[Antennas] Coaxial Sleeve antenna ?!

[George, W5YR]

Just one small reminder: a resonant horizontal dipole has a driving-point
resistance which depends upon the height the antenna is mounted above
ground. It can range from 45 or so ohms up to nearly 100 ohms at low
elevations settling down to about 70 ohms as the height becomes large
compared to a wavelength. 

The classic diagram illustrating this is found in almost every edition of
the ARRL Antenna Book; in my 18th Edition, it is on page 3-1. The plots
show that a resonant horizontal half-wave dipole over real ground will have
a 50-ohm input resistance at about 0.13 wavelengths height. Over perfect
ground, the height would be about 0.17 wavelengths. With a vertical antenna
over perfect ground - which is the usual mounting for the Coaxial Sleeve
antenna - the input resistance starts off at 100 ohms with the center at
0.25 wavelength and decreases with increasing height until in the range
from 0.45to 0.55 wavelengths it is 70 ohms.

So, if the antenna is mounted horizontally, the 50 ohm coax feedline will
have about a 1:1 SWR if the mounting height is about 1/8-wavelength. At
about 1/3-wavelength, the SWR will approach 2:1. Mounted vertically, the
SWR will range from 2:1 at 1/4-wavelength height for the dipole center to
about 1.5:1 at heights of 0.45 wavelengths and above.

Ron has a good point when he suggests that using 75-ohm coax for this
antenna is a better idea than using 50 ohm coax if line SWR is likely to be
a problem. For vertical mounting, it would be preferred at center heights
above about 1/2 wavelength and for horizontal mounting for heights of 0.2,
0.5 and 0.7 wavelengths.

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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Ronald KA4INM Youvan wrote:
 
>    No, this is a dipole, feed in the center `through one element'.
> I have not been able to make one of these that operates on 2 meters
> as it should, a ferrite bead on the coax (or several) at 1/4" from
> the feed point might make the radiating end (high impedance) resonant.
>    As with all center fed dipoles it is a 75 Ohm antenna that needs
> to be fed with 75 ohm cable.