Article on Sleeve Dipole Mobile Antenna Using Plumbing
and Electrical Parts available at your local hardware store.

Copyright July 2000 by John Dewey (KA9CAR) AND July 2001
by the ARRL. Published in QST August 2001 pg 68. May not be
republished with out permission.  You MAY web link to this page
for non commercial purposes.

BEGIN ARTICLE

Home Brew Mobile Antenna for a car with no horizontal metal
surface.

THE PROBLEM
My car (Tracker) is a small SUV with a convertible top.  The only
place I could easily attach a standard mobile whip and have metal
all around it was on the front hood, a position that had several
disadvantages.  I considered a "cowl mount" antenna bracket but
that would put the antenna adjacent to passengers and require a
hole in the car.

While preparing for a public service event, N9REP showed me a
450 ground plane he had made for the base operation using
welding rod.  Inspiration struck and I made a similar ground plane
antenna on a 7 foot mast strapped to the bicycle rack at the back of
the car.  The 6 foot mast was necessary to keep the ground radials
above the heads of pedestrians but it also it gave me 15 mile
simplex distances with my HT.

While this antenna was good for acting as a chase car at bicycle
events, it had two disadvantages.  I could not open the tail gate of
the car or drive into the garage when this ungainly looking
contraption was attached to the rear of the car.

THE SOLUTION
I pondered my options and studied the various spare parts laying
around my garage.  The solution I arrived at is a Sleeve Dipole
attached to the spare tire mount on the rear of the car.

HISTORY
The sleeve dipole is simply a dipole antenna with the feed line
entering through one of the elements instead of at right angles to
the elements.  It has previously been described for use as an easy to
transport a portable antenna when made from a piece of Coax
cable.

An amateur base station version is the AEA ISOPOLE antenna. 
There is some times a description of the theory of the Isopole
antenna at

http://www.rimtai.com/tw/chtr/Products/ANTENNA/AEA/coupling.htm

There is some discussion of the theory in the ARRL Product
Review, available to ARRL members at:
http://www.arrl.org/members-only/prodrev/pdf/pr8004.pdf

FEATURES
The features for this sleeve dipole are:
All parts where obtained at ham fests or the local hardware
store.
Common hand tools were used for construction
Mounting is independent of the vehicle "ground plane".
Can be readily scaled for other frequencies.


DESIGN AND CONSTRUCTION

The ARRL antenna book has a chart that shows how much to
shorten an antenna based on the ratio of the diameter of the
element to a wavelength.  Considering that this is a whip of
approximately 1/8 inch diameter and a sleeve of approximately 2
inch diameter, I chose to use 96% as a compromise.  For a
frequency of 146 MHz  this yielded an element length of 19.4
inches.

For the sleeve, I used a piece of 2 inch copper drain line.  Solder a
cap on the line, and drill the proper size hole in the cap.  Measure
the sleeve from the point where the inside of the coax is connected
to the outside of the sleeve, across the end cap then down the
length of the pipe to 19.4 inches, and cut to length.  Depending on
the type of  "mount" you use to connect the coax to the whip and
the sleeve, the point to measure from may be near the center of the
cap, or could be an inch above that.  I used a Radio Shack 21-961
which makes the connection at the top surface of the copper pipe
cap.

A piece of 1/2 inch electrical conduit is approximately the same
outside diameter as the sleeve of a PL259 connector.  I used a
piece of conduit and a conduit coupler to hold the PL259.

A supporting bushing is needed to keep the sleeve centered on the
mast and electrically insulated from it.  In the PVC pipe section of
the hardware store I found a coupling for 1 1/4 inch pipe was a
good fit inside the 2 inch copper.  A plastic bushing to adapt the 1
1/4 to 1/2 inch will fit over the mast after a little work with a rat
tail file.

Place the bushing on the mast, run the coax up the center of the
mast and through the coupler then attach the PL259.   Attach the
259 to the antenna mount, then put it inside the emt coupler and
tighten.   Now slip the sleeve over the mount and assemble the
insulating washer and tighten down the nut.

Slide the plastic bushing into the bottom of the sleeve, and secure
with a small sheet metal screw.  Be sure you don't short the sleeve
to the mast with this screw.

Attach a whip to the mount.  Remember to measure and set the
19.4 inch length from the same point you measured the sleeve.

MATCHING
A dipole is usually a 72 ohm device and fed with 75 ohm coax. 
Most amateur transceivers are 50 ohm devices.  I identified 3 ways
of feeding the antenna.  1) Use 75 ohm line for a good match to the
antenna and allow the mismatch to occur at the radio.  2) Use 50
ohm feed line and cut it to a length that a smith chart shows should
be a reasonable match.  3) Use a random length 50 ohm feed line
and see how well it matches.  I chose option 3.  I used a friends vhf
analyzer [N9REP at Field day 2000) to check the match, and found
it to be 1.7 to 1 across the 2 meter band.  This is not high enough to
hurt the transmitter, and the feed line length is short enough that
"SWR losses" are not important.  You should use a method that
you like.

MOUNTING TO THE VEHICLE
On my "SUV" there is enough room to attach the mast to the spare
tire mount between the tire and the tail gate.  On some you may
simply use hose clamps to attach to the spare tire rack.  On plastic
body cars you will need to find a suitable way of mounting to the
frame or trailer hitch.  This antenna could also be attached to a
wood or metal balcony rail on your deck.  Whether mobile or fixed
you should be sure that physically and electrically it is mounted so
that it does not endanger people or risk contact with electric lines
or lightening.


RESULTS
Standing on the front porch with my HT I am marginal into a
repeater 12 miles away.  When hooked to this antenna with the
antenna held overhead, I got Q5 reports.  Reports from the car
using the mobile radio indicate that this antenna meets my needs
well.

Email  ka9car@arrl.net

END ARTICLE

Copyright July 2000 by John Dewey (KA9CAR) AND July 2001 by the ARRL.
Published in QST August 2001 pg 68. May not be republished
with out the authors permission.  You MAY web link to this page
for non commercial purposes.


See the Antenna Specialist Web site for a Commercial Sleeve Dipole.

130/174 MHz
3 dB Gain Coaxial Mobile Antenna
Providing Artificial Ground Plane
ASP-358
http://www.antenna.com/lm_cat/lmrpg10.html