our March 5 column,
Buc Fitch wrote about a simple adaptor that every engineer needs.
It’s a simple “H” pad that matches 150-ohm mike level to a 600
ohm line level. Longtime equipment manufacturer Harold Hallikainen
comments that, although the pad provides a differential gain of –55
dB, its common mode gain is 0 dB. This means the common mode
rejection ratio of the input will be 55 dB worse than it was. If the
shunt resistor were split, with the center grounded, the common mode
and differential mode gains would be the same, maintaining the input
Thanks, Harold, for
making a good adaptor even better.
engineer Paul Sagi sends in a useful site:
It’s a selection of impedance calculators that Mantaro engineers
use and freely share. Paul used the twisted pair calculator to
calculate impedance for some new UTP cable he used to replace the
telephone wiring of his ADSL Internet service. Now the SNR margin is
better by 5 to 6 dB with the new cable.
calculators for both straight wire and strap, and the calculations
make it easy to see why strap is good for high frequency and impulse
currents. It’s neater than paralleling many round conductors,
although Paul has added some round conductors to fool and satisfy
copper thieves, putting the round wires above the strap. You can also
use many round wires instead of strap, with the loss of a few to
theft not having much effect.
* * *
Another example of an
old pole, this one in San Bruno, Calif.
It’s not just in
the wilds of Montana that sections of wooden phone pole are left
hanging until the cables can be transferred!
Dane E. Ericksen,
P.E., is a consultant to Hammett & Edison in Sonoma, Calif. Dane
just read the April 9 issue of Workbench, and saw Greg Muir’s
“old pole” pictures. They reminded him of a similar replacement
pole that was at a friend’s house in San Bruno, Calif.
Seen in the image
here, it was this way for about six months. The “air pole” made
Dane real nervous when the only available parking spot put his car
* * *
Schardin shares a Pubtech listserv post written by Dave Barnett, who
is constructing a building and learned some things about electronic
ballasts for fluorescent lights.
His station monitors
an EAS signal that is already pretty weak; any added electronic noise
in the facility would be a concern. For standard four-foot light tube
applications, Dave tried Philips ballasts that had been specified by
the contractor, but found them to be noisy. A single light would
render his test radio useless for picking up the EAS primary within
20 feet of the ballast — and plans called for about 30 of these
lights in the building.
Dave then tested a
similar type of light that uses a GTE Sylvania ballast. By
comparison, the radio worked quite well beyond about three feet from
the light. He went with that model.
Talking to the light
supplier, Dave learned that common fluorescent products use a
half-dozen ballast models interchangeably. It’s wise to be aware
and try various models.
Dave also tested a
dimmable recessed can fluorescent. It produced interference somewhere
between the levels generated by the Philips and GTE ballasts for
He further tested an
LED retrofit for recessed cans, made by Sylvania. It caused no
interference even when the radio’s antenna was held up to the
light. It fades nicely on a dimmer and is brighter than the
incandescent version it replaces. It cost $25; so he has what he
needs for the recessed lights.
fixture, made by Lithonia Lighting, replaces a two-tube fluorescent.
It looked acceptable and the light was nice and dispersed, so Dave
decided to test this too. A local community center has these lights
in one of its meeting rooms; Dave took a radio in and asked them to
turn on the lights. Ultimate failure! His signal strength meter
pegged and the FM signal — way stronger than the EAS primary —
went away completely.
The lesson to be
learned from Dave’s limited tests is that engineers can expect wide
variation in RFI generated by various brands of fixtures and
Hal Schardin has
been down this road too and writes that if you want lower EMI/RFI
from an electronic ballast, specify a “residential” ballast.
Ballasts for residential applications must meet the lower consumer
limits of EMI/RFI. Electronic ballast emissions are covered by FCC
A further lesson:
It’s not enough to leave lighting up to the architect or
contractor. Engineers should be involved in the entire project from
planning to completion.
Hal Schardin is
engineering supervisor for the Minnesota State Services for the
Workbench. You’ll help your fellow engineers and qualify for SBE
recertification credit. Send Workbench tips to firstname.lastname@example.org.
Fax to (603) 472-4944.
Author John Bisset
has spent 44 years in the broadcasting industry and is still
learning. He handles West Coast sales for the Telos Alliance; he is
SBE certified and a past recipient of the SBE Educator of the Year