Getting on 12/17M again:
After replacing a Hex Beam (20/17/15/24/28/6M) with a Cushcraft A3S tri-bander, I began missing access to the 12M and 17M bands. So, some digging in the junk box and a bit of 3D printing yielded a pretty good result that I thought others might be able to use. The basic structure of the antenna is as follows:
The Design Process:
The trap was designed using the VE6YP Coaxial Trap Design tool. (https://www.qsl.net/ve6yp/coaxtrap.zip)
By setting the ID of the coaxial turns to 1.5 inches (the OD of the form), the tool arrived at a nearly exact 4-turn solution, making the coil form design straight forward. Using chatGPT, and feeding it some parameters like wall thickness, type of coax and the fact I was using FreeCAD, it gleefully generated a python script that I could import directly into FreeCAD, generating this 3D result. (Pretty wild!)
To save you some time, I have posted the STL file needed to recreate this on GitHub. I used PETG for this part because it does well in the weather and is really tough. You can see the winding and connecting method for applying the coax to the form at https://www.qsl.net/ve6yp/CoaxTrap.html.
Construction:
The antenna itself was constructed with DX10 wire with the dimensions as shown above. The 49:1 UNUN in my installation is about 30 feet off the ground. For line-isolation, which is necessary to create a good counterpoise, I used an MFJ-915 RF Isolator, (I had one of these lying around.) You can get the same result by wrapping the coax, 6 feet from the UNUN, through an FT240-31 toroid 8-9 times (as shown below):
Results:
So my final installation looks like this (maybe a little tough to see here. I also have a trapped 80/60/40/6M dipole in the view) The antenna rating us a solid 100W (limited by the coaxial traps), and the match on both bands was better than 1.2:1 SWR.
Hope this post finds you doing well and helps you to get on these bands! Lots of DX on FT8 and CW, and not bad for SSB either.
73 de WR9R