Why - Designing/Math - Building one - Stacking - Applications - The idea
 

Also see my first article on eHam.net.
 

(printable images)

Gain like a parabolic dish but omni-directional with the wide bandwidth of a discone antenna. A dipole or other driven element could also be used at the focus. Vertical polarization if vertically polarized at the focus. May be stacked for gain improvement, but bandwidth issues result. Overhead reflector can be raised higher, like a periscope antenna. Idea has been proven with sound and light for omnidirectional gain.

Why make a Parabolic Discone antenna?

The first advantage of the Parabolic Discone is wide bandwidth plus gain in an omni-directional antenna.  Traditional omni-directional antennas with gain can only operate over a narrow bandwidth effectively. Another advantage of the Parabolic Discone is that other omni-directional antennas with gain require tuning and phasing to work well, but not this antenna.

The gain vs. size of the Parabolic Discone grows like a traditional "Parabolic Dish."  The difference is that gain in the Parabolic Discone causes the RF field to flatten out into a "pancake" at higher levels of gain.   The gain increases with frequency like a dish antenna.  Doubling the size causes 3 db of gain, doubling the frequency results in 3 db of gain.

The length of the overhead cone is roughly equal to a fixed frequency omni of similar gain.  However, fixed frequency omni antennas have limits of useful gain and highest frequency that they can be made for.  The Parabolic reflector in conjunction with the overhead cone reflector are only limited by how big and precise you can build them!

You could alter the angles of the overhead cone reflector if your wanted to "steer" the signal up or down from horizontal.  This would be helpful if you wanted to use a Parabolic Discone as a repeater antenna and did not want gain that was purely horizontal.

You can also put the parabolic section on the ground and mount the cone reflector higher, like a periscope antenna.  The overhead cone reflector may need to be made bigger, depending on the beamwidth of the parabolic section.  The cone could be composed of many evenly spaced rods attached to a point because the RF is verticality polarized.
 

Designing a Parabolic Discone - the Math

Henry O'Tani (G8OTA) suggested that the gain should be the square root of a "normal" parabolic dish, because of the spreading from one dimension to two dimensions caused by the cone (half the dB gain value).

Instead of making a single Parabolic Discone bigger, it may be possible to use stacked Parabolic Discones. The problem is that gain and pattern distortion can result.  This is because you have some of the same considerations that influence dipole based collinear antennas.

Calculated omnidirectional gain over isotropic for an ideal Parabolic Discone:
(note: These figures still need to be verified as of 4-4-06.  One db of illumination loss has already been subtracted.)
 

	18 inch	3 foot	6 foot	12 foot	24 foot
144 MHz	.	.	.	6	9
220 MHz	.	.	5	8	11
440 MHz	.	5	8	11	14
900 MHz	5	8	11	14	17
1200 MHz	6	9	12	15	18
2.4 GHz	9	12	15	18	21
5.0 GHz	12	15	18	21	24
10 GHz	15	18	21	24	27
24 GHz	19	22	25	28	31
47 GHz	22	25	28	31	34
149 GHz	27	30	33	36	39
250 GHz	29	32	35	38	41

(Let me know if you need over 250GHz.)

I am asking for help to develop the equations for this antenna.  Unfortunately medical problems from being paralyzed, including fatigue, have prevented me from figuring it out for myself.  I will also provide links.

One thing to note is that the some of the RF from the driven element does not get reflected by the parabolic section, it escapes at higher angles.  A compromise is necessary for how high you make the parabolic section vs. the end results.

Too much gain may cause a problem in some applications.  As the field narrows with increasing gain, the antennas at the other end might be to high or low for the best gain.  This is true for any omni directional antenna with gain.
 

Building a 24 Inch Stressed Parabolic Discone

The pictured two foot wide parabolic discone was constructed out of 16 pieces of steel rod covered with aluminum window screening for the parabola.   Galvanized sheet steel was used for the overhead cone reflector.  The discone element at the focus is made out of wire, cut for a 1200 Mhz low frequency.
 

The parabolic section was built on the end of a pipe fitted with a piece of hardwood dowel inside.  Using a drill bit slightly larger than the rods, 16 evenly spaced holes were drilled around the pipe one inch from the top.  Pieces of rod 18 inches long are then stuck into the holes.  The rod is 0.078 ‘'music wire'' from K&S Engineering, # 505 (bought at a toy/hobby store).

A central "mast" made from a fiberglass rod is stuck slightly off center into the wood fitted in the pipe.  A short piece of PVC pipe is attached on the fiberglass rod using nylon wire ties, by putting the rod up though the ties and pipe.  Fishing line is used to pull each of the steel rods into a parabolic shape by tightening and attaching the line to the piece of PVC pipe.  After tightening all the rods a line was run along the outside edge.  "Plastic Dip" was used at all the attachment points to keep them from slipping.

Aluminum window screening is attached by tightly weaving a thin copper wire between the screening and the rods.  The necessary precision of the resulting parabolic section depends on the gain and highest frequency desired.
 

The overhead cone is cut from sheet metal by making a circle with an 18 inch  radius as long as the intended distance from tip to edge.  A notch is cut from the edge to the center then the metal pulled into a cone.  It is best to practice on a scale model cut from a piece of paper first.

Update on May 11, 2006:
The driven element for the 24 inch Parabolic Discone was constructed out of wire using VE3SQB's discone design program for 1200 Mhz operation.  Download the program from his website:
http://www.ve3sqb.com/

Remember to design the discone for the lowest frequency of desired operation.  Also note that good transmitting SWR usually exists up to 3x the lowest frequency, though reception should be good up to 10x the lowest frequency.

The entire Parabolic Discone has a lowest frequency that it will work at, so look at the table in "Designing a Parabolic Discone" for an idea.  Gains of less than 6 db may require changing the height of the focus and the height of the parabolic section.  The 24 inch Parabolic Discone in theory has a gain of appx. 7db at 1200 MHz.  I verified over 6db of gain at 2.4 and 5.8 GHz based on transmission distance.

(Sorry for forgetting the obvious step of making the discone part.  Without a driven element the other parts will not do much!)
 

Equation to verify the parabolic shape:
y= x² / ( 4 * focus distance)  ==>  y= x² / ( 4 * 3)  ==>  y= x² / ( 12 )

Graph for confirming the curve of the parabola:
(3 inch high focus point and 12 inch radius)
 

x y 1 0.083 2 0.333 3 0.75 4 1.333 5 2.083 6 3 7 4.083 8 5.333 9 6.75 10 8.333 11 10.083 12 12

 

Stacking Parabolic Discones
 

Collinear stacking may be an option instead of building larger parabolic discones.  In theory you could achieve 3 db of gain by stacking two identical parabolic discones.    I believe this should provide better results than stacking regular discones because the radiated waves are already "straighter" over a wider bandwidth. I have experimented with a set of one foot parabolic discones at 5.8 GHz and observed appx. 1 db of gain.  My limited results are likely due to loss and impedance issues in my test setup.  I simply split the RF from one 50 ohm coax into two equal length 50 ohm coax using a BNC "T" splitter. More experimentation with stacked parabolic discones may be worth the effort because of the space savings.  A low loss way to drive the stacked antennas may make stacking useful.

Applications:

• Wireless Networks- see Wlan.org.uk for license exempt Wireless Networks.
• Multi-band Amateur Radio Repeaters using one antenna
• "Police Scanner" antenna with gain.
• Military applications, like as part of a phased direction finding antenna with gain.
• Anywhere a high gain with wide bandwidth omni-directional antenna could be useful

The Idea for the Parabolic Discone

I like the Discone Antenna for it's wide bandwidth, but gain seemed impossible.  Arrays of discones would not have useful results because the phase and necessary distances will change with frequency.  Only by using the discone with a parabolic reflector was it possible to achieve predictable and useful results.

The idea for the Parabolic Discone antenna came to me because "dish antennas" are best understood using the "ray" principle, like light.  So, anything you can do with light you can do with radio waves (in theory).  Looking at antenna design like it was optics can inspire new ideas.  The downside is that antennas must be large in terms of wavelength for ray principles to apply.
 
 
 

Let me know if you build a Parabolic Discone, and the results.  I will add links on request for anyone who builds one of these antennas.

e-mail:  admin@H-a-m-Domain.com (without the dashes).

(I added the dashes to reduce spam.  Let them spam a non-existant website!)
 

See my other efforts:
1. Liquid Metal Antenna and Elastic Stretchable Liquid Metal wire
2. About me on  hamdomain.com

For a website with links to other antenna projects, I recommend:
AC6V.com

PS:  Yes, the para-discone appears to be an original concept by myself.  I am paralyzed and did not have the money, time, and energy to get a patent. Fatigue and other medical problems limit my free time to what I am desperate to get done.  (It is hard to focus when your bladder feels like it is turning inside out.)  Give me the respect of being the "inventor" and that may be better than nothing.  I might keep giving away patentable ideas for free.
 
 

* I am being called a terrorist, please help!: It is hard to talk about what happened because it started with over six months of harassment while trying to keep my job in Bowling Green, Ohio.  Beginning in 1998 a coworker convinced me to join the Jaycees civic group he was in.  It  ended with harassment that I was a criminal and a terrorist. Why?!!!  I upset VIP's from Bowling Green who were in the group by complaining about their conduct.  So they exploited my weakness diagnosed in the army.  I had bipolar disorder, a mental illness.  This gave the VIP's the ability to attack me as a nutcase "waiting to go postal" and kill them all.  Their revenge was very detailed! The prosecutor of Bowling Green Ohio was a member of the Jaycees looking for status, so he assisted the group in what appeared to be criminal harassment against me.  When I tried to make criminal complaints no one took me seriously because the group had planned ahead.  See the website below for the letter the prosecutor wrote me saying that he was not interested. I am lucky that out of fear and anger about the harassment I had written many letters which were documented as evidence to show how crazy I was becoming.  Those officially recorded letters show my mental distress, and the group makes it clear that it was not concerned if it was causing me more psychological harm.  That demonstrates criminal intent! See my WhyHope.com website for details of the harassment by the Jaycees. I had kept it secret from my ham radio activities, but now I have put in a criminal complaint to stop the conspiracy and menacing by stalking that the prosecutor and group committed.  This is certain to upset VIP's, so you might never see me again.  The nightmares from the harassment keep me from enjoying life, so I must take the risk.