When I did my Inverted L with the elevated radials, I think the impedance was about 38 ohms.
I decided to wind a 1.5:1 Unun based on Cecil's book on a K Mix. That brought it nicely in line to about 55 ohms if I remember correctly. That was about 2 years ago. Balun Designs also has them for sale https://www.balundesigns.com/model-16132-1-1-56-unun-32-to-50-ohms-2kw/ Mike va3mw On Sun, Sep 1, 2019 at 7:54 PM Wes <[email protected]> wrote: > That's exactly what I do; shorten the radiator to create a capacitive > feedpoint > Z and shunt that with inductance to make a resonant impedance step-up at > the > desired frequency. > > That said, my original loading wire was some stranded super flexible bare > copper > 12 AWG that I pickup up at a swap meet. It's about 90 feet from my > vertical to > my tower so I ran the wire from the vertical to near the tower and had > resonance > at 1825 kHz, but with a feedpoint Z of about 25 +/-j with my radials. > Over the > band of 1800 to 1850, the Z almost perfectly overlaid the 2:1 circle on > the > Smith chart. A few months later I noticed that the beautiful shiny > copper was > now severely tarnished. Since I was laying down more radials with 14 AWG > THHN I > replaced the top-loading wire with a similar length of that. Imagine my > surprise when I found that the wire was now too short and I didn't have > physical > room for more. I expected a push going from 12 to 14 but adding > insulation at > the same time. > > Using the old engineering adage that if you can't fix it, feature it, I > decided > to shorten the wire even more to add capacitance to the feedpoint Z and > shunt > that with an inductor to created an impedance step-up L-network. Success, > a > perfect 50 j0 match at 1825 kHz. > > But, is it really an operational improvement? Not necessarily. Before > matching, a hundred-twenty-five feet of LDF-50 coax rotated the feedpoint > Z > around the Smith chart, but it was still a 2:1 mismatch for the tuner in > the > shack to flatten. Not a problem. > > Now with matching, I have a "perfect" match at 1825 kHz, and if you don't > have > tuner you can stop here and claim success. The problem is, this is a > relatively > narrow-band solution. At approximately +/- 25 kHz, the feedpoint Zs land > back > on the 2:1 circle on the Smith Chart, except one point is near 25 ohm and > the > other near 100 ohm. I'll leave it to the reader to decide whether this is > an > improvement. I happen to think it is because: 1) I have enough space for > the > loading wire, and 2) the shunt inductor is a DC short across the > feedpoint. > Goodbye static build up. > > Wes N7WS > > > On 9/1/2019 11:46 AM, N4ZR wrote: > > The other day a ham friend suggested using a coil ("hairpin") to match > the low > > impedance of a well-radialed inverted L to 50-ohm coax. This struck me > as a > > potentially-attractive alternative to a series vacuum capacitor, but I > don't > > know enough to evaluate it. Thoughts? > > > > _________________ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
