[K3CAL] Off Center Fed 2m/70 cm Sleeve Dipole

[Karl Long]

For those at the ARES meeting last night (and others) who are interested in the antenna I plan to build, here are some links. 
 
The antenna is a vertical, center fed (well, off center) 2m/70cm dipole.  It is made with 1/2" CPVC pipe with the feed line coax running up through the pipe to the feed point.  The dipole elements are made by wrapping aluminum duct tape (not the cloth tape) around the CPVP pipe. The upper element is longer than the lower element, due to the interaction of the lower element with the coax inside. The element size was determined by experimentation. 
 
It was designed by Harold Melton, KV5R and can be found at http://www.athensarc.org/sleevedipole.asp.
 
Geoff Haines, N1GY has made a couple of antennas using aluminum pipe to the same dimensions. The first one is at http://mysite.verizon.net/cpthaines/id14.html and the other is at http://mysite.verizon.net/cpthaines/id17.html.
 
Warning: Technobabble beyond this point.
 
If you do a Google search on "OCF sleeve dipole," you will find several different designs.  I have not been too happy with what I have seen.  Most of them say you have to trim for best SWR (in other words, the antenna is not the right size to begin with).
 
At least one web site gives a formula for making OCF antennas for any frequency, giving (in my opinion) incorrect calculations for the length of the bottom element, using the velosity factor of the coax (based on dialectric between the shield and the center conductor).
 
The actual interaction is between the coax shield and the "sleeve" conductor of the bottom element.  The actually VF depends on the dialectric coeficient of the outside wrapper of the coax, the CPVC pipe and the air inside.  In practice, this is not easy to calculate, though it is almost certainly not the VF of the coax.
 
And finally, a sleeve is not an effective balun unless it is is much larger than the feedline with a lot of air inside.  This is not the case with this antenna, so a choke "balun" of 2 - 4 turns (depending on who you believe) of the feedline coax is necessary to help prevent common mode current in the feedline (feedline radiation).
 
</soapbox>
 
Karl
KG1L 		 	   		  
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