On 1/23/13 8:50 PM, Jim Lux wrote:
On 1/23/13 7:26 AM, [email protected] 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
forgot.. 1-1.8 GHz measurement span
It meets the spec everywhere.
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