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

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

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

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

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

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

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

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

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

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.