How to test an antenna without using VNA or spectrum analyzer?
Hello,
Yes you can, but you need good knowledge on EM propagation, antenna theory, electronics and cable transmission. I know several people who do it like this (as they have insufficient demand to justify the high price of a VNA or spectrum analyser).
Complex impedance you can measure with a slotted line (on FR4 or air line) and a diode detector by measuring the VSWR pattern along the line. For the diode detector you need to make a correction when you measure around the quadratic to linear RF to DC characteristic.
VSWR you can measure by using a commercial directional coupler (as this is faster then with the slotted line).
At UHF, you can also do your own gain measurements on antennas with not too high directivity. You can use a tuned dipole as reference antenna and a diode detector as RF measuring device. The challenge is to reduce influence of reflections, but at 13 cm wavelength this is not difficult as your measuring distance between DUT and reference antenna can be less then 0.5m (for low gain antennas).
As source I would suggest a constant carrier source, as a WIFI pulsating source may introduce additional errors because of the IF bandwidth of your diode detector. You might use some VCO with buffer. If you really want to use WIFI, you should use a small capacitor following the detection diode and add your buffer very close to the diode. From there you can go to the oscilloscope to observe the pulsating output.
A complete other solution for the detector is by using a mixer with LO and convert the 2.45 GHz to a frequency that you can display on your oscilloscope. As low noise is not of importance, you may use an harmonic mixer, so you can use an LO that is far below 2.45 GHz.