Home |
Search |
Today's Posts |
|
#1
![]() |
|||
|
|||
![]()
By way of agreeing with what Reg posted about capacitance indirectly
adding to the loss, consider that for any TEM line (twin-lead and coax being two examples), the impedance, Zo, is sqrt(L/C), and the propagation delay, tau, is sqrt(LC) [neglecting the very small contribution of R and G for practical lines at HF and above]. From these two, you can see that C=tau/Zo. If the velocity factor is unity, then tau for a foot of line is one foot divided by the speed of light, about 1.017 nanoseconds. If Zo is 50 ohms, then C for that line would be 20.33pF/foot. If you have line which you know to be 50 ohms and 31.0pF/foot, then you know the v.f. is 20.33/31.0 = 0.656, and by my other recent posting in this thread, you know that its attenuation will be about 1/0.656 = 1.52 times as many dB/unit length as the same line with air dielectric (which would be 50 ohms times 1.52 = 76 ohms). (The interrelation of tau, Z, C, L, line physical length and velocity factor suggests that you can determine Z, for example, by measuring C and v.f. accurately. Some line configurations let you accurately measure conductor diameters as well. You end up with lots of ways to determine a set of line parameters.) But note that a 50 ohm air dielectric coax using the same outer conductor diameter would have a larger inner conductor, but MORE loss than the 76 ohm air dielectric line because of the higher capacitance. Quantitatively, it will have about 1.1 times the dB/unit length loss compared with the 76 ohm line...so the difference in loss between air inslated 50 ohm line and solid polyethylene dielectric 50 ohm line (same OD) will be a ratio of 1.1:1.52, or 1:1.38. Going from 50 ohm line insulated with solid pe to 50 ohm line of the same OD with air insulation will cut the dB loss by about 27%. Going from solid to foamed pe will get you about half that much. There's a bigger effect going from a solid pe 75 ohm line to an air dielectric 75 ohm line, cutting the dB loss by over 42% (assuming I didn't screw up the calcs too badly). Cheers, Tom |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
Inverted ground plane antenna: compared with normal GP and low dipole. | Antenna | |||
Phone line as SW antenna [04-Apr-00] | Shortwave | |||
Comet B-10 VHF Antenna Question | Antenna | |||
QST Article: An Easy to Build, Dual-Band Collinear Antenna | Antenna | |||
Complex line Z0: A numerical example | Antenna |