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

Thu Feb 21 07:45:59 EST 2013

Not sure of the question exactly, but if we're talking about RF connector compatibility for ARES/RACES, I propose everyone get whatever adapters they need to mate to a male BNC connector like the one in the picture below.   Then if we can make sure all our antenna coax terminates in a male BNC, everything will be compatible.  Why BNC?   UHF connectors (PL-259) are not that great above 10 meters.  Type N connectors are much better, but bulky.   SMA connectors are good from an RF standpoint but very difficult to install on normal size coax cables. 

Best way to adapt to your HT is to use a short adapter coax "pigtail" with the proper connectors on both ends. That way the mechanical strain of the antenna coax and its connectors is removed somewhat from the HT body. An example of a SMA to BNC female is in the picture below. 

N3AE 

----- Original Message -----

From: "Dennis N3QHC" <n3qhc at verizon.net> 
To: "Calvert Amateur Radio Association - K3CAL" <k3cal at mailman.qth.net> 
Sent: Wednesday, February 20, 2013 10:36:59 PM 
Subject: Re: [K3CAL] Off Center Fed 2m/70 cm Sleeve Dipole 

I do not recall.  I seems to me it the opposite on these newer HT’s than was on the HT’s of the past.  Shawn saw the connector last night.  Perhaps he knows.  Dennis 

From: k3cal-bounces at mailman.qth.net [mailto:k3cal-bounces at mailman.qth.net] On Behalf Of Karl Long 
Sent: Wednesday, February 20, 2013 4:08 PM 
To: k3cal at mailman.qth.net 
Subject: Re: [K3CAL] Off Center Fed 2m/70 cm Sleeve Dipole 

I believe it was BNC.  I thought I heard "female", but a male BNC would fit all the HTs I have seen, at least the ones that use BNC. 

Dennis, can you clarify this? 

Karl 

KG1L 

To: k3cal at mailman.qth.net 
From: davehardy0101 at aol.com 
Date: Wed, 20 Feb 2013 14:13:13 -0500 
Subject: Re: [K3CAL] Off Center Fed 2m/70 cm Sleeve Dipole 

Thanks for the antenna plans.  Good meeting last night.  Ordered some powerpole connectors from Power Werx this morning to be compatible with others.  What was the antenna connector?  I can't remember the configuration... 

Also have a 3 mtr beam coming soon so I can hit the repeater from Ranch Club, maybe..... 

KB3RAN 

David Hardy 
davehardy0101 at aol.com 

-----Original Message----- 
From: Karl Long < karl_g_long at hotmail.com > 
To: k3cal < k3cal at mailman.qth.net > 
Sent: Wed, Feb 20, 2013 1:23 pm 
Subject: [K3CAL] Off Center Fed 2m/70 cm Sleeve Dipole 

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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