> At 04:44 PM 11/28/2003, Jim Reid wrote:
> ><snip>However, I don't use it for twin
> >lead fed balanced antennas, for that I use my
> >modified Johnson kW Matchbox.<snip>
>
> Care to share the mods?
My adaptation/realization of L.B.Cebik's (W4RNL)
optimization of Link-Coupled Antenna Tuners, see:
1. Five part tutorial,
http://www.cebik.com/link/link0.html
2. Updated discussion of one recently available
such tuner from Eu.:
http://www.cebik.com/link/link.html
Basic mod. is to add a series tuning C with the link coil,
and add switching to allow taps to each of the five
turns of the kW Matchbox link coil. The optimum
component values depend upon the band being
used, see the Cebik table on page 2 of 9 of Part V
of the five part tutorial. There you learn that the
series tuning C for the link needed has quite a
spread of capacitance going from about 120 pF
for 10 meters, up to around 900 pF for 80 meters
(note, the kW Matchbox does not have enough
L or C in the BIG secondary components to handle
160 metes, the secondary coil would have to be nearly
twice as long!). To provide the necessary C sub p
range needed, I have added two variable C's to
my unit, and one doorknob. These must be pretty
good sized if running max. legal power into the primary
coil of the tuner. I bought a BIG C from Surplus Sales
of Nebraska for the "main" C sub s; the doorknob is
switched in for 80 meters. Doorknob C is 500 pF,
smaller variable is another 0 to 500 pF. For full
power on 80, I open the small C so no plates are
meshed (they will arc as too close spaced) and
use the larger C from SSoN, which has never
arced, hi.
The large variable C is mounted in its' own box atop
the Matchbox. The smaller variable, the five
position switch for the link taps, and a three position
switch to select among the C sub p options plus to
add in the doorknob as needed are all mounted on the
front panel directly above the big Tuning C dial for
the secondary dual variable C. This is the same space
in which an SWR meter is mounted in the later models
of the kW box.
The modified unit is MUCH easier to tune than the original
First resonate the primary circuit; I use my MFJ 259 to
accomplish this step in seconds. The two big
variables of the secondary circuit are easily brought
to minimum SWR/ match to the antenna once the primary
link circuit is tuned. A tiny bit of retweaking of the primary
may be needed as the secondary nears min reflection.
But the whole process is very fast as compared to the
original Matchbox set up.
Per Cebik, the optimum range of primary link coil
inductance is such that the "stock" kW Matchbox
link coil must be tapped; a tap selection to each
turn. Optimum inductance runs from 0.20 uHy for
10 meters, up to 2.2 uHy on 80 meters.
Interesting that the brand new price for the kW Matchbox
was only "$124.50 Amateur Net.", but that was nearly
50 years ago.
Well, believe that summarizes the mods I made to
this box. Excellent performance as far as I can
determine.
73, Jim KH7M
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