected. Of course, even with 26-30 dB gain in the antenna, the SA did not have enough gain nor low enough noise floor to see any GPS signal from the antenna. But, each gain stage seemed to be working ok. So, what was the failure?
> Upon removing the radome, one unexpected thing was seen. The construction uses a short coax cable up from the N connector, through a hole in the circuit board, where it is bent over and finally soldered to circuit board pads for the shield and center conductor. There was a great deal of very dark flux residue around this coax solder connection. The appearance was so bad it even looked like a cracked solder joint, though that proved not to be the case when the flux residue was thoroughly removed. It did not occur to me to functionally test the antenna at this point. Later, it was necessary to unsolder this coax so the board could be removed to access the components on the underside for detailed testing. But, stage-by-stage RF gain testing did not reveal any problems, so the antenna was reassembled for actual field testing.
> The result? The antenna now works ok; locking sync to the GPS GNSS. I gotta conclude the flux residue was attenuating the signal out of the antenna. Careful inspection of that coax solder joint absolutely did not show any problem after the flux was removed so I believe continuity was ok.
> I next removed the radome from one of my (new) Symmetricom antennas to inspect its coax solder joint and discovered this (perhaps newer) version has a metal shield-can soldered over the coax solder pads; I am loathe to remove that shield just to inspect the solder joint flux. However, there is no flux evident on the solder tabs where the metal shield-can is soldered to the circuit board so the whole thing must have been defluxed after soldering. That would be a better process anyway.
> To make this very long story into a short one, I learned that the HP/Symmetricom 58532A GPS Reference (timing) antennas use a simple patch antenna instead of a quadrafilar antenna and that old solder flux residue will attenuate the even amplified GPS signal out of this antenna.
> I welcome your constructive comments.
> Best wishes,
> Larry McDavid W6FUB
> Anaheim, California (SE of Los Angeles, near Disneyland)
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