antenna-theory.com

[mattdon111]

I am building a receiver device that is using three dipole antennas in RF (microwave) range. I am trying to measure power of signals of three different bandwidth of 900MHz, 2.4GHz and 5.8 GHz in dBm, I build the PCB and antennas are placed parallel right next to each other on one side of PCB and are connected SMA adapters. while measuring power if I shoot 2.4GHz with horn antenna, other two antenna also pick up the signals.
What could be the reason ?
what is the effect of one antenna to another when they are of different bandwidth ?

[admin]

Antennas are not perfect filters in that they receive only one frequency.

If you have a small antenna (let's say resonant at 5.8GHz), it may have an efficiency of -1 dB (or a gain of + 2 dB). At 900 MHz, the antenna is too short for this wavelength, but the efficiency is probably -20 dB (or a gain of -17 dB). So there should still be energy at 900MHz.

If an antenna is tuned to 900MHz though, it may very well also radiate well at higher frequencies (because it is large enough for the 5.8 GHz wavelength). So if the antenna is -1 dB efficiency at 900 MHz, it may be -3 dB at 5.8 GHz (very close).

Basically, antennas are not filters

[malekabadi]

As admin mentioned, the 900MHz and 5.8 GHz antennas are receiving some portion of 2.4 GHz signal.

For instance, if the 5.8 GHz antenna has -0.5 dB S11 at 2.4 GHz, it means that it is 10% efficient (e = -10dB) and can receive power. If you have estimation of your antennas S11 and gains at different frequencies you can find a relation between the antennas receive power and see if you measurement readings are consistent!