Home |
Search |
Today's Posts |
#1
|
|||
|
|||
Effective length of braid dipole leg?
I want to make a flexible, portable, vertical 10M dipole from coax by
folding back roughy 1/4 wavelength of the sheath and taping it in place. I would not expect the sheath to perform like an equal length of solid or stranded wire because if the extra length of wire. Is there any theoretical way to compute a factor, or do I just keep snipping until I get lowest SWR at the desired freq? Ken KC2JDY |
#2
|
|||
|
|||
Effective length of braid dipole leg?
snipping is the easiest way.
"Chevy454" wrote in message ... I want to make a flexible, portable, vertical 10M dipole from coax by folding back roughy 1/4 wavelength of the sheath and taping it in place. I would not expect the sheath to perform like an equal length of solid or stranded wire because if the extra length of wire. Is there any theoretical way to compute a factor, or do I just keep snipping until I get lowest SWR at the desired freq? Ken KC2JDY |
#3
|
|||
|
|||
Effective length of braid dipole leg?
Ken,
You'll have to do some 'tuning'/trimming no matter what you make the thing out of, wire/braid. So, why not make that braid adjustable? Let it slide up/down just a bit (carry lots of tape). No two installations are ever the same so being able to 'adjust' things is reasonable. - 'Doc |
#4
|
|||
|
|||
Effective length of braid dipole leg?
Chevy454 wrote:
I want to make a flexible, portable, vertical 10M dipole from coax by folding back roughy 1/4 wavelength of the sheath and taping it in place. Be careful about the end of the coax sheath where the voltage can reach levels high enough to arc and/or give severe RF burns. I would not expect the sheath to perform like an equal length of solid or stranded wire because if the extra length of wire. Is there any theoretical way to compute a factor, or do I just keep snipping until I get lowest SWR at the desired freq? A web page gives 460/f for the overall length. I would use the standard 468/f and trim to resonance. -- 73, Cecil http://www.w5dxp.com "According to the general theory of relativity, space without ether is unthinkable." Albert Einstein |
#5
|
|||
|
|||
Effective length of braid dipole leg?
Chevy454 wrote:
I want to make a flexible, portable, vertical 10M dipole from coax by folding back roughy 1/4 wavelength of the sheath and taping it in place. I would not expect the sheath to perform like an equal length of solid or stranded wire because if the extra length of wire. Is there any theoretical way to compute a factor, or do I just keep snipping until I get lowest SWR at the desired freq? Ken KC2JDY I've found this method to be very ineffective in choking common mode feedline current -- an effective sleeve requires a high impedance for the outer coax structure, which this doesn't provide. So your dipole impedance and performance will depend on the length and orientation of the feedline and the ultimate path to ground for the common mode current. You'll probably need to trim it when connected to the rig you'll be using, grounded (or not) as it will be when used. Roy Lewallen, W7EL |
#6
|
|||
|
|||
Effective length of braid dipole leg?
On Fri, 03 Oct 2008 10:20:44 -0700, Roy Lewallen
wrote: I've found this method to be very ineffective in choking common mode feedline current -- an effective sleeve requires a high impedance for the outer coax structure, which this doesn't provide. So your dipole impedance and performance will depend on the length and orientation of the feedline and the ultimate path to ground for the common mode current. You'll probably need to trim it when connected to the rig you'll be using, grounded (or not) as it will be when used. That's unfortunate, because I am looking for a simple arrangement. What if I just hang a wire dipole vertically from the mast (= bendy Black Widow crappie pole) with a PL-259 at the feedpoint? How much would the coax feedline have to stand off from the dipole to keep from interfering with its operation? I can't pull it too tightly away or the pole will bend. Ken KC2JDY |
#7
|
|||
|
|||
Effective length of braid dipole leg?
On Oct 3, 3:35*pm, Chevy454 wrote:
On Fri, 03 Oct 2008 10:20:44 -0700, Roy Lewallen wrote: I've found this method to be very ineffective in choking common mode feedline current -- an effective sleeve requires a high impedance for the outer coax structure, which this doesn't provide. So your dipole impedance and performance will depend on the length and orientation of the feedline and the ultimate path to ground for the common mode current. You'll probably need to trim it when connected to the rig you'll be using, grounded (or not) as it will be when used. That's unfortunate, because I am looking for a simple arrangement. What if I just hang a wire dipole vertically from the mast (= bendy Black Widow crappie pole) with a PL-259 at the feedpoint? *How much would the coax feedline have to stand off from the dipole to keep from interfering with its operation? *I can't pull it too tightly away or the pole will bend. Ken KC2JDY The vinyl on the outside of coax has some fairly poor RF insulating qualities. A number of CB antennas were built like this and the end of the braid would often arc through the vinyl if illegal power was used. One of the manufacturers changed the way they were making them by embedding wires on the inside of the fiberglass tube that was used to house the antenna. The wires replaced the shield. Another design is to replace the folded back shield with a trumpet shaped tube. This is common today on WiFi antennas. I have one for 6M that has the coax running up through some EMT tubing. The coax is held centered inside the tubing with little blocks of styrofoam. A few years ago the PVC coupling I was using to fasten it to the mast cracked. I repaired it by wrapping with fiberglass and have had no problems since. Jimmie |
#8
|
|||
|
|||
Effective length of braid dipole leg?
Chevy454 wrote:
That's unfortunate, because I am looking for a simple arrangement. What if I just hang a wire dipole vertically from the mast (= bendy Black Widow crappie pole) with a PL-259 at the feedpoint? How much would the coax feedline have to stand off from the dipole to keep from interfering with its operation? I can't pull it too tightly away or the pole will bend. Ken KC2JDY A very long distance I'm afraid. You might be able to live with the common mode current. Otherwise, a larger diameter air-insulated sleeve or some other decent isolation method like a J-pole are about all I know of that would reduce it. Even those methods, though, can result in some common mode current depending on the feedline length due to mutual coupling between the feedline and intentionally radiating part of the antenna. If you really want to avoid common mode current on a high, elevated vertical, you need to first effectively choke the conducted current with a high Z sleeve or J-pole type decoupling stub. Then you need to take care of the coupled current. One effective way is to use two common mode chokes ("current baluns") spaced about a quarter wavelength apart along the feedline. The Isopole took care of both by using two flaring sleeves. There are a lot of antennas being used out there which have no special effort taken to suppress common mode current, and they seem to function well enough to satisfy most users. But it can be a problem in some cases. Roy Lewallen, W7EL |
#9
|
|||
|
|||
Effective length of braid dipole leg?
On Fri, 03 Oct 2008 14:47:12 -0700, Roy Lewallen
wrote: ...You might be able to live with the common mode current. Otherwise, a larger diameter air-insulated sleeve or some other decent isolation method like a J-pole are about all I know of that would reduce it. Even those methods, though, can result in some common mode current depending on the feedline length due to mutual coupling between the feedline and intentionally radiating part of the antenna. The large-diameter sleeve is impractical in my app because it make the antenna non-flexible for storage. Also expensive. My principal use will be QRP. I will stick some ferrite beads on the feedline and hope for the best. The most I would use non-QRP would be 25 watts SSB. Ken KC2JDY |
#10
|
|||
|
|||
Effective length of braid dipole leg?
"Chevy454" wrote in message ... On Fri, 03 Oct 2008 14:47:12 -0700, Roy Lewallen The large-diameter sleeve is impractical in my app because it make the antenna non-flexible for storage. Also expensive. My principal use will be QRP. I will stick some ferrite beads on the feedline and hope for the best. The most I would use non-QRP would be 25 watts SSB. Ken KC2JDY One design that I saw that you may try it to just use a 1/4 wavelength of wire at the end of the coax. Then about 1/4 of a wavelength down the coax you use a choke. It could be a coil of coax or it could be the beads at your power level. I have not tried it, just saw it in one of the ARRL books. |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
dipole length vs db | Antenna | |||
i want to know the length of a 6m dipole | Antenna | |||
extra length?? 2 dipole | Antenna | |||
Dipole Length Question | Antenna | |||
Unequal length dipole arms | Antenna |