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

[Dennis N3QHC]

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 <mailto:k3cal at mailman.qth.net> 
From: davehardy0101 at aol.com <mailto: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 <mailto:davehardy0101 at aol.com> 

-----Original Message-----
From: Karl Long <karl_g_long at hotmail.com <mailto:karl_g_long at hotmail.com> >
To: k3cal <k3cal at mailman.qth.net <mailto: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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