antenna-theory.com

[coal21]

Can a 75 ohm GPS antenna be connected to 50 ohm receiver? I have installed this receiver at least 20 times with a 50 ohm antenna and LDF4-50 cable. I just received drawings for the install and they show a 75 ohm antenna, 75 ohm cable and 75 ohm "N" connector going to a 50 ohm GPS unit. I am stuck using the antenna and cable(Crappy RG-59). Any suggestions on how to hook this up?

[admin]

If you have a 75 ohm antenna hooked up to a 50 Ohm system, than the reflection coefficient will be (75-50)/(75+50) = 0.2, which means your VSWR will be (1+.2)/(1-.2) = 1.5.

A VSWR of 1.5 is pretty low, and shouldn't cause much of an issue at all (mismatch loss will be <1 dB, so definitely won't make or break your system).

[coal21]

VSWR isn't really an issue it's GPS so it's recieve only. I know I can't use the 75 ohm connector to connect the antenna cable to the 50 ohm input on the GPS receiver, the center pins are different sizes. I was looking at an impedance matching gizmo that has the correct connectors on each end at : http://www.pasternack.com/50-ohm-n-male-75-ohm-n-female-matching-pad-pe7070-p.aspx I'm just wondering why would they want to use RG-59 75 ohm when RG 58 is about the same price and the same form factor, higher freq(actually in the GPS L1 band) and longer cable legnth is possible. It makes no sense to me.

[la7no]

Even on receive only, there can be reflections and standing waves.

Since GPS is a digital system relying on measuring pulse delays and phase differences, I would not be surprises if unwanted reflections could introduce measuring problems. Maybe that is why purists would like to see impedance matching all over?

Interesting subject.

P-T

[admin]

There is no debate on VSWR for receive: it is just as important to match the antenna for receiving as it is for transmitting.

If you think different, you don't have an understanding of what is going on.