Spiral Conical Antenna - the Form

SuperKuh and Andrew McNeil inspired me to build a larger size spiral conical antenna form to test three different designs I plan to develop on the form.

The three spiral conical designs I plan to make and test are:

1. Bottom fed single spiral conical
2. Bottom fed double spiral conical
3. Top fed double spiral conical

I started off using a street cone I had stored in my parents garage that was still around since where I last left off on this project in the early 90's... or was it late 80's?

Regardless of when I started and left off; at first I decided, I'll just go right ahead and make with the cone using some 14 gauge household wire I had left over from my last investment.

Then after thinking about some more to expand the frequency range to get down to ~400MHz and searching for more inspiration on the internet, I determined I needed a larger cone base diameter and decided to move forward with using delrin, foam or wood with more consideration of the forms dielectric properties.

After a trip beyond memory lane with some more modern approaches considered along with the extra inspiration as motivation... I headed down the street to the local Fab Lab to see what they had for materials and equipment.

Typically, there are limiting factors with any project where the size of the equipment that can be used to make the parts has to be considered. Thankfully, there was a large enough laser cutter at my disposal sizing in at 18" x 24".

Knowing my limits for material that can be placed on the laser cutter I then asked around the Lab regarding how the materials they have, or suggest can be used, cut.

I also asked what materials they recommend to use since laser cutter's manuals read like they're picky regarding cutting parameters and materials that can be cut depending on the type of system specifications.

Then I asked what materials the Lab had to offer, and found that they had nothing... other than the idea to use cardboard for prototyping my design.

Of course the Fab Lab also has the people resources to support the use of the equipment being that I've never used a laser cutter until this gig.

With a fresh idea for making a form cost effectively using cardboard, I sprung into action and headed over to the local home improvement box store and bought a box for $1.78 and some tax.

1. Cardboard Box (Make sure to use a box with side walls the size either in the attached Adobe Illustrator file or scaled accordingly)
2. Laser Cutter and/or Utility Knife (If you don't have access to a laser cutter large enough you can just use a utility knife or X-Acto Knife)
3. Titebond 2 Premium or other wood glue or even resin (I went the less toxic route to avoid resin)

Since I already designed the form, you can use the spreadsheet I used or make your own based on your own dimensions.

You can also start off with the Adobe Illustrator file I made and used since the Fab Lab had the application.

If you don't have Adobe Illustrator, Inkscape or another application that can be used, then I suggest just use the old fashioned draw a plan and transfer the plan to the cardboard material method.

Unfortunately, I didn't snap any photos of the laser cutting process since took a little trial and error with either the laser cutter connecting to the computer, the laser cutter settings to make the cut complete or the laser cutter settings to not set the cardboard on fire.

Once the laser cutter was fine tuned for the cardboard material I had... we wound up cutting four patterns for four forms with the first having a spar in three pieces that needed to be taped temporarily.

With the cut parts in hand, I began assembling and took a photo since I found out I needed to edit the Illustrator file since I didn't leave enough space for the top loading conductor to feed through the center for the top loaded spiral conical antenna design iteration. I then disassembled the form to trim the spars.

After using the utility knife to trim the spars tip end to make space, I determined I'd save time that way and not use the laser cutter, I re-assembled the form and voilà.

A word to the wise... don't use scissors since they can deform the cardboard and crush the corrugate if you didn't know.

The title sums it up. Glue the joints with a coat or four of the Titebond 2 Premium or whatever glue she's using. I read some people spray or paint a light layer of water before they apply glue. Be careful if you do this as can warp the cardboard, like I know and you can see.

The one spar that needed attention was glued one side at a time, carefully peeling the tape back that I made sure to cut ahead of time and not wrap around all four sides. I made sure to leave one end exposed to be able to peel back later.

With the two taped spars glued together I went ahead and painted the glue on the whole shebang.

After way more than four layers... here we stand.

Enjoy!