Topband: Hairpin match to inverted L?

[Wes]

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?
>