ond, collapse the antenna to minimum
length and rested position. Finally,
connect the center conductor of the
cable to the rod antenna through a 100
pF or similar low-value capacitor.
If the signal strength of the cable
is too strong, the TV’s AGC (automatic
gain control) won’t be able to cope and
some channels will distort. If this
happens, place a cable attenuator ( 20
dB) in line with the cable before the
capacitor take off.
QI’m trying to make a UV watermark detector (using UV LEDs)
to help find watermarks
on stamps. I’ve tried to use
the basic LED flashlight approach to
— Phil Perez
AWatermarks are placed on
business letterhead, bank
notes, currency, and postage
stamps. Originally, watermarks meant the areas of a document
where the paper was pressed thinner
than the whole — usually in the form of
an image or initials. In recent years, watermarks have given way to fluorescent
paper/inks and phosphor tagging.
Fluorescence — Stamp paper or inks
containing fluorescent dyes that glow
when exposed to long-wave ultraviolet
light with a peak wavelength of 365 nm.
There is no afterglow. Fluorescent
papers usually glow a bluish-white;
stamp inks containing fluorescent dyes
can glow in a variety of colors.
Phosphor Tagging — A clear phosphor colloidal solution (tagging ink)
applied over a stamp, to its paper, or
mixed with the ink. Tagging glows when
exposed to short-wave ultraviolet light
that peaks at 254 nm, and has a brief
afterglow that is used by automatic
canceling machines to find, face, and
cancel an envelope’s stamp. Reddish
tagging was only used for air mail
stamps from 1963 to 1978. Beginning
1978, bluish-green tagging was used
for all subsequent air mail stamps.
Now for the bad news. There are
very few ultraviolet LEDs on the market
that emit at 380 nm, fewer at 365 nm —
and none that emit below 365 nm,
certainly not at 254 nm. Which leaves
you with fluorescent tubes.
Fortunately, blacklight tubes — as they
are called — are readily available at
these wavelengths. And they don’t
require a special light fixture. A
battery-operated, fluorescent lamp
often sold for emergency lights in auto
glove compartments is perfect. You can
find the blacklight tubes at gem shops
or stamp supply stores. NV
It seems like every week I
discover another way to do an
Internet search. For instance, I
just ran across Lexxe — an
ask-a-question site at
www.lexxe.com It lets you surf
the net using plain language.
Got a question? Lexxe has the
answer — but it can be slow.