Topband: Air Wound Coil

[Dan Maguire]

Grant wrote:
>>> Or the inverse as I did, cut my T for the low end of the band. Then three series capacitors with PCB relays to short each individually (none, 1, 2, or 3) yielded nearly full band coverage <1.5:1 swr.  My T is 85' to top and a 50:25 ohm TLT is a close match, then the capacitor stack follows.

I modeled Grant's configuration with AutoEZ, mostly to see how the optimizer would handle things.  I ended up with capacitor values quite different from Grant's 4000/2000/1333 pF, but part of that could be because my modeled conditions do not equal Grant's reality.  Here's a full-band SWR sweep which also shows the values I used for the variables.

https://postimg.cc/image/6o9yxez4h/

Bob wrote:
>>> A simple T antenna will have a radiation resistance of 10-12 Ohms with electrical height of say 43ft with a Good ground system of quarter wave radials.

So then I went back and modeled something similar to what Bob described.  For this one I first optimized on both the height of the vertical portion and the length of the T arms to get a feedpoint Z very close to 12.5+j0 at 1975 kHz with no series coil.  Then I optimized the inductance values for the three lower band segments all at once.

https://postimg.cc/image/3u6tk1241/

AutoEZ users, if you'd like to see the optimizer setup and the trick for making the capacitance/inductance values frequency-agile just drop me a note via private email and I'll send you the models (*.weq files).

Dan, AC6LA
https://ac6la.com/