G4E: Mobile and Portable Operation – Ham Radio General License Study Guide

G4E covers the practical challenges of operating HF from a vehicle or portable location. Topics range from mobile antenna design to DC power wiring, vehicle interference sources, and alternative energy systems including solar panels.

The exam draws from topics including the purpose of a capacitance hat on a mobile antenna, the purpose of a corona ball, the best DC power connection for a 100-watt mobile transceiver, why the auxiliary power socket should not be used, what most limits an HF mobile installation, the disadvantage of a shortened mobile antenna, sources of vehicle receive interference, how solar panel cells are connected, the open-circuit voltage of a silicon photovoltaic cell, the purpose of a series diode between a solar panel and battery, and the precaution required when charging a lithium iron phosphate battery from a solar panel.

HF Mobile Antenna Design

An HF antenna for a vehicle must be physically short relative to a full-size resonant antenna at HF frequencies. A full quarter-wave antenna on 40 meters would be about 10 meters tall — impractical on a vehicle. Electrically short antennas require special design techniques to function effectively.

A capacitance hat is a disk or radial structure placed near the top of a mobile whip antenna. Its purpose is to electrically lengthen a physically short antenna. The capacitance hat adds capacitance to the tip of the antenna, which lowers the resonant frequency and allows a shorter physical length to resonate at the desired frequency. This improves the efficiency of the antenna compared to using a loading coil alone.

A corona ball is a smooth, rounded metal ball placed at the very tip of a mobile antenna. At high RF voltages — which occur at the tip of a high-Q mobile antenna when transmitting — sharp points cause corona discharge (ionized air glowing around the tip) which wastes power and can cause RF burns. The corona ball presents a larger smooth surface that reduces the electric field gradient and prevents this RF voltage discharge from the tip of the antenna while transmitting.

The single most important limitation of HF mobile operation is the efficiency of the electrically short antenna. Mobile HF antennas are much shorter than a full-size resonant antenna, which means they have a high reactance that must be tuned out and a low radiation resistance. The ratio of radiation resistance to total resistance (including loss resistance in the loading coil and other components) determines efficiency, and for short antennas this ratio is poor.

A shortened mobile antenna also has the disadvantage of very limited operating bandwidth. High-Q short antennas have a very narrow resonance. Even a small change in frequency causes the SWR to rise sharply, requiring retuning when moving across the band.

DC Power Wiring for Mobile Operation

A 100-watt HF transceiver can draw 20 amperes or more from the vehicle's 12-volt DC system during transmit. The best power connection for this installation is directly to the battery using heavy-gauge wire with in-line fuses at both ends of the run. This provides the lowest resistance and highest current capacity path for the transceiver.

The vehicle's auxiliary power socket (cigarette lighter socket) must not be used for a 100-watt transceiver. The wiring to these sockets is typically designed for low-current accessories and may be inadequate for the current drawn by the transceiver. Undersized wiring can overheat, creating a fire hazard, and will cause voltage drops that degrade transceiver performance.

Vehicle Interference Sources

Modern vehicles contain many systems that can generate interference to HF receive operations:

• The battery charging system — the alternator generates RF noise, especially if the diode rectifiers are aging or if the alternator whine couples into the antenna
• The fuel delivery system — electric fuel pumps and fuel injector solenoids generate switching noise
• The control computers — engine control units and other microprocessor-based systems radiate broadband digital noise

All of these systems can cause receive interference in a vehicle-mounted HF installation. Addressing them may require RF filtering on power leads and careful routing of antenna cables away from vehicle wiring harnesses.

Solar Power for Portable Operation

Solar panels convert sunlight into DC electrical power. Understanding how they work internally and how to integrate them safely with batteries is important for portable and emergency operation.

The individual photovoltaic cells within a solar panel are connected in a series-parallel configuration. Cells are connected in series to add their voltages, and groups of series cells are then connected in parallel to increase current capacity. This combination produces the panel's rated voltage and current output.

A fully illuminated silicon photovoltaic cell produces an open-circuit voltage of approximately 0.5 VDC. Multiple cells in series are needed to produce the 12 volts or more required to charge a battery.

A series diode should be connected between a solar panel and the battery being charged. Its purpose is to prevent the battery from discharging back through the solar panel during periods of low or no illumination. Without the diode, at night or in low light, the battery voltage would be higher than the panel output, and current would flow backward through the panel, draining the battery.

When connecting a solar panel to a lithium iron phosphate (LiFePO4) battery, a charge controller is required. Unlike lead-acid batteries, LiFePO4 batteries require precise charge voltage limits and must not be overcharged. A charge controller monitors the battery voltage and regulates the charging current to keep the battery within safe limits. Connecting a solar panel directly to a LiFePO4 battery without a charge controller risks damaging the battery or causing a safety hazard.

G4E Practice Questions