FIGURE 5
box and were cut to proper length. I paid $20 for the
remaining parts needed. The wall plug transformer was the
most expensive at $10. The probe components are installed
in a modified Sharpie felt tip pen housing. The cap was
removed and the pocket clip snipped off flush with the
body. Then, a 1/8” hole was drilled into the end to accept
the RG-174 cable. I then removed 1” of material from the
end of the pen body, reached in with needle-nose pliers,
and removed the ink cartridge. The protruding felt tip was
then snipped flush with the end of the body. I stood the
body up straight on a firm surface and got the remaining felt
out by driving it back into the housing with a hammer and
punch (a flat end nail of the proper size will also do). A little
cleanup work was needed inside the housing, so I used
various drill bits and rotated them by hand to
remove the remaining plastic nibs. The housing will
need a thorough cleaning to remove any remaining
ink residue; the process can be a bit messy. I used
a piece of plastic laminate (Formica) shaped to fit
the interior all the way to the tip. Although Figure
4 shows this as a rectangular shape for drawing
simplicity, the finished piece will be more bullet
shaped near the tip end. The components C1, R1,
R2, and R3 are all thru--hole components. Since a
copper-clad board has too much stray capacitance,
these parts lend themselves to better support in
this application.
After I was satisfied I had a good fit in the
housing with the laminate, I pre-drilled all the
mounting holes. The actual probe tip was a broken
needle from my wife’s sewing machine and it takes solder
quite readily. This was soldered to one lead of C1 with its
other lead dropped through its board hole and yanked back
to hold it in place. The probe tip is then positioned on
center with about 1/2” exposed beyond the laminate. A
blob of epoxy cement is dropped on it to preset it. When
cured, another blob is smeared over it again and half of C1,
and is allowed to flow around both sides of the board. This
gives the tip adequate support when cured. The component
leads are dropped through their corresponding holes; their
leads bent back and joined with a healthy dose of solder.
When cooled, snip these flush with the board. The exposed
section of the cable braid is actually pinched back to form
two solder nibs where R3 and the ground lead connect;
then get it pre-tinned.
Attach the RG-174 cable and secure it down with
dental floss (the waxed type is best). Drill a thru-hole in the
probe body at an appropriate spot for the ground lead to
protrude. Push the ground lead through it from outside the
housing and far enough so that the protruding end sticks
out far enough to allow soldering to the cable’s ground
braid when the probe board is partially inserted into the
housing. When completed, the probe board is pushed in all
the way until it is snug. Apply a slight pulling force on the
ground lead while doing this. There should be about 3/8” of
probe tip protruding from the housing end. In my probe,
friction was all that was needed to keep it in place. Some
wadded, non-conductive material would have been jammed
into the open end had this not been the case.
The back of the probe body needed a little abrading to
accept the cap with a nice snug fit. The finished active
circuit is shown in Figure 5. This circuit starts out with a
piece of 1” x 2” single-sided copper-clad board. A trace
3/16” wide x 1-1/4” long was etched right down the center
of it. A couple of small islands were also etched out at the
upper right corner to accept the regulator (TO-92) and its
associated components.
On the long trace, a 1/8” hole is drilled where the
MMIC amplifier will reside. This is located about 1/4” off
center to the left along the axis of that trace. The remaining
copper area at the edge of the hole was then removed to
completely break the trace’s path. Also, two breaks were
etched out where the chip capacitors will be installed. (A
note here: When I refer to etching, I mean etching with a
50
November 2010
