Ham Radio 2000/Ham Radio 2000.iso/ham2000/antenna/discone1/discone1.txt

Text File | 1993-12-30 | 11KB | 203 lines

DISCONE1.TXT
by Frank Kamp
K5DKZ
DEC 1993

                    A DISCONE ANTENNA PROJECT


     If you are not familiar with this type of antenna, don't feel
bad.  It is not one of your more popular HF or even VHF radiators.
It is widely used as a base antenna for emergency services (police,
fire, ambulance).  It's primary claim to fame is it's very
broadband performance.  A discone designed to cover 144 mhz can
work equally well up through 1296 mhz.  Feedpoint impedance is 50
ohms unbalanced.  Ideal for 50 ohm coax or hardline.  It is
vertically polarized, omnidirectional, and can best be compared to
an extremely broadband ground plane antenna.  Not much good for UHF
DXing, but it makes a dynamite scanner antenna.

     As the name implies, this antenna includes a disc which is
horizontally oriented.  The disc is driven at it`s center by the
center conductor of the coax feedline.  Immediately below the disc
is the cone which is connected to the coax shield.  The disc is
centered over the apex of the cone.  Disc diameter is 0.7 times the
free space quarter wavelength at the lowest frequency of interest.
The slant height of the cone is equal to a full free space quarter
wavelength at the lowest frequency of interest.

     If you take a vertical cross-section through the center of the
cone, the bisecting plane will form an isosceles triangle with the
cone.  All sides of the triangle are equal.  Each included angle is
60 degrees.  The vertically running feedline exits at the apex of
the cone and describes a 30 degree angle to the cone's surface.
Okay, enough geometry.  What we have here is a frisbee balanced on
top of a dunce cap.

     The mechanical difficulties involved in constructing this
thing for HF become obvious when we consider a cone slant height of
about 32 feet for 40 meters.  AT 50 mhz this dimension drops to
five feet.  At 2 meters we can easily get by with 2 feet.  We can
also simplify the construction by simulating the disc and cone with
a skeletal wire frame.

     The discone I built was constructed of #12 copper clad steel
wire salvaged from an old antenna project.  Slant height of the
cone was chosen at 2.5 feet and the final design included eight
wires to simulate the cone; eight wires to simulate the disc.  Disc
diameter was 21 inches.  Since this particular discone had a low
frequency cutoff of around 100 mhz, none of these dimensions proved
critical.  Good results were obtained on 2 meters with variations
as much as +- 1.0 inch.  With this antenna sitting on it's radials
(oops, I mean skeletal cone) on the shack floor, I got an SWR of
1.1 to 1 all across the 2 meter band.  That was with eight wires
each for the disc and cone.  With six wires each, the SWR was 1.2
to 1.  Four wires gave an SWR of 1.4 to 1.  Two wires resulted in
an SWR of 2.5 to 1.

     This test was only done on 2 meters.  I suspect that the
higher frequency performance of this antenna would be much more
adversely affected by the lower wire counts.  I settled on eight
wires for that reason.  Besides, that use up all the wire I had.

     There is nothing sacred about using wire.  Tubing; copper,
brass or aluminum would serve as well.  Since the entire structure
is supported at the apex of the cone, all the weight is supported
in compression by the mast with no unbalanced cantilevered loads.
This makes the weight problem become less of a factor.  Obviously,
the lighter the better as long as the skeletal structures still
simulate disc and cone.  I would not recommend soft drawn copper
wire as a material unless the cone slant height was less than 1.5
feet.  That would put the cutoff frequency at about 160 mhz.  Too
low for 2 meters but good for 220, 440, and up.  At UHF this thing
might even make a decent mobile antenna.

     My final construction progressed as follows:

A 2 X 2 inch copper plate, 0.250 inch thick, was drilled for
mounting a standard SO style coax connector in it's exact center.
Eight holes, to pass the #12 wire were drilled, equally spaced, on
a 1.75 inch bearing circle around the coax mounting.  A one inch
long section of copper pipe (approx 1.50 inch inside diameter) was
soldered to the copper plate so that it was concentric with the
coax connector mounting.  You will want to remove the coax
connector before soldering the pipe to the plate.  Use some kind of
fixturing to hold the pipe in alignment with the plate.  After the
pipe is bonded to the plate and before the assembly has had a
chance to cool off completely, mount the coax connector by
soldering it to the plate.  Use of the higher quality SO connectors
that use high temperature dielectric (like teflon) is highly
recommended.  (But then, we wouldn't use anything less in a VHF/UHF
application, would we?)   Tinning all parts before assembly also
helps.  The 2.50 foot wires were then soldered into the eight holes
in the plate and to the copper pipe resulting in a very rugged,
electrically sound assembly.  Unless you have access to a very
large soldering iron, you will need to use a torch to solder this
assembly together.  Take care when soldering the wires not to
reflow the pipe to plate joint.

     Two 3 X 3 inch scraps of thick printed circuit board material
were etched to remove the copper and drilled to put a half inch
hole in their centers.  One of these insulating spacers was
attached to the copper plate with 4-40 machine screws.  The copper
plate is easy to work and sufficiently thick to drill and tap for
the screws.  Use flat head screws and recess them into the
insulating plate so that the second insulating spacer can be
epoxied flush to the top of the first.  Rough up the interface
between the two insulators with sandpaper before gluing.  Allow the
epoxy to cure overnight before continuing.  I used 100 mil thick G-
10 glass epoxy circuit board because that was what was available.
A quarter inch thick section of Delrin or Teflon would do even
better and not require epoxy.  The objective here is to
electrically insulate the cone from the disc.

     Solder a two inch (or so) length of solid copper wire into the
center connection of the SO connector.  Drill a hole in the exact
center of a 3 X 3 inch copper or brass plate to pass the unsoldered
end of the solid copper wire.  Brass shim stock 0.060 inches thick
makes an adequate plate here.  The plate is bolted to the top of
the insulator with 4-40 hardware (stainless if possible).  Locate
the holes carefully to avoid shorting the thin plate to the thicker
one.  Solder the wire at the center of the thin plate and solder
your disc skeletal wires to it as well.

     Using some sort of template or gauge, bend the skeletal cone
wires out so that they form a cone with a base diameter of 2.5
feet.  You now have a discone antenna that will work well on 144,
220, and 440 mhz.   If you contemplate using it on frequencies
higher than that, you will want to add a circular hoop of wire to
the base to stabilize it.

     I found that adding the stabilization hoop was as challenging
as building the antenna.  First, we need to get 94 inches of
copper-clad steel wire into as perfect a 2.5 foot diameter circle
as possible.  That is the easy part.  The hard part is getting it
all to lie in one plane.  The wire was joined by overlapping it by
a quarter of an inch, wrapping it with buss wire, and soldering.
Similar buss wire wraps were used to connect the hoop to the base
of the cone.  Almost any size buss wire will do.  Leads cut from
half watt resistors are ideal.

     Don't rely on sight alone before doing the final soldering.
I found that I could be off as much as two inches before the thing
didn't look right.  Measure both the length of the radials as well
as their spacing on the hoop.  A cloth tape measure of the kind
found in most sewing boxes makes the measuring task much easier.
I found that attaching opposing wires first made the job easier
too.

     Once all the wires are attached and soldered, re-measure again
to make sure it is right.  You may have to adjust the bends in the
radials to ensure that the feedline drop is as vertical as possible
and perpendicular with the horizontal at all angles to the cone.
One caution.  Don't try to bend the ends of the radial wires around
the hoop.  Soldered buss wire connections will prove more than
adequate and prevent a lot of frustration.  If you really want the
maximum performance from this design, you may want to consider
adding a second hoop halfway up the cone for added stabilization.
I have not used my discone above 900 mhz and can't say if the added
stabilization is really needed.  The finished product looks like an
unsuccessful attempt to build a tomato tree, but it is fairly
stable and best of all, it works.

               This is certainly not the only way to build a discone antenna.
I used wire because it was cheap and easy to work.  I prefer a
soldered assembly for it's ruggedness and electrical integrity. The
wire stabilizing hoop could be eliminated if tubing were used or a
higher cutoff frequency were desired.  I also like to use
connectors at both ends of the feedline to facilitate antenna
maintenance without having to run extension cords for soldering in
difficult places.

     Installing this antenna is particularly easy.  A length of RG
52 coax with PL-259 connector is inserted up the center of the
mast, holding my beams.  Connection is made to the SO connector at
the base of the discone and the discone is dropped into the  center
of the mast.  A couple of self tapping screws into the copper pipe
hold the antenna to the mast.  I just realized that may not be so
easy for folks who do not have tilt over towers.  Oh well, you get
the idea.  The discone should be mounted as high on the tower as
you can get it.  Don't forget to provide a rotator loop for the
discone feedline should your mounting require it.

     As a final thought, I wonder what the surge impedance of #12
wire inside a 1.5 inch copper pipe is?  Wouldn't it be neat to use
sections of copper pipe for the mast and have it act as a homebrew
hardline as well as support mast?  Why, if we chose the height of
our mast judiciously we might even be able to use it as a vertical
on the HF bands.  Wonder how good a top hat the discone would make.
Wonder if we could use such a system simultaneously on HF and 220?
Anyone for a 14/220 repeater?

     Serious prospective builders may have noticed the lack of
sketches and diagrams in this short article.  That is because ASCII
graphics is a conflict in terms.  If you do need a sketch, or if
you have any comments, you may write me at 907 Dumont, Richardson,
TX 75080.  I will send you a sketch if you enclose an S.A.S.E.


                               END