EMRFD Message Archive 1975

Message Date From Subject
1975 2008-08-20 10:21:52 bkopski ADD REACH TO THE AADE LC METER
I, as so many others, very much like the AADE LC meter. This is one
great asset for any shack – especially the shack of an HB'er. But
often I wished for just a bit more – like being able to easily
measure some "C" on a board or within some ugly construct. I wished
for a probe extension for my AADE.

Some posted photos (in the K3NHI folder) illustrate one approach to
such a probe. It's conceptually simple and in my case all parts and
supplies already existed in my shop. (Some of these came from my
aero modeling hobby and are typically found at hobby shops, or as

One photo ("probeparts.jpg") shows much of the stuff that's needed.
The basics include a sewing needle about 1 ½" – 1 ¾" long, a plastic
insulating tube to slip over the needle, a slightly un-wound spring
liberated from a ball point pen, and a piece of RG-174 stripped about

Probe assembly begins with dressing the cable braid back on itself,
shortening the coax center conductor, and soldering the needle to the
latter. What you see in "solderneedle.jpg" is not a sloppy solder
job. Rather, solder bumps placed along the needle length were
intended to provide something for epoxy to hold on to when the
insulating tubing was glued in place – but gluing turned out not to
be needed – so no solder bumps are warranted!

This step is followed by sliding the plastic tube over the needle
right up over some of the coax center conductor insulation
("needlecover.jpg") and then dressing the coax shield back over the
plastic tube ("dressbraid.jpg"). The braid is trimmed back from the
tube front end about 3/8". The last step in this sequence is to
slide the modified spring over the dressed braid
("placespring.jpg"). Some heat shrink tubing is placed as a "back
stop" to prevent reward travel of the spring. DO take particular
note of the relative positions of the spring extension, needle,
plastic tube, and the final braid position.

One more note: the spring inner diameter should permit easy
installation over the re-dressed braid. The spring must still be a
little compressible but not so large in inner diameter to permit poor
or intermittent contact with the braid – this is the "ground" contact
of the finished probe. There is a lot of braid / spring overlap, and
so adequate contact should be assured with a reasonable sliding – not
sloppy - fit of the spring-on-braid.

Finally, a loose fitting plastic tubing / handle is slid over the
whole works. Mine is 3" long and I used some hot glue to adhere this
tube to the coax outer cover at the rear of the tube – which keeps
everything together.

The bent-out extension of the spring – the ground contact - is quite
slinky in a firm kind of way and easily moves some within the
handle. It is even compressible back even with the needle tip. The
needle tip is the "hot" end of the probe and of course is the other
contact point. It is normally about 3/16" behind the ground
contact. In use, the ground contact is placed as appropriate and the
needle tip is maneuvered into contact as needed. In effect, the
ground contact is used as sort of a pivot point while the needle tip
is being aimed. A more-vertically held probe seems to give better
repeatability than a lower angled approach to the target component.
I'm guessing this has to do with the few front spring coils that are
not in contact with the coax braid. Anyway, this is easier to do
than to describe.

My RG-174 is approximately 8" long, and the entire probe assembly
indicates about 22 pF - which is then zeroed-out on the AADE. This
probe typically indicates capacitance a fraction of a percent higher
than the same capacitor placed in the meter terminals. I did learn
that a longer cable tends to slightly increase this difference,
although I'm not sure why.

Because there are several physical variables – such as exactly what a
retrieved ball pen spring may measure, or what plastic tubes are
handy, etc., I've avoided detail dimensions. And while my "inner"
and "outer" tubes were hobby shop supplies, I can also imagine using
the spray spout tube from an aerosol, and a ball pen, or plastic
pencil for the respective tubes. There are probably numerous other
choices I've not thought of. Just pay attention to the relative
positioning of the various parts as in the photos, and all should be
well. I believe you will find the added reach of your AADE well
worth this easy assembly chore!

Cordially and 73,
1980 2008-08-20 18:00:35 victorkoren Re: ADD REACH TO THE AADE LC METER
My explanation of the capacitance reading of the meter when using a
long coax as probe is simple:
The coax is a transmission line and being so it is not just an added
parallel capacitor because it has distributed series inductance with
distributed parallel capacitance. for very short pieces of transmission
line it is approximated well as parallel capacitance but as it gets
longer the series inductance can't be ignored.
The easiest way to see how the coax affects the capacitor measurement
is by using the smith chart.
A capacitor at a specific frequency will have a negative reactance
which will be located at a point on the bottom perimeter of the smith
chart. Connecting a short coax in series with the capacitor will move
the reactance to the left, still on the smith chart perimeter. The new
point is a lower reactance point which means higher input capacitance.
Making the coax even longer will move the input impedance towards the
left end of the smith chart which represent zero impedance or in other
words infinite capacitance. No real capacitor added in parallel to the
measured capacitor could have done that, so the conclusion is that a