Resonant frequency sensing sensor
If you don't need a vector measurement, a scalar network analyzer setup will be the cheaper alternative. VCO/DDS, directional coupler, detector, that's rather simple.
What frequency range? And what is "low cost"?
I think the answer really depends on the antenna type, in particular the frequency range you are talking about.
For <500MHz a VCO and an in-line SWR type of meter would suffice. If you are measuring millimeter wave feed horns the implementation is quite different even if the underlying theory is the same.
Brian.
Brian, for such scalar measurement you always need
- a signal source
- a coupler to separate incident wave and reflection
- a detector or receiver
where your VSWR meter implements the last two items, and the signal source is separate. But overall it's the same setup.
It's no clear why shanmei writes "no VNA", if he excludes an expensive lab instrument or the VNA design principle (which can be built an low cost for less demanding specs).
Antennas don't necessarily feature a resonance frequency. I presume you are either looking for the maximal radiation resistance or observing the transmission between two antennas.
Thank you for your replies.
The antenna is a senor that its resonant frequency would change in the range of 1GHz-5GHz. For example, if the environemnet pH value changes, the resonant frequency of the antenna changes.
The goal is to build a readout chip to wirelessly sense the resonant frequency of the antenna so that the pH value can be wirelessly read.
The coupler idea is good, but it requires the readout circuit close to the antenna. Is it possible to wirelessly read the resonant frequency of the antenna with a longer distance? For example, 10m.
No, there is no easy way to implement that as a passive device with that range.