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delay line? velocity factor???
On Feb 14, 3:21 pm, ml wrote:
I.....verse with it; it would work to fill a volume around the antenna. But to get the full effect, it should be a pretty large volume, containing the electric field in the neighborhood of the antenna. Not very practical. In coax, the electric field is between the wires; in the dipole, it's also between the wires, but the volume is very much larger. On the other hand, people have been shortening resonant antennas for a long time by increasing the inductance: thus, loading coils and "slinky" antennas. Similarly, people make "slow" coax by making the center conductor a helix, and thus make delay lines. Cheers, Tom ok what is a 'delay line'?? i would think that would just increase the surface area and therfore sorta increase performance Wikipedia gives a definition of delay line; a length of transmission line is technically a delay line, but often for longer delays, a special line is made in which the center conductor is a wire wound around a core (often of the same material as the dielectric between center and outer). The winding should be done with space between the turns, not close-wound, to give more uniform delay versus frequency. For a uniform TEM transmission line, the delay time is the square root of the total capacitance between the conductors times the total net inductance of the length of the conductors: Tau=sqrt(L*C). Many E&M texts go into how to accurately calculate the inductance and capacitance for coaxial line with straight conductors. In an antenna, you can increase the inductance by adding a lumped inductance, commonly called a loading coil, or you can replace the straight wire with a wire formed into a helix. Google "slinky antenna". You'll find lots of info. I'm not making any claims that a slinky antenna is either a good antenna or a poor one; it's just one way to make a shortened dipole or monopole antenna, or even a shortened Yagi. Cheers, Tom |
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