contributor Charles “Buc” Fitch, P.E., had a client who lost a
24VDC logic supply at the transmitter site. The power supply loss
prevented logic for the backup system to operate. The customer had
installed dual 24 VDC power supplies but needed some way of linking
the two supplies in a main/standby mode.
Fig. 1: The schematic
illustrates how to control dual 24VDC supplies.
(Click to Enlarge)
came up with the controller schematic shown in Fig. 1. Parts are not
critical. The panel has an audible alarm, which can be a Sonalert or
a less expensive audible alarm. The only requirement is that the
alarm use 24 VDC.
The Electronic Goldmine or Marlin websites have alternative buzzers,
though not always in stock. Consistent supply of material is
certainly an issue these days for the do-it-yourselfer.
uses MPJA as an acronym. Their site is www.mpja.com.
two relays are standard 24 VDC DPDT relays. If all you have are 12
VDC relays, you can certainly use a 12V zener diode ahead of the
relay coils to drop the voltage.
completed project will control two 24 VDC supplies operating in
parallel, which provides constant backup. If either supply fails, the
audible alarm provides flexible alerting to staff.
Roberta Barmore has been following our discussions about replacement
insulators. She’s surprised that no one has mentioned an “old
standby” replacement for HV/RF insulators: glass soft-drink
days, soda pop is more likely to be sold in cans. But there was a
time when every radio station worthy of the name had convinced at
least one local bottler to install a cola machine at the station. The
accompanying rack of empties was an attractive source of five-cent
wash and dry the bottle, then epoxy a bolt in the narrow end, along
with some kind of mounting flange or clamp to the wide end. Even
without that refinement, they’d work.
the tip of isolating a shorted AM modulation transformer from the
ground, using some sort of insulator? Many a local AM station stayed
on the air with a shorted-to-frame modulation reactor or transformer
sitting on four empty “shorty” glass soda bottles while waiting
for replacement parts to arrive.
for the memory, Roberta.
you’re using PVC or glass bottles as a temporary insulator, always
consider the voltages.
with that in mind, consider this comment from engineer Henry Downs.
worked with me at Dielectric. After reading our PVC pipe insulator
tip, Henry thought readers would be interested in the following:
Henry used numerous plastics, such as PEEK and ULTEM, two “low RF
loss” materials, in his filter and antenna designs while at
discovered that the plastics were relatively low loss at room
temperature, with loss tangents on the order of 0.0014 to 0.002.
However, as the temperature rose, the loss tangents rose
dramatically, such that a temperature increase from 23degrees
Celcius to 120 C yielded a loss tangent increase of approximately 10
Fig. 2: The finished
power supply control project is shown.
a result, the application of RF power can, under the right
circumstances, cause thermal runaway of the insulating part.
you feel you have to use a polymer material, Henry recommends Teflon,
where this runaway does not seem to occur.
wrote recently about towers “in the crosshairs.”
ago, engineer Jim Appleton and his team were installing two new
towers for an AM station to go DA. They had installed 60 feet of the
first tower and were up on the structure when they heard a bang
and a twang below them.
Jim and his crew started screaming, looked around and saw a hunter
running up over the hill. Gone!
guy had shot the tower about 20 feet below them — a little too
close for comfort!
make matters worse, during that same project someone stole nine
200-pound rolls of copper ground radial wire!
discussed the differences between 50- and 75-ohm Type N connectors,
and the fact that 75-ohm is costly and hard to find.
Muir of WolfRam Engineering was building a translator for a client.
He thought the antennas terminated in 50 ohms, but once on site, he
found the antennas needed 75-ohm connectors, and all he had were
50-ohm Type N connectors. So Greg got to work.
the risk of violating the connector’s electrical characteristics a
bit, Greg — who has machining ability — chucked the 50-ohm center
pin in a lathe and took 30-thousandths of an inch off the pin. The
result was a 75-ohm connector to mate with the antennas.
mod worked, even though Greg hasn’t yet put one of these modified
connectors on his network analyzer.
with the PVC insulator, when you are in a corner with no connectors
and under pressure to get something done, quick fixes can save you.
This one seems to work well.
adds that he’s out of the captive broadcast employment trenches; he
says he prefers to work at his own engineering company where he can
help clients in a quality manner.
Greg, for a unique temporary solution.
to Workbench. You’ll help your fellow engineers and qualify for SBE
recertification credit. Send Workbench tips to
email@example.com. 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.