Some time ago I ordered a lot of (cheap) and small GPS patch antenna from Aliexpress.
DUT:
I was very interested in the resonant frequency so I decided to connect it to my HP8753C to evaluate the match.
Result:
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- Minimum S11 -13.4 dB (VSWR: 1.544:1, reflection coefficient gamma: 0.214) at approx. 1070 MHz
- Antenna resonant peak far off GPS L1 center frequency (nominal 1575.42 MHz). Usually, a sufficient ground plane is required for patch antennas,
but these usually shift the resonant frequency to lower values. - Nevertheless the match looks better than 10dB at the 1575.42 MHz which is sufficient
Side Note: A good match does not necessarily lead to a “good antenna”. Although a good match is a basic requirement for all energy being transported from (and to) the antenna it is not guaranteed that the energy will be radiated in an effective way. S11 (or VSWR) only quantifies the amount of energy being reflected back from the antenna into the transmitter. It is up to the antenna (material and geometry) to efficiently radiate the delivered energy. Just think of a dummy load as an extreme of this phenomena: Nearly all energy is transferred (very good match, S11 minus many dB) but nearly nothing is radiated. Beside impedance mismatch losses, ohmic conduction losses (copper etc.) and mostly dielectric losses of surrounding material (like FR4 etc.) massively degrade antenna efficiency. The belonging measure is called “antenna efficiency” mostly given in %. (For RX operation the same applies analogous due to the antenna reciprocity theorem).
Equipment:
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- HEWLETT PACKARD 8753C, SW: 4.02
- HEWLETT PACKARD 85047A
- APC-7 – SMA (f)
- SMA (f) – U.FL (m) MULTICOMP MC000984
- RG 58A/U 0,5m SMA (m) – SMA (m)
- Kirkby Microwave 85033, ‘50Ω SMA Calibration & verification kit’
Download:
Touchstone file (*.S1P)