14 January 2017
improve the pattern. The shield actually looks like two
conductors because of the skin effect, so you can increase
the inductance of the outer part near the feed point to
keep current from flowing from the outer side of the shield
to the dipole. If you don’t do that, the coax is part of the
antenna and radiates along its length.
This could be good or bad, depending on what gain
pattern is desired. It sounds a little crazy that the coax has
two effective conductors along the shield, but you really
see it in practice, even below 30 MHz. Of course, at lower
frequencies like 160 meters or 1.8 MHz, you need lots of
ferrite beads to make enough inductance.
There are other antenna designs that could change the
gain pattern for different results, but dipoles are simple to
build and they conveniently have a feed point impedance
close to common RG- 6 coax. I hope she gives it a try.
PBX and Auto Dialer
QI love my small PBX system and want to add an auto dialer between the CO line and the PBX. I want to detect outgoing digits (if more than two digits, no action). If they are 1, 1, then the
dialer should add prefix 9 and dial 911 (or disconnect
and dial 911) as is required by new laws. Are there simple
commercially available devices or can you suggest some
simple circuit to achieve such a thing?
AThis seems like a bit of a complicated device that will require some careful programming to make sure that the right thing happens. It’s possible to detect DTMF (dual-tone multi-frequency) dial signals in software if the audio signal is
digitized, using what’s called the Goertzel algorithm. That
is a very sharp IIR (or infinite impulse response) digital filter.
It’s also possible to build some hardware band pass filters
that could be monitored with a controller. Lastly, there are
some specialized DTMF decoder devices like the MT8870
from Mitel. Then, you’d need some isolation transformers
and relays to switch the circuit from pass-through to a way
to inject generated DTMF signaling into the CO line. The
algorithms for what you want to do could be written for a
microcontroller like an Arduino or Raspberry Pi.
Sadly, I think this is a rather complicated thing to both
construct and implement both in hardware and software
— particularly as an add-on. Unless your PBX system has
a software upgrade to provide the desired functionality,
constructing an add-on won’t be simple. That’s not to say
it’s impossible by any means. How to build such a device
does exceed the extent of what I could write here, though
it would be a fun undertaking if you had the time.
It turns out, though, that building up a new PBX from
scratch isn’t hard. There’s an open source PBX called
Asterisk ( www.asterisk.org) that runs on an ordinary PC
running Linux. Asterisk is designed to work with VoIP
phones, but there are also POTS interface cards available.
The website has a section under “Products” that can
direct you to manufacturers of hardware POTS interfaces
compatible with Asterisk. Being open source, if you aren’t
happy with how it works, you can change it in software.
While you’re on the Web, if you are plagued by
unwanted marketing calls, be sure to search for “PBX” and
“Lenny” ... NV
n FIGURE 4. Schematic representation of a Passive
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