A Clearly Better Baby Monitor
QWe have a baby monitor system and it works pretty well — except when I move it to the garage. Then, it becomes very “static-y.” Is there a way to legally modify it to have greater range?
The unit is a VTECH DM221.
Herb Ruiz
Seattle, WA
ATo steer clear of violating FCC Part 15 regulations, it’s probably best to try to improve the receive side as opposed to the transmit side of the monitor. The transmitter is probably
very low power, so any significant attenuation through your
house will cause the receiver’s digital demodulator to not
be able to decode the modulation without errors.
This particular system has a “parent unit” that can
transmit as well as receive, using its Talk Back feature, so I
don’t advise transmitting with any modifications in place.
The thing that would be easiest to modify is the antenna
system. Looking at the user manual, this device operates
just below the 2 GHz band. There is no specified power for
the transmitter.
Antennas for that band can be quite small since the
wavelength is only about 16 cm (about six inches). This
means a full wave dipole would be only 8 cm, which is
pretty small. We can perhaps enhance the amount of
captured electromagnetic wavefront, and by that improve
the gain by attaching a resonant wire.
I managed to find an article on iFixit that describes
fixing the antenna. You can check it out at https://www.
ifixit.com/Guide/VTech+Safe+And+Sound+DM221+An-
tenna+Replacement/52562. It tells us where the antenna
is. The antenna is a bent piece of wire that’s near the
handle and is mounted roughly horizontal when the device
is standing upright. With that in mind, let’s get some wire.
Try cutting a piece of wire (anything convenient is fine
— the type isn’t that important) for a full wavelength (about
six inches long). We’re going to try to couple some of the
RF to that at a low impedance part of the antenna, which
should be about one-quarter wavelength in from one side.
Affix the wire with tape or some other suitable adhesive at about 1.5 inches from one wire end and centered
on the top of the parent unit, and then raise the long
end. Either hang it from something or attach it to some
non-metallic support — even a wall. That should passively
couple the radiator to the internal antenna. Ideally, that
will improve your signal-to-noise ratio enough to make the
audio decode better. Playing with the orientation of the
wire or its height might also yield improved results. NV
QUESTIONS and ANSWERS
8 February 2018
