At an SBE meeting I
attended last year in Missouri, I had a brief chat about preventing
copper theft with Engineer Dave Buell, who mentioned an article in a
city of Independence newsletter discussing Independence Power &
Light’s use of DataDots to mark the utility’s assets.
Fig. 1: A promotional
image for DataDot Technology USA makes the salient point.
DataDot technology is
an identification and anti-theft security system that its maker says
can be used on all kinds of assets.
consists of polyester substrate micro-dots, each the size of a grain
of sand, onto which information is etched by laser. The dots are
described as virtually invisible; they typically are mixed with a
clear sealer and brushed or sprayed on to the object you want to
protect. They can be applied to equipment and even copper. The
ID information can be entered into the company’s database
for retrieval should a stolen item be recovered.
The city of
Independence is using the product to protect copper systems. You can
find out more about the Australian manufacturer at
Dave also passes along
information about using a constrictor knot to attach sash cord or
thin nylon rope to a cable (for instance, to use as a pull string
when you want to “fish” the cable through a conduit). Dave uses
his index finger to form the knot. Once you know it, it takes only a
few seconds to permanently attach a piece of rope to a cable. Dave
cautions not to cinch this knot on your finger. Once cinched down,
you will have to cut the rope to remove it.
After eight years
supporting an academic department at a nearby university, Dave is a
freelance television engineer once again, his profession for the 20
* * *
50- and 75-ohm Type N connectors is a point under scrutiny.
2: This is a female N connector, 75 ohms, in chassis mount
configuration. Note the small center female ‘pin’ or receptacle.
courtesy of Frank Hertel.
Yes, a male 50-ohm plug
may fit into the female 75-ohm Type N; but damage to the latter may
result. A male 50-ohm Type N has a larger pin diameter than a 75-ohm
Type N. Also consider the converse; mating a 75-ohm Type N male
connector with a 50-ohm female Type N will not yield a reliable
connection (if you make a connection at all).
The 50-ohm Type N
center pin is approximately 0.065 inches in diameter. The 75-ohm
connector pin is 0.036 inches in diameter. In the case of the BNC
connector, in most cases the center pin diameter is identical for
either 50- or 75-ohm connectors. The impedance conversion is
accomplished by changing the diameter of the insulator/pin passage
hole within the connector’s body.
Assuming you have not
mixed connector parts, you can determine if the connector is 50 or 75
ohms by looking at the size of the hole in the compression nut.
Assuming you are using RG-58 or RG-59 cables, 50-ohm cable will be a
sloppy fit through the compression nut; and 75-ohm cable will not fit
through the compression nut hole of a 50-ohm BNC compression nut.
At the transmitter
site, pay attention to cabling. The FM exciter wants to see 50-ohm
impedance. A mismatch will occur if you use 75-ohm cable. You’ll
see this as reflected power on the exciter, which cannot be tuned
out. So don’t just grab any old piece of coax for this critical
Thanks to Frank Hertel
for this important reminder. He is with Newman Kees Engineering.
* * *
Fig. 3: This is a
50-ohn N connector in a right-angle configuration. Again, note the
center female receptacle ‘pin’ diameter, compared to Fig. 2.
An engineer called
recently about what appeared to be a shorted winding to frame in the
modulation transformer in an old 500-watt tube transmitter. Since the
transformer frame is grounded, the transformer is shorted and will
not operate. His predicament reminded me of a conversation with
engineering sage Ira Wilner.
Such shorts can be
repaired temporarily by isolating the transformer from the chassis
ground, using any insulating material, typically a piece of wood. I
say temporarily; this “fix” can represent a hazard to an
So some precautions are
in order. First, order the replacement transformer; the fix is not
intended to be permanent. Second, make sure the transmitter door
interlocks are operational — the high voltage should cut out if the
door is opened. Third, place paper warning-labels on both the
transformer and the door to the transmitter. I’d use a broad-tip
Sharpie so that the notice is obvious. It wouldn’t hurt to put the
same warning on the front of the transmitter, too.
The transformer must be
unbolted to set on the insulating material; so if the transmitter is
moved, the transformer will move, too — possibly damaging the
wiring or winding. It’s a good idea to add that to the note and
to Workbench. You’ll help your fellow engineers and qualify for SBE
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Fax to (603) 472-4944.
Author John Bisset
has spent 44 years in the broadcasting industry and is still
learning. He is SBE certified and is a past recipient of the SBE’s
Educator of the Year Award.