Home |
Search |
Today's Posts |
|
#1
![]() |
|||
|
|||
![]()
Think of the coaxial line formed by the pulled-back braid (outer
conductor), the vinyl jacket (the dielectric), and the braid of the feedline going up (the inner conductor). It's shorted at the top end, and you want it to be 1/4 wave long so it reflects an open circuit at the bottom where the feedline emerges. But that's 1/4 wave considering the velocity factor of that line. And the vinyl jacket has a fairly high dielectric constant, so the VF in that section may be about 0.5. If you cut it for 1/4 wave in freespace, that makes it close to 1/2 wave considering the VF. That's really bad: it reflects a short instead of an open, and there's no decoupling between the antenna and the feedline. If you want to do it right, use a tube for that sleeve whose ID is a few times the diameter of the coax feedline, and it would be ideal if you remove the jacket and just have a very few thin dielectric "washers" to keep the feedline centered in the sleeve tube. Then it's tough to mount! So another way is to make a rigid feedline (like from copper pipe), maybe 40 inches long, with a rigid sleeve (which of course is the lower half of the dipole, too) over it and then you can mount it by the rigid section of feedline. At about this point, isn't it easier and more reliable (potential serious safety issue here for a boat, eh?) to just buy a commercial antenna? Cheers, Tom (VE8AE Andrew) wrote in message om... I want to build a nice small antenna for my boat. I had a J pole in a PVC tube, but I thought about a vertical dipole of the sleeve type. Can you make such a beast with the coax and just remove outer insulation and then pull braid back over the coax until they are both the same length? I want it for 156.8 MHz so it should be around 1.5 ft per side. But I have seen an article that says the top portion should be smaller than the lower braid portion to balance things out electrically. Can anyone shed some light on this for me? I have tried it and my analyzer doesn't show it resonant anywhere near that freq. Hmmmm. Help please. Any other good marine antennas that are easy to build would be great if you have plans or ideas. Thanks folks. Andrew VE8AE |
#2
![]() |
|||
|
|||
![]() "Tom Bruhns" wrote in message m... Think of the coaxial line formed by the pulled-back braid (outer conductor), the vinyl jacket (the dielectric), and the braid of the feedline going up (the inner conductor). It's shorted at the top end, and you want it to be 1/4 wave long so it reflects an open circuit at the bottom where the feedline emerges. But that's 1/4 wave considering the velocity factor of that line. And the vinyl jacket has a fairly high dielectric constant, so the VF in that section may be about 0.5. If you cut it for 1/4 wave in freespace, that makes it close to 1/2 wave considering the VF. That's really bad: it reflects a short instead of an open, and there's no decoupling between the antenna and the feedline. If you want to do it right, use a tube for that sleeve whose ID is a few times the diameter of the coax feedline, and it would be ideal if you remove the jacket and just have a very few thin dielectric "washers" to keep the feedline centered in the sleeve tube. Then it's tough to mount! So another way is to make a rigid feedline (like from copper pipe), maybe 40 inches long, with a rigid sleeve (which of course is the lower half of the dipole, too) over it and then you can mount it by the rigid section of feedline. At about this point, isn't it easier and more reliable (potential serious safety issue here for a boat, eh?) to just buy a commercial antenna? Cheers, Tom Tom, I never tried this, but always wondered about using a 1/4 wave section of triaxial transmission line, Should be able to get the length right with an MFJ 259/269. Another idea would be a piece of 1/2 inch ID copper braid. Tam/WB2TT |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
Inverted ground plane antenna: compared with normal GP and low dipole. | Antenna | |||
40m 1/2 wave coaxial sleeve antenna | Antenna | |||
Mobile Ant L match ? | Antenna | |||
Poor quality low + High TV channels? How much dB in Preamp? | Antenna |