Nuts & Volts
Nuts & Volts
Nuts & Volts
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ond, collapse the antenna to minimum<br />
length and rested position. Finally,<br />
connect the center conductor of the<br />
cable to the rod antenna through a 100<br />
pF or similar low-value capacitor.<br />
If the signal strength of the cable<br />
is too strong, the TV’s AGC (automatic<br />
gain control) won’t be able to cope and<br />
some channels will distort. If this<br />
happens, place a cable attenuator (20<br />
dB) in line with the cable before the<br />
capacitor take off.<br />
24 March 2006<br />
STAMP COLLECTING<br />
SECRETS<br />
QI’m trying to make a UV watermark<br />
detector (using UV LEDs)<br />
to help find watermarks<br />
on stamps. I’ve tried to use<br />
the basic LED flashlight approach to<br />
no avail.<br />
— Phil Perez<br />
USMC, Ret.<br />
AWatermarks are placed on<br />
business letterhead, bank<br />
notes, currency, and postage<br />
stamps. Originally, watermarks<br />
meant the areas of a document<br />
where the paper was pressed thinner<br />
than the whole — usually in the form of<br />
an image or initials. In recent years, watermarks<br />
have given way to fluorescent<br />
paper/inks and phosphor tagging.<br />
Fluorescence — Stamp paper or inks<br />
containing fluorescent dyes that glow<br />
when exposed to long-wave ultraviolet<br />
light with a peak wavelength of 365 nm.<br />
There is no afterglow. Fluorescent<br />
papers usually glow a bluish-white;<br />
stamp inks containing fluorescent dyes<br />
can glow in a variety of colors.<br />
Phosphor Tagging — A clear phosphor<br />
colloidal solution (tagging ink)<br />
applied over a stamp, to its paper, or<br />
mixed with the ink. Tagging glows when<br />
exposed to short-wave ultraviolet light<br />
that peaks at 254 nm, and has a brief<br />
afterglow that is used by automatic<br />
canceling machines to find, face, and<br />
cancel an envelope’s stamp. Reddish<br />
tagging was only used for air mail<br />
stamps from 1963 to 1978. Beginning<br />
1978, bluish-green tagging was used<br />
for all subsequent air mail stamps.<br />
Now for the bad news. There are<br />
very few ultraviolet LEDs on the market<br />
that emit at 380 nm, fewer at 365 nm —<br />
and none that emit below 365 nm,<br />
certainly not at 254 nm. Which leaves<br />
you with fluorescent tubes.<br />
Fortunately, blacklight tubes — as they<br />
are called — are readily available at<br />
these wavelengths. And they don’t<br />
require a special light fixture. A<br />
battery-operated, fluorescent lamp<br />
often sold for emergency lights in auto<br />
glove compartments is perfect. You can<br />
find the blacklight tubes at gem shops<br />
or stamp supply stores. NV<br />
COOL WEBSITES<br />
It seems like every week I<br />
discover another way to do an<br />
Internet search. For instance, I<br />
just ran across Lexxe — an<br />
ask-a-question site at<br />
www.lexxe.com It lets you surf<br />
the net using plain language.<br />
Got a question Lexxe has the<br />
answer — but it can be slow.