Topband: matching an inverted-L

[EUGENE SMAR]

Bruce:

     I think the term "Largish" is mine.  By that I mean an air variable
capacitor that looks like it came from a linear amp or high-power tuner.
Usually these kinds of caps are more than two or three inches long (the one
I use is about 8 inches long) with plate spacing big enough for you to get a
pencil between the plates when the capacitor is fully open or unmeshed.  The
one I use I bought at a hamfest many years ago.  I finally found a use for
this ancient junk!  I measured the capacitance value at 25-635 pF.

     My inverted L is larger than a quarter wavelength at 160M.  It measures
about 170 feet.  It's because of this excess length that I need a series cap
to cancel the inductive reactance of this long wire.  (But this longer
length gives me a better match to 50 Ohm coax.)  If you were to build a
simple quarter wavelength L (130 feet +/-), you wouldn't need the cap.  But
you'd have a feedpoint impedance of somewhere around 25-35 Ohms or so, plus
some small reactance.  This gives rise to a worse SWR than the longer L with
series cap.  I had used one of these shorter designs for about three years
on 160M.  It still worked, but I wanted to play around with the longer L.

     When I installed my L, I used an MFJ antenna analyzer with BCB
(broadcast band) filter to measure the feedpoint impedance.  Yes, the cap is
connected in series with the coax center wire and the L wire, and you tune
the cap for minimum SWR at around 1.840 MHz - the middle of the DX window.
My L's SWR is below 2:1 from 1.80 to around 1.90 MHz.  YMMV.  My rig's tuner
lets me operate anywhere in the band.  Plus I have about 2500 feet of radial
wire in the grass underneath the feedpoint.  (It's hard to believe there's
that much copper, but it's taken me four seasons to put it all down.)

     This basic design for the longer L is from W1BB, a guru in the early
days of 160M operations (and now a SK).  He recommended that the series cap
be anywhere from 500-800 pF.  If you can find one in this range with
sizeable plate spacing that's what you'd need.  You might try Surplus Sales
of Nebraska (www.surplussales.com ) for photos of some of these caps.
You'll also see what I mean by pricing.

     GL and let us know if this helps.  Hope to hear you on TopBand soon.

73 de
Gene Smar  AD3F