GPS antenna in small device.. A few questions. 1

[mattisback]

Hey Everyone,

I am designing a small GPS tracking device and I had a couple of questions regarding selection of a GPS antenna. I've done a fair bit of research into this but I really don't know all that much about RF which is why I'm here asking for assistance.

First things first, the device I’m designing (a simple GPS logger) will be contained in plastic housing with internal dimensions of approximately 38x21x24mm. It will be oriented with the 24mm dimension pointing towards the sky. The internal space available for an antenna is probably about 29x21x8mm and there’s a little more space below that for a ground plane. All of this (along with my extremely shitty MSPaint skills) is shown in the image below.

I’ve been looking at a few different GPS modules whose acquisition sensitivities range from -145dB to -148dB. I’m not sure how much of a difference this change will make. On one hand, a few dB is a pretty significant margin increase however it seems like it might be small/insignificant in comparison to the tolerances in gain I might see at the antenna stage. Does anyone have any insight into this? (Probably not the right forum area for this question)

I originally wanted to go with a patch antenna because of their increased directivity and therefore better gain however I only have limited room for a ground plane and so I know that a patch antenna’s performance will be reduced compared to its spec. What I’m not sure of is how to compare a patch antenna without the recommended ground plane with some other type of antenna such as a chip-scale or PCB antenna. Does anyone know much about using GPS patch antennas without the recommended ground plane or any information on what the optimum type of antenna for a small package GPS device is?

There are also other environmental factors which will affect the gain of the antenna. There will be a battery in the case (as well as some other circuitry), this as well as the metal screws shown in the picture above will surely have an impact on the performance of a patch (or other) antenna. I’m just not sure how much of an impact. How can I get a good idea of this? Also, how much of an impact would putting an antenna up against the walls (with no air gap) of the plastic case have on performance? I wonder if using a bigger antenna with no air-gap would be better than using a smaller antenna with an air gap or not? Lastly on the topic of environmental factors, the device, in its enclosure will be attached to an animal (eg. A dog) via a collar around its neck. The device will sit at the top of the collar, behind the animal’s head (pointing towards the sky). I wonder if the animal itself might act as a reflector, increasing the gain of a patch (or other?) antenna, maybe the animal would act like a ground plane? Does this sound plausible or would it more likely just cause more signal attenuation? Maybe it would work if other items (eg. the battery) weren’t in the case. Would the other items being between the antenna and the ‘ground plane’ (the animal) have a significant impact on antenna performance?

The other thing I was curious about relates to active vs passive antennas. I was looking at some active (single stage LNA) patch antennas and I wondered... why? What is the need for amplification at this stage? Surely another stage of amplification could just be integrated into the GPS module? I hear that the gain of the integrated LNA in a GPS is already automatically adjusted based on the level of the received signal. I understand that amplifying the signal at the antenna could be used to combat attenuation in a connecting coax cable but I plan on using a very small transmission line from the antenna to the GPS module so there shouldn’t be too much of this. If you apply 15dB of gain at this stage then should you not also be introducing a lot of noise?

Anyway, as you can see I am full of questions on the subject. I’m not so skilled in the black-arts of RF engineering, although I would love to learn more. If anyone has answers to some or all of my questions, or has any information on where I could learn more (eg. books/web pages to read or anything else) so that I can answer some of these questions myself I would really appreciate your help.

Thanks in advance and apologies for the fairly long-winded post.
Best regards.
Matt

 

[IRstuff]

There are plenty of GPS receivers with no external antennas:

http://www.sparkfun.com/products/8975,

which seem to work reasonably well.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Higgler]

You will have alot of trouble making a good antenna at those dimensions.
If this is for a collar on an animal, you really need to metallize the collar somehow to increase the ground plane size.

Based on a quarter wave patch I made previously, this will fit on your antenna size. A PIFA antenna is a modified quarter wave patch, and look up this type antenna too. Here is a link showing both;

http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/12385/1/01-0484.pdf

One thing about very short and efficient antennas, it's the ground plane that actually radiates more energy and not the "antenna". Hence my strong suggestion to metallize the collar somehow (wires can work, doesn't have to be full width metallization).

The ground plane size will tune your frequency, so beware. Batteries do radiate energy. The animal can have an effect on the antenna frequency too. Animals are lossy, so they'd look like an absorber lowering your gain, though they will block energy going downward, and hence might give you some directivity.

thickness of the antenna = bandwidth of the antenna, so go as thick as possible to help the other environmental factors when they shift your frequency.

Not a simple design.
good luck,

 

[VEBill]

The Microsoft Pharos GPS-168 series of GPS dongles are pretty small. Not only has the GPS antenna built in, but also the very good SiRF III GPS receiver and USB interface. I don't have the exact dimensions, but they're about an inch square. They're highly sensitive and will track multiple satellites even when placed about 30 to 50 cm from the (civilian/commercial/passenger) aircraft window.

GPS Trackers (with real-time GSM links) are already available on the market. Just one example:

http://www.dealextreme.com/p/quad-b...acker-bug-with-sos-850-900-1800-1900mhz-11314

GPS Loggers (not real-time) are also readily available and are much cheaper.

Note that GPS receivers vary widely in sensitivity. A good, highly sensitive GPS receiver (SiRF III technology, or equivalent or better) could easily compensate for a less than ideal antenna.

 

[VEBill]

PS: For GPS you obviously don't want "better gain". Your antenna should more-or-less cover the upper hemisphere and thus it should be intentionally low gain (wide beamwidth).