Topband: Hairpin match to inverted L?

[Michael Walker]

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 <wes_n7ws at triconet.org> 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
>