UWB Antennas group delay measurements are carried by keeping antenna separated by 30 cm ftf or e2e configuration. The literature survey reveals it to be the abeyance of far-field condition.... Please elaborate this 30 cm distance !!!
A good rule of thumb is that for a small antenna (D=lambda/2 or so), if you are more than 2 wavelengths away, you are pretty much in the far field zone. The actual distance is a bit arbitrary and depends on the antenna structure somewhat. For UWB, the antenna dimension D will probably be somewhere in the range of lambda/2. If we are assuming a 3-10GHz UWB frequency range, D = 5cm at the lowest frequency and lambda = 10cm. If we are further away than 20cm, we are in the far field region. At 30cm, we are sure to be in the far field at all frequencies of operation.
That said, you are certainly free to perform measurements at other distances: 1m, 100m. I think the 30cm distance is convenient to many researchers who perform measurements without the use of an anechoic chamber. The effects of secondary reflections from objects in the lab are reduced if the direct beam is very strong. The 30cm represents a tradeoff in this regard.
If you are too close to the other antenna then capacitive and inductive coupling takes place which changes the transfer of energy between the antennas in a way that does not scale with distance in the same way that radiation does. There can also be significant electric and magnetic field components in the direction of energy transfer, which is poorly defined anyway when the antennas are close together. In the far field the inductive and capacitive terms become insignificant, as do fields in the direction of propagation, so the power coupling then becomes proportional to 1/range squared and the dependence on frequency does not change with range, so the dispersion (group delay) curve remains constant with range.
Look at the field terms for radiation from a small dipole, and you will see field terms that fall off differently with range. Only the 1/range field terms (1/range squared for power) represent radiation.
See the equations for a Hetzian dipole near the bottom of this page.
https://en.wikipedia.org/wiki/Dipole_antenna
- Badges
- Science topic