GPS active antenna conversion.

Discuss commercial and home made antennas.
trevatxtal
Posts: 6
Joined: Sat Sep 05, 2015 6:27 am

GPS active antenna conversion.

Post by trevatxtal » Sat Sep 05, 2015 6:46 am

Hello folk I have been hovering for some time.
I Hang about in South Devon, maybe see you at the Rally on the 13th Sep ,Newton Abbot.
I would now welcome opinions on GPS active antenna conversion.
Ebay have them at 99P inc postage, I purchased eight for the cables and plugs.
I took one apart and was pleased to find the two transistor + IC, wide band amplifier was full accessible for conversion to other bands.
At that price I can afford to screw a few up.
To find them search Ebay "GPS Aerial Signal Active Antenna"
Your input greatly appreciated.
Trev

iScan
Posts: 13
Joined: Wed Sep 09, 2015 10:11 pm

Re: GPS active antenna conversion.

Post by iScan » Sat Sep 12, 2015 8:42 pm

Hello Trev,
Are these amplifiers or antennas with amplifiers? My concern is what makes an amplifier band specific? Could be filters. Could be traps. Could be simply that the antenna is tuned or cut for that band. Maybe I am wrong but if the antenna is simply cut for that band, wouldn't you just need different antennas for other bands?

iScan
Posts: 13
Joined: Wed Sep 09, 2015 10:11 pm

Re: GPS active antenna conversion.

Post by iScan » Thu Sep 17, 2015 4:34 am

Hello Trev,

Thanks for your tip. I ordered some of these through eBay for 0.99 USD for the coax and curiosity. I wanted to see what you were talking about. Mine had two bar magnets in the base of the active antenna. These are marked 3V but there is no documentation so I'd guess they are fed 3 volts DC for operation. These I ordered have microscopic parts about what you described and are built with a heavy chunk of ceramic on one side and a chunk of aluminum around 2mm x 4mm x 4mm on the other. I think their physical antenna is a simple printed circuit path the reverse side of the printed circuit board is solid foil and a jumper may run through the board to connect it. But I don't think so because paths on the front could have gone to the edge which does connect by the mounting screws. Perhaps too, if it existed, I may have broken connection to the foil on the outside. I can't imagine changing much in the ones I bought due to their tiny size. Like you said, the coax is likely to be useful and these priced even lower than the active antennas you found.

trevatxtal
Posts: 6
Joined: Sat Sep 05, 2015 6:27 am

Re: GPS active antenna conversion.

Post by trevatxtal » Mon Sep 21, 2015 3:32 pm

hello iscan
The magnet is to fix on to a car top as these are satellite navigation amplifiers and tuned cavity receiver.
The ceramic device is the tuned (tuned cavity) antenna, as far as I can see there ar two transistors and an ic/fet 4 pin device.
I will take one apart to be able to draw a cercuit diagram, I had hoped to find on on line but no luck so far.
The gain at the sat frequency is suposed to be over 25 Db so worth playing with and at 99P what is to loose.
Trev

iScan
Posts: 13
Joined: Wed Sep 09, 2015 10:11 pm

Re: GPS active antenna conversion.

Post by iScan » Wed Sep 23, 2015 6:30 pm

Hi Trev,
Non-destructive reverse assembly is a problem. I found screws that held the magnets, then I quickly ripped into the first unit, breaking the ceramic out in pieces. The ceramic was riveted into place and hid circuit components. I have no experience working with such sub-micro discrete components as were found in the lot of 8 that I purchased. Signal inputs are not likely to overload with power ranges these can handle. But, I don't see myself as able to make changes on these circuit boards. Were you thinking to try some other tuned antenna/s? There is that new article on a log-spiral active antenna (the www.RTL-SDR.com home.) It suggests a low noise amplifier to replace the part found in that discontinued active antenna.
I surely agree with you about their cost. For 99 cents and time it takes to dissect their output feed points, you get of whatever quality at least a nice length of shielded coax with a male SMA connected at one end. At the minimum these are good for 8 projects. Then there's the fun of some reverse engineering. Regardless of any success, the cost is worth all that!
iScan

trevatxtal
Posts: 6
Joined: Sat Sep 05, 2015 6:27 am

Re: GPS active antenna conversion.

Post by trevatxtal » Thu Sep 24, 2015 4:02 pm

Hello IScan
I agree the amplifier board components are very small but I will persevere as I have nothing to loose the cable and plug is worth the money.
The miniaturisation of electronics is something we will have to come to terms with.
I recently purchased a soldering station with a hot air blower and very fine jets, it enabled me to remove individual surface mount components and re solder them under a large magnifier.
I am practising on old boards (disk drive and other miniature things) before dissembling the uhf amps.
When I have anything that is useful I will let you know.
In the mean time many thanks for the Link lots of lovely things to think about.
Regards.
Trev

Ebay search for " 2 in 1 Soldering Rework Station Hot Air & Iron 10 Tips + 4 Nozzles"

iScan
Posts: 13
Joined: Wed Sep 09, 2015 10:11 pm

Re: GPS active antenna conversion.

Post by iScan » Mon Oct 05, 2015 4:37 am

Hello Trev,

You might not need to work on the PC board. I had a break through, maybe. I looked more closely and I think the plate on top of that ceramic is just a patch antenna. Look at the series of one corner and corners removed on two corners. Opposite the corner diagonally the corner is rounded off. Look at what I thought was a rivet is probably the soldered probe to connect to the patch. See it is off-centered slightly? If all that is so, desoldering the probe and removing the ceramic, could give you easy access to replace with a patch antenna tuned to receive L-Band ADS-B? There's an L-Band Patch antenna here (see the news article) on RTL-SDR.COM and this looks so much like it.

rtlsdrblog
Site Admin
Posts: 2330
Joined: Mon Nov 19, 2012 11:54 pm

Re: GPS active antenna conversion.

Post by rtlsdrblog » Tue Oct 06, 2015 12:25 am

Got any images of your GPS antenna? Recently there was a thread on Reddit (https://www.reddit.com/r/RTLSDR/comment ... _inmarsat/) where someone had a different type of GPS antenna, with a smaller filter. That one had no DC block caps between the resistors, so instead of adding a short, it is recommended to add a 100pF+ capacitor.

iScan
Posts: 13
Joined: Wed Sep 09, 2015 10:11 pm

Re: GPS active antenna conversion.

Post by iScan » Wed Oct 07, 2015 5:40 am

Yes. But, not such great photos. The first one I admittedly ripped right into it. You can see what was under the ceramic spacer in one. The other only has the cover removed though, I scratched the top element to be sure it was metal. I hope these are a help. Here is the listing... http://www.ebay.com/itm/121758469993?_t ... EBIDX%3AIT
and photos...
http://www.dumpt.com/img/viewer.php?fil ... 0tfnz0.jpg
http://www.dumpt.com/img/viewer.php?fil ... zgwpob.jpg
http://www.dumpt.com/img/viewer.php?fil ... xq2ffe.jpg
http://www.dumpt.com/img/viewer.php?fil ... 5o9yyx.jpg

iScan
Posts: 13
Joined: Wed Sep 09, 2015 10:11 pm

Re: GPS active antenna conversion.

Post by iScan » Wed Oct 07, 2015 8:15 am

You both know these things. Since we now have all these cheap surplus cables that happen to all be of similar lengths, I thought to put my question here.

If I use two of these coax and connect the center conductors one to to the other's braid on each end, connect biconical dipole elements one side, connect the receiver on the other, I would likely improve these coaxial cable signals having constructed better cable and at the same time not need any balun at all, right? Seems it could be a way to have advantages of balanced lead and yet use coax cable too. Usually might be expensive to run coax this way, but there is so much of it and whomever knows what kind of quality it is may not want to say. Then my sideways thought comes at the receiver when the two reconnect at the SMA terminal THAT is unbalanced. But then how significant a loss could THAT be?

Thanks for your thoughts!

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