GPS antenna question...

white feather

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The company that I work for makes hand held "detectors" that have the capability to connect to GPS. They have small screw on plastic covered antennas that are about 2.5" long that have a connection much like a coaxial cable. The antennas are made in house after our original supplier stopped making them. I get to test them on the detectors to make sure that they will "hook up" to GPS. This sometimes takes a while and I suspect that some of the antennas are not very good. If they don't connect fast enough, I just change them out and put them in a bag with a question mark on it. Not saying they are bad but... The bag is getting pretty full. It seems to me that there must be a way to bench test them to see if they are going to provide enough (gain?) to work before they go on the unit. I just had the bright idea to ask the experts on NES. What do you think, is there a way to build a (relatively inexpensive/not super complicated) device to test them? Thanks in advance for the help.
 

Another_David

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not a lot of info about what the "detectors" detect or what not.

I would start with a simple resistance test on the in house antennas. maybe some EE can comment on a better metric but you can tell a lot by resistance like if there is a bad solder or crimp causing a short (low resistance) or open circuit (infinite resistance).

beyond that, a more sophisticated test fixture could have a signal emitter on a similar range frequency of GPS then the antenna would connect to a receiver fixture and measure signal strength.
 

Kevin_NH

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Whatever you do, don't make a test source at 1575Mhz, that's frowned upon by the FCC

Would need to know more specs.

You could test impedance, unless the GPS antenna is 'active', has an LNA that require a power source and would be damaged by impedance testing. But if you're making them in-house then they're probably just simple patch antennas?

beyond that, a more sophisticated test fixture could have a signal emitter on a similar range frequency of GPS then the antenna would connect to a receiver fixture and measure signal strength.
That would work fine, assuming you have a RF-proof enclosure to test in, otherwise you will eventually meet some very nice gentlemen from the FCC.
 

Skysoldier

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Deleted..

You need a test jig......that you know works......and then test them.

Antenna design is one of the last "black arts" of electronics!
 
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Another_David

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aside from going the make a transmitter route, you could get two simple GPS devices that let you measure the signal reception per satellite. Then attach a known good antenna to one device and then attach the test antenna to the other device and compare results.

not sure what devices are available these days that use an external antenna but any droid phone with GPS will do this. you would just need to mod the droid phone to accept an external antenna

to get decent results you'd want to be near a window. my screen shot:

 
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Zappa

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I use two of these NMO stubbies on my vehicle, drilling holes doesn't upset me. [laugh]

One for GPS, the other for Sirius/XM.




Both bands use "active" antennas, there are 30 tiny amplifiers in the circuitry (one for each GPS sat frequency), which are powered by 5 volts going up the coax from the receiver.
I've seen antennas fail, but it's not very common.
 

Spanz

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if you are making your own antennas...then you should have a test fixture.

in this test fixture you would have a place to mount your antenna, have a known good GPS antenna maybe a foot or two up above it, some absorber foam around the fixture to not let your hands be part of the test, a signal source, and a receiver. You could tell then if an antenna was completely broken, slightly broken and capable of rework, etc.

You test equipment can be really cheap stuff, like a fixed oscillator hooked to the transmit antenna, and a coaxial microwave detector hooked to a DC voltmeter. if you want to get fancy, you could buy a used "network analyzer", which would offer not only a go/no-go test, but ALSO test if the antenna impedance is bad (due to a solder blob, open wire, etc) to help you debug/fix the antenna in the bag that do not work.

the former solution might be $1000, the later more like $5000

http://www.ebay.com/itm/Agilent-871...397?pt=LH_DefaultDomain_0&hash=item541b4047dd



https://www.coppermountaintech.com/store/itemDetail.cfm?prodID=25&catID=0



you can use just a cheap gps antenna for the transmitter, but you will get beter test repeatability with a right hand circular polarized RHCP transmit antenna

http://support.spectracomcorp.com/a...PS?retURL=/apex/KnowledgeSiteHome&popup=false


one benefit of doing the test fixture would be improved test time. probably 1 minute per antenna, rather than waiting for a gps receiver to lock onto a signal from a satellite.
 
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