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RE: Patch plans anyone?


> Does going to teflon (Er=2.1), shrinking the patch 2x, make that much
> difference in the pattern?  Or just the gain?  Has anyone
> been able to model this?

The physical size scales as sqrt(epsilon). For example at GPS, the
free-space wavelength is 19cm so an unload patch will be ~ 1/2 wavelength =
9.5 cm = 3.8 inches on a side. The smallest of the patches are about 1.2
inches on a side and use Er ~ 10 ceramic.

As you load the antenna with dielectric to make it smaller, it becomes a
higher Q device. I have seen small GPS patches with an intrinsic bandwidth
<10 MHz, i.e. a Q >60. As the Q of the antenna increases, it becomes much
more sensitive to ohmic losses. In the transmit case, the patch gets hot and
your signal is attenuated. In the receive case (like 2,4 GHz), you take a
double "hit" -- the losses soak up the signal and the losses add thermal
noise. Every dB of internal loss in an antenna (and the associated cables)
adds 1 dB to the noise figure.

The pattern of a patch is essentially a dipole (or its circularly polarized
equivalent). even the pattern of an infinitesimal dipole is nearly the same
as a real half-wave dipole. So the pattern is unaffected.

73, Tom

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