Vectronics VEC-1402DK - Manual (Page 17)

VEC-1402DK Instruction Manual High Performance 2 Meter Pre-Amp Kit
15
Outdoor Preamp Installations:
Although your VEC-1402DK may be used at the "radio" end of the feedline with
good results, you'll obtain best performance by mounting the preamp at the
antenna. This type of installation overcomes the effects of feedline loss, so your
radio can recover weaker signals that might otherwise be lost in the coax. When
mounting the unit outdoors, make sure to protect the PC board and preamp
components from exposure.
Rugged, well-shielded, cast-aluminum boxes make the best preamp enclosures.
However, you can use a tight-fitting molded plastic box, or a casing made from
1-1/2" ID Schedule-40 plumbing materials. With any enclosure, it's best to
mount connectors and mounting brackets on the bottom to prevent water from
seeping in around hardware. While it's best to use stainless-steel hardware to
prevent corrosion, rust, and potential electrical or mechanical failure, anodized
hardware for TV antenna installations can be adapted for use. Although the
enclosure should be designed to keep water out, drill a few small weep holes in
the bottom to allow condensation to drain or evaporate.
Sunlight can be as problematical as rain. Paint the box white (or another light
color) to prevent solar heating and damage to heat-sensitive components.
If the preamp is installed in a metal box, there may be a slight detuning of the
preamp. A final "touch-up" alignment may be needed.
There are two common methods of powering a preamp at a remote location.
You could power it over a separate power cable; but there is a better way.
Building a Bias-T:
A technique called "phantom powering" lets you feed the operating voltage
through the coaxial line between the operating location and antenna. It's done
with a simple device called a "bias-T”. The bias-T piggybacks DC power onto
the coaxial feedline without interfering with VHF signals. Components are
provided in this kit should you wish to build your own bias-T.
As shown below, the bias-T consists of a 2.2-uH choke plus a 100-pF multilayer
capacitor. The 2.2-uH choke (red-red-gold) allows DC current to pass onto the
feedline and, at the same time, prevent VHF signals from getting lost in the
power supply circuitry. Conversely, the 100-pF multilayer capacitor (marked
"101") passes VHF signals into the receiver while blocking DC power from
back-feeding into the radio's antenna jack. Mount bias-T components in an
external project box powered by a small 9 or 12-volt wall adapter (see following
diagram). Use a shielded metal enclosure.