On 1/23/13 7:26 AM, li...@lazygranch.com wrote:
For a single frequency use like GPS, the impedance should be close to the target. It is true for scanners and such, 50 ohms is quite nominal. (This notion of DC to daylight and maintaining 50 ohms is fantasy. ) But for a GPS, you know exactly the application.
You assume that the antenna designer actually tried to hit a particular Z. They may have been going for "about 60 ohms" which would be about 1.2:1 for both 50 and 75 ohms. Or, they were more worried about optimizing the pattern or axial ratio, and the Z could be "good enough".
I've got a SWR plot from a passive L1/L2 antenna here (specified as better than 1.5:1) and it varies somewhat randomly with no apparent pattern between 1.5:1 and 1.1:1, and is about 1.3:1 at L1. Another antenna (same mfr, same model, slightly different installation and cable) ripples between 1.45:1 and 1.05:1, mostly oscillating around 1.2:1. At L1 it's about 1.1:1
It meets the spec everywhere.
The loading could effect the antenna pattern if it were not for the preamp.
It is unlikely that the impedance presented at the feedpoint of an single feed antenna will change the pattern, particularly for something low gain like a GPS. That is, there's no physical way it "could" affect it. The pattern is determined by the currents distributed around the physical antenna, and they have a fixed (complex) ratio to the current at the feedpoint. all an impedance change would do is change that current, but then everything changes together and the pattern is unchanged.
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