Viper SC+™ IP Router for Licensed Spectrum PN 001-5008-000 Rev. C
| Page 23
Figure 15 – Voltage transient immediately after the gas tube turns on
During testing, transients were measured on the antenna port of CalAmp’s Viper at voltage levels up to +/-280V. These
voltage transients often have high frequency content that can easily pass through any filtering in the radio and damage
components in the transmitter and receiver circuitry.
2.5.4.
GOOD DESIGN PRACTICES
There are two relatively easy ways to avoid creating large transients due to static buildup on an antenna and the
subsequent firing of the gas discharge tube in the lightning arrestor. Following either or both of the recommendations
below will eliminate this potential problem.
1.
Use antennas with a DC grounded active element. Antennas can easily be tested, by using an ohm meter, to
measure the resistance from the center conductor to the ground of the RF cable that is directly attached to the
antenna. The ohm-meter should indicate a short. (Some antenna designs, such as folded dipole or folded dipole Yagi
antennas, inherently have a DC ground on the active element due to the nature of the antenna design.)
2.
Use a lightning arrestor that does not have a gas discharge tube. PolyPhaser® makes several DC-blocked lightning
arrestors that have an inductor to ground instead of a gas tube. These lightning arrestors will not allow the static to
build up on the antenna, and there is no gas tube that can trigger causing a transient into the antenna port of the
radio. The following lightning arrestors, manufactured by PolyPhaser, have inductors to ground instead of gas tubes:
a.
PolyPhaser® Part Number: VHF50HN Frequency Range: 100MHz - 512MHz, 750W
b.
PolyPhaser® Part Number: TSX-NFF Frequency Range: 700MHz - 2.7GHz, 750W
Tip:
Lightning arrestors that use gas tubes will normally specify a “Turn-On Voltage” in the data sheet. If you see this
specification in the datasheet, it is very likely that the lightning arrestor has a gas discharge tube. If you are still unsure,
contact the manufacturer.
