Vectronics VEC-1402DK - Manual (Page 18)

VEC-1402DK Instruction Manual High Performance 2 Meter Pre-Amp Kit
16
2.2 uH
100 pF
Receiver Antenna
Power (9-15 VDC)
Warning--do not reverse direction when installing
Warning: Never reverse-polarize the supply voltage to
your preamp or reverse the direction of
installation of the bias-T. Damage to
electronic components may result.
Important Note: Never connect your bias-T module to a VHF antenna unless
the preamp is also installed in the line. Most VHF antennas are DC-grounded to
prevent static buildup on the elements. This provision will short circuit the
preamp power supply to ground if the preamp is removed!
Finally, choke RF1 (2.2-uH, red-red-gold) must be installed on the preamp's PC
board whenever the VEC-1402DK is phantom-powered over the coaxial
feedline! This choke, in combination with C9, C10, provides the necessary
network at the preamp end of the feedline to separate DC and RF signal paths.
Balun Option T1:
Virtually all balanced antennas, and many so-called low-cost "unbalanced"
antennas, perform better with a balun installed. Baluns block unwanted
common-mode noise pickup on the outer surface of the coaxial feedline shield.
They also prevent the feedline from interacting with the antenna elements and
disrupting the desired pick-up pattern. The balun used on the VEC-1402DK is
a low-loss, simple coaxial choke that works much
the same as the 2.2-uH
molded choke used in the bias-T circuit. The unique thing about a "coaxial
choke" is how it blocks RF energy from flowing on the outside of the cable,
while still allowing desired signals to pass freely inside. Interference and
residential noise traveling up the outside of the feedline are blocked before
reaching the antenna, while weak radio signals picked up by the antenna
elements easily pass down the inside of the cable without attenuation.
To prepare the balun, do the following: