Sleeve Dipole

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David Mayes:
Has anyone had any experience with Sleeve (or coaxial) Dipoles for HF.

I'm intending on installing a dipole for 30meters.
I've seen a couple of designs for a sleeve type dipole where the antenna is fed from the end. That is, the coax passes through one side of the antenna and is connected to the center feed point (as per normal dipole).

I thought this might be better than having the coax hanging from the middle.

First design (I saw) :-
- One half of the dipole is the outer shield of RG213 (the sleeve) , through which is passed RG58 feedline.
- The other half is HDC wire.
- The feedline braid is connected to the sleeve and the center conductor to the other side. (HDC)
- An RF chock is placed at the end of the sleeve.

Second design:-
- Does not use the sleeve and I assume the TEM coaxial transmission line propagation property. (See VK1ODs comment below)
- The outside of the outer conductor (braid) forms the other half of the antenna.
- An RF chock is placed at the end of the 1/4 section (the feeline side).

The 'W2DU type balun' would very convenient here. Both the above designs used the large coaxial air coil type.

Some other considerations are the feed point connection strength, weight and securing the feedline end of the antenna so at not to damage the coax.

David Mayes.



To quote VK1OD:

"Consider the detail of the end of the coax where it connects to the base of the vertical. The current on the outside of the inner conductor is accompanied by an equal but opposite current on the inside of the outer conductor... this is a property of TEM propagation on a coaxial transmission line. (Note that at HF skin effect isolates the inside surface of the outer conductor from the outside surface of the outer conductor.)

    But where does this current on the inside of the outer conductor flow?

    It flows around the end of the shield and onto the outside of the outer conductor. So, if there is a current I flowing into the base of the vertical, there is an equal current on the outside of the coax towards the end of the shield. The shield is as much a current carrying and radiating conductor as the vertical."

:
   I've homebrewed quite a few coaxial dipoles for 20 meters through 440.  They are all easy and inexpensive to make, are simple to raise to fairly good heights, are resonant without a tuner, and work quite well!  Just constuct a simple homebrew balun by wrapping the coax around a plastic soda bottle, and you're good to go!

Dale Hunt:
I've used a coaxial dipole on 20m at least, and may have built a couple
of others.  I've made several for 2m.

First, to make sure we are using the right terms.  A coaxial dipole
generally has the coax running down inside the bottom half of the
antenna (and very often the support mast as well.)  A "bazooka" dipole
is made from coax cable (configured as a pair of resonant stubs
across the feedpoint) and is fed in the center like a normal dipole.  A
"sleeve" dipole usually has some sort of sleeve over the center portion
of the antenna:  imagine a dipole made from 1/4" think material with
a copper plumbing "T" around the center, but not necessarily connected
to the antenna.  The arms of the T may be adjusted to broaden the
operating bandwidth of the antenna, but the feedline still comes away
at a right angle (inside the base of the "T".)

While is is possible to make a coaxial dipole with the shield from a
larger size of coax (I've used RG-58 braid around RG-174 coax, for
example) you can also build it with a single piece of coax.  For 2m I
measure back about 24 to 28" rather than the expected 18", then
bunch up the braid and turn it inside out over the standing portion
of the coax.  Bunching up the braid makes it larger diamete, but
shorter at the same time.  While this is fairly easy to do for 2m, for
HF is is much easier to use a separate piece of braid.  (Attach the
coax to the inner conductor of the coax you are stripping, so when you
pull out the inside it pulls the new feedline in to replace it.)

Coaxial dipoles aren't perfect:  there is a high voltage between the bottom
end of the antenna and the feeline, and in this version the only insulation
is the PVC outer sheath of the coax, which isn't designed for good RF
properties.   Increasing the spacing between the inside of the lower
radiator and the outside of the feedline will improve the efficiency.
You may have current flowing on the outside of the coax
as well - a choke balun may come in handy if this is a problem.  (It may
make the antenna hard to tune.)

The version that uses an RF choke instead of a separate bottom part
around the coax doesn't work as well:  the RF voltage across the choke
is very high, so even a very good choke doesn't provide good isolation.
Some folks have tried a parallel-tuned circuit instead of just a wound
choke.  Even with that you may need a second choke a quarter wave
further down the feedline to be more effective.

They are simple to make if all you have on hand is coax (or if you have
a large supply of coax you are looking for uses for.)  Personally I'd
probably tend to use an end-fed half wave wire and/or a J-pole because
the antenna is lighter and I usually have more wire on hand than coax
that I am willing to cut up.  But I am comfortable matching the high
impedance of such an antenna.  Certainly knowing how to strip back
a piece of coax to make a coaxial antenna is a useful skill, especially
for improvising antennas in an emergency.

Good luck!

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