(Off Topic) Power for GPS Active Antenna Question?

i'm taking a break from my preamp this week. I wanted to get my new GPS installed in the truck... so here's my problem:

I want to use a handheld GPS on my active antenna that requires 5vdc at no more than 1 amp. So I built a power supply with a 7805 regulator and two BNC connectors.

The bnc connectors need to be coupled for the signal to pass and the power will need to be injected right into the antenna wire, but the GPS cannot handle the 5v. So I need the power to only go out via the Antenna BNC.

I guess my question is: What will block DC and still let a 1500MHz signal pass. I don't want to use a transformer if i can help it because I'm trying to keep the BNC jacks close together in order to keep my 50ohm signal path.

-mike

You need an inductor (maybe 10 turns of wire, air-core) between the 7805 and the antenna line, don't forget the output capacitors on the output of the7805 - on the regulator side of the inductor. You need a capacitor in series with that line, probably something like a 100 or 1000 pF ceramic chip capacitor. It probably needs to be a ceramic chip capacitor at these frequencies. If you can at all pull it off, the interconnects should be 50 ohm (I think that's what GPS uses) microstrip on a PC board, not wires or anything like that. Obviously the inductor/capacitor junction goes to the antenna, the capacitor feeds the signal to the GPS receiver. Good luck! I thought most GPS receivers already fed power to an active antenna, though.

i'm using a 10uF 16v cap on each side of the 7805 regulator. i also have an air core inductor at 5 turns with .5mm wire.

my interconnects are 50ohm BNC's, but i was going to just use the capacitor to link the BNC's together instead of microstrip on a pc board...?

I thought most GPS receivers already fed power to an active antenna, though.

Click to expand...

most recievers do... but own the only model (Garmin MAP76) that puts out 2.5v instead of 5, well below the tolerance.

If the BNC jacks are really close together it might work ok. Maybe if you have problems, put another wire in parallel and in close proximity to the other wire to approximate a 75 ohm twin lead sort of thing ???

-Dale

i'm not sure how capacitors act with frequency's. Can you explain how this series capacitor is blocking DC and still allowing the signal to pass? Tthe capacitor is frequency dependent... so how do I calculate the value or type of capacitor?

That's pretty much the definition of a capacitor- blocks low frequencies while allowing high frequencies to pass. Any cap will block DC. The frequency at which a cap starts to attenuate a signal is easy to derive with a simple formula, but if I type it out from memory I'll get it wrong!

It's something along the lines of 1/2piCf, but that's probably not right :/

No, that's correct, although it's usually written out as Xc = 1/ (2pi)FC

Assuming you're working into a 50 ohm transmission line, correctly terminated, the "corner frequency" for the low frequency rolloff will be at the point where Xc = 50 ohms. Keep this corner frequency below 1500MHz and you'll be fine.

The RF choke that goes between the 5V supply at the transmission line should have a reactance of 500 ohms or more at 1500MHz.

Xl = (2pi)FL

Luckily, a small aircore inductor will do the job at these high frequencies.

Layout is going to be pretty critical at 1500MHz. Unless you want to get into using stripline techniques on a PCB, your best bet is to cram the power injection circuit into the smallest area possible, built "ground-plane" style on a small piece of unetched PCB, and use short lengths of low-loss 50-ohm coax to the BNC jacks. Of course, it should all be housed in a shielded enclosure.

yup you got it

Xc=1/2piFc

oops hheheh

nm

My GPS needs to sense a load in order for the antenna port to activate, switching from intrenal antenna to external.

The antenna port puts out 2.5vdc. How shall I make a dummy/fake load?