rf - Free and Open Source Software

scriber line in the United States now has a
nominal 900-ohm impedance. This fact will
probably be a surprise to most pho ne patch
manufacturers, also. However, t he 600-ohm
equipment has worked well in t he past.
There are other restrictions on what yo u
can fced to the jack supplied by the telephone
company.
...Power of the signal at the central office
(must) not exceed 12 db below 1 milliwatt
averaged over any 3 second intcrva!....The
power of the signal which may be applied by
the customer-provided equipment to the
Telephone Company interface located on the
customer's premises will be specified for each
type of co nnecting arra ngement bu t in no
case shall it exceed one milliwatt.
In other words, you are not allowed to
over-d rive the telephone company lines. T his
is a reasonable provision, and should be t he
basis for providing a meter on your phone
patch, alt hough most patches don't have meters.
It is not too like ly that your patch will
provide more than a legal signal. If it does,
the "network control signaling unit" has
some built-in compensation. Of co urse, the
telephone company could specify an un·
reasonably low input level. Line loss can be
as high as 10 db. Be suspicious in this area,
but don't leave yourself open to that $10
charge.
The power in the band 3995 to 4005 Hertz
shall be at least 18 db below the limit (that
is. 18 db below the 12 db below 1 milliwatt
at
the central office)....T he power in the
band 4 .000 Hertz to 10.000 Hertz shall not
exceed 16 db below I milliwatt....l 0.000
Hertz to 25,000 Hertz- 24 db below ...
25,000 Hertz to 40,000 Hertz- 36 db below
...above 40.000 Hertz- 50 db below...
These figures give the " low-pass" filter
req uirements which your phone patch must
meet. They should not cause any difficulty.
To prevent interruption...(the) signal (wi ll)
at no time have energy solely in tile 2450 to
2750 lIertz band. If signal power is in (the)
band, it must not e xceed the power prese nt
at the same time in the 800 to 2450 Hertz
band.
This rat her simple restriction is the reason
for the occasional unexplained disco nnection
of an apparently-good phone-patch circuit.
The telepho ne companies use special signals
in the 2450-2750 band which are the equivalent
of a " hang up" command, and the call is
cut off. An occasional hetrodyne at the
wrong freque ncy can cause this problem, bu t
there is not mu ch you can do about it.
Other Methods
There is one other way to accomplish a
phone patch, and it can be done at a saving.
58
Section 20 of the tariff provides restrictions
on " Inductive or Acoustic Coupline," which
can be used without charge. I suspect that
there will be quite a bit of amateur develo p­
ment effort toward developing inductive or
acoustic phone patches. The restrictions are
as follows : " ...Network control signaling shall
be performed by equipment furnished, installed,
and maintained by the Telephone
Company.
In other words, you have to have a telephone.
No fair just inductively coupling the
patch to the telephone company lines.
.....Connection is made externally to a Telephone
Company net work control signaling
unit."
You can't put part of the patch inside the
telephone. The inductive coupler or acoustic
arra ngement must be outside the phone.
...Equipment mu st comply with the following
minimu m network protection criteria:
...The power of the signal which is applied
by the customer-provided equipment to the
network co ntrol signaling unit located on the
customer's pre mises be limited so that the
signal power at the output of the network
control signaling unit (i.c. at the input to the
telephone company line) docs not exceed 9
db below I milliwatt when averaged over
any three second intcrval....(and)....Ifrequcncyjpower
limits).
Notice that the signal power restriction is
9 db below I mw at the line input. rather
than at the cent ral office. If your line happens
to be among the worst, your signal could
be 19 db down at the central office, rather
than the 12 db allowed for an electrically
(QKT) connected system. This is rat her restrictive,
and again industry has objected. The
frequency{power limits are the same as
listed before, except that they are referred to
the minus 9 db figure.
Now What?
With pho ne patches now legal, the amateur
"fraternity" should take so me positive
action. Each of us with a phone patch must
make sure that the patch is operated properly
and that it is not harming the telephone e­
quipment.
Since a tariff now exists for customerprovided
equipment, the telephone companies
can no longer t urn a blind eye o n
phone patches. The cost is really very reasonable,
and the peace of mind is worth it.
Call the local company and get " legal" yourself.
Since the ARRL now recognizes phone
patches, we should urge the ARRL and any
other interested amateur groups to present
the amateur views on this subject to the FCC
73 MAGAZINE