Home |
Search |
Today's Posts |
#19
![]() |
|||
|
|||
![]()
On Mar 31, 10:58*am, "Richard Fry" wrote:
A normal-mode helical with a radiating aperture of 5.25' is not a "270 degree" radiator on 26.5 MHz. *It is a ~ 51 degree radiator on that frequency. This is making no sense to me so I fear we have some sort of semantic problem. I'm now not sure what you mean by "a radiating aperture of 5.25 feet". "The IEEE Dictionary" says: "In some cases, the aperture may be considered to be a line." I was assuming that the 5.25 feet aperture was akin to a line of straight wire 5.25 feet long or a 5.25 foot long (end to end) helical monopole. If that is not the case, please enlighten me on your definition of "aperture". EZNEC says my 5.25' (end-to-end) physically tall helical monopole is electrically 270 degrees long. I assumed that 5.25' is the length of a straight wire or the physical end-to-end length of the helix itself (not the linear length of the wire). The velocity factor of a helix is a function of the helix geometry and *varies widely with diameter and turn spacing*. The helix I designed using EZNEC has a current maximum at the feedpoint, a current minimum 1/3 of the distance up the helix, a current maximum 2/3 of the distance up the helix, and a current minimum at the end of the helix. That's 270 electrical degrees any way you cut it because *there is always 90 electrical degrees between the current maximum and current minimum in a standing wave*. The requirement that a 5.25' tall helical monopole has to satisfy to be 270 electrical degrees long on 26.5 MHz is to have a velocity factor of 5.25/27.85 = 0.1885 which is a piece of cake. The 5.25' is the actual end-to-end height of the helical monopole and the 27.85' is 3/4 of a wavelength in free space at 26.5 MHz. Note that the velocity factor is the distance a traveling wave travels in the helical medium in unit time compared to the distance a traveling wave travels in free space in the same unit time. Richard, this is giving me a headache - what am I missing? -- 73, Cecil, w5dxp.com |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
Fractal Sleeve for Monopoles? | Antenna | |||
Magnetic monopoles | Antenna | |||
Helically-wound Monopoles | Antenna | |||
Vertical dipole, helically wound - comments? | Antenna | |||
End Effect on folded dipoles/monopoles? | Antenna |