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- Last Updated: 16 March 2014
- Created: 16 March 2014
Once settling on 4NEC2 as the modeling software, I started fiddling with the new antenna itself. The design elements of a 160m Half sloper are very simple. 
The reduced half sloper reduced half sloper adds a loading coil and shortens the wire accordingly. The reduced length is of course to support reduced space availability.
Some of the things I wanted to find out: Was the 1/8 wave length of the wire portion something magical, or could any length be used as long as the loading coil was correct? What effect does the height of connection to the tower have on the tuning, and efficiency? What difference would placement of the loading coil make?
I started with my OCF Dipole model, and removed the source generator from the feed point, so it becomes a passive element, like the others that I included in the model (tower, satellite antennas, and rain gutter). I place the end of the sloper a good ten feet from the end of the OCF, since I really didn't want a huge amount of coupling between the new radiator and the OCF. Sometimes I'll use both antennas, one on receive on another radio, so some degree of isolation was desired.
Two problems immediately came up. The first, was that the model of the tower with actual diameters (2 feet at the base, tapering to the top), produced strange results. The NEC2 modeling engine has various limitations, one of them is modeling both large diameters or patches right next to or connected to small diameters (like wires). In fact it will give warnings using some built in sanity checking routines. I solved this by modeling the tower as a thick wire (about 1 inch diameter). The second problem is the loading coil model has two options: Parallel RLC and Series RLC (of course R=0 and C=0) in this case. There is no choice for just a coil. My first models were using the Parallel RLC, not really knowing which would be appropriate. I struggled to get this to give acceptable values, but I'm not really sure why. Eventually I changed to Series RLC, and thats when things started coming together.
For this model I ended up with 73 feet of wire, 36 feet up the tower, with a loading coil of 20uH. I experimented with moving the coil to the center of the wire, but that required a loading coil of 45uH.
4NEC2 has a handy optimization feature where you set a symbol for any model parameter (inductance of the coil, xyz position of a wire end, virutally anything). I created 2 symbols, one for the inductance of the coil, and another for the height of the attachment point on the tower. The optimization runs iterates through values above and below the starting value, running the model each time, doing something like a binary search to find the best values. This optimization function found the right coil value, and height to produce the desired low SWR point.
It appears that the characteristic impedance of this design is 70ohms, resulting in a 1.5 SWR. A little bit of matching at the TX end will fix this up.
I'm sure I could change the angles to change the impedance, but the angles are pretty much determined by the my lot size and support points.
So here we have the design that I have so far, in a nice 3D image. I still wonder what the easiest way to insert the house and the fences and such, for a better visualization.. maybe someday.
Its the middle of March, the snow is melting... The roof is already clear of snow.... I can't wait to get started on the physical part of this project.