[Antennas] Coaxial Sleeve antenna ?!

[George, W5YR]

Demetre,

The dipole antenna that is formed with the coax center conductor and
folded-back braid is just like any other dipole. Its feedpoint impedance
depends upon how it is mounted and how high, etc. You will *probably* end
up with an antenna which is pretty close to matching 50-ohm coax without
using a tuner, but this would not be a direct result of making the antenna
out of 50-ohm coax.

BTW, you should insulate the folded-back braid very well since it will be
at a high r-f potential near the end. Probably the most effective way to
insulate the antenna and prevent water from getting into it would be to
cover all of the exposed braid with heat-shrink tubing and to put some sort
of sealing compound around the area where the braid is folded back.

A choke or current balun in the feedline at the braid end of the antenna is
a good idea as well and if common-mode current on the feedline outer braid
is a problem even with the choke near the end of the antenna, a second one
1/4-wavelength closer to the transmitter end will help.

72/73/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121  Kachina #91900556  IC-765 #02437

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Demetre Koumanakos wrote:
> 
> Hi All,
> 
> I'm about to start construction of a sleeve antenna.
> Just a short description of the antenna: 1/2 wave length coaxial
> cable, at about the center of it the braid of the upper part is pull
> down over the lower half leaving the center conductor free from
> shielding.
> Now for the question... If 50 ohm coaxial is used, would this make
> the antenna 50 ohms and no extensive tuning will required (if
> frequency/lenght calculations are correct) ???