Antenna size and frequency
Now for communication we say that antenna size should be comparable with the signal wavelength, and by increasing frequency we can reduce antenna's dimension. What if we don't? i mean if i design an antenna for let's say 2 GHz, how will this antenna function for higher frequencies (for e.g 40 GhZ) and for that matter at lower frequencies( 200MHz)?
Regards,
Hello,
Antennas with small electrical size (or better volume) have strict relation between useful relative bandwidth, and wavelength with respect to size. The smaller size/wavelength, the smaller relative bandwidth will be. You may look for: physical limits small antennas Wheeler in a search engine (don't use quotes around the words).
In real world, electrically very small antennas have such unfortunate Im(Z)/Re(Z) ratio, that matching will introduce significant loss, reducing overall antenna efficiency and hence actual gain. If you are able to convert all electrically energy to radiation energy, even electrically small antennas have gain > 0 dBi for certain directions. For the reception case electrically small antennas can be attractive as long as overall S/N ratio isn't degraded too much. This is used in may active antennas for MF and HF reception (AM/SW broadcast/utility/amateur/etc).
For electrically large antennas you have more flavors. Antennas with large standing wave ratio in their conductors have a radiation pattern with many lobes, while well-designed structures with traveling wave behavior may show good directivity to a predefined direction, over large relative bandwidth (horns, flared transmission lines, flared coaxial structures, etc).
Practical example: when you have a thin half wave dipole at 1 GHz (about 13 cm long), it will perform very bad at 200 MHz because of the huge mismatch (the antenna behaves like a capacitor). Matching for this frequency will increase the gain with similar shaped radiation pattern as for the 1 GHz case. At 2 GHz, you will also have mismatch because of the high input impedance, but radiation pattern has similar shape as at 1 GHz. At 3 GHz it will have a reasonable match, but the radiation pattern will have strong lobes and will no longer have same shape as around 1 GHz.