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Amateur Radio Info & Exams - Amateur Practices 3 - HF Mobile and Field Operations

Both mobile (vehicle, vessel, or aircraft based), and field based operations usually make use of non-mains power sources, and may involve compromise antennas, although a field with large trees can provide some great options for full-scale antennas.

HF Mobile

HF can be operated from vehicles, although this can be a little more challenging than VHF or UHF.


Many mobile HF operations operations use a vertical antenna, but the problem is that a there is a limit to the height of antennas, due to low flying tree branches, telephone and pay TV cables, and saggy power lines, not to mention the potentially most "exciting", uninsulated rail and tram overhead supply systems, with between 600 volts DC, and 25 kilovolts AC. In Australia rail and tram power can be 4.3 metres, occasionally less.

One antenna option is the 108" (2.7432 metres), or 102" (2.5908 metres) stainless steel whip, the latter designed to be used with a spring, bring the overall length close to the former. These are designed for use with 27 MHz CBs, where they are around a quarter-wave long. The can be trimmed to use on 10 metres, or may work on 12 metres, although them may be 10cm short. Any spring you use should have an internal braid link between the base and top. A tuner of some sort can match these for use on lower frequency bands, with decreasing efficiency, as the frequency drops. SGC sells several "Smartuner" products designed for use with 4WDs and pick-ups. The also offer whips to use with them, and longer whips for use on larger boats and yachts.

Not on the exam, the backstay of a yacht, with appropriate insulators, also makes a great wire antenna, but you MUST also have a whip for use on marine channels if you lose your mast.

Several single-band helically wound antennas of around 1.5 metres are available, on a fibreglass rod or heavy tube. They usually include a short steel rod at the top, which can be adjusted to tune the antenna.

Multiband HF antennas also use a helical winding, but with sockets to allow a "wander lead" to to link the bottom point with the relevant tap. They include taps for all ham bands from 80 to 10 metres, sometimes also 160 metres. Terlin / Outbacker, and FAMPARC are examples. The Terlin ones can be custom made with extra (non-ham) channels, if needed.

Lightweight Diamond antennas are wound on a thin rod, with a longer, thin wire whip, and several optional sections for extra bands. This is the HV7A and coils.

FAMPARC, a radio club in Melbourne have also produced a tapped HF antenna, although these are no longer listed on their website, so may be something to find at flea-markets or ask around your local radio clubs.

"Screwdriver" antennas, so named because the home-build ones use the motor and gearbox of a battery screwdriver, vary in quality, but most are designed for use on a soft-suspension car on Japan's smooth city roads, and fail if used on a 4WD and/or on Aussie roads. The ATAS-120 from Yaesu, parts of which are distributed on roads such as the Barkley Highway, are not for rural driving. These work by having a long coil with a strip where a contact (or the coil), is driven up and down by the motor, finding a tune point. There is a thin stainless whip above this. Bullbars apparently amplify vibrations, contributing to ATAS failures.

The heavy-duty Codan and Barrett antennas include tuners in the base. Perhaps the long ones include a long tapped coil, but the shorter are quite possibly relay switched inductors and capacitors, like many auto-tuners. The top element is either a steel whip, or a braid covered fibreglass whip with heat-shrink.

The more flexible antennas can be pulled back to a cord at the back of the vehicle, either to reduce the height, or so the antenna acts as an NVIS antenna, for effective shorter range operation on lower HF bands. They can also be pulled forward from a rear mount.

The auto-tuners should be able to also tune a length of wire thrown into a tree, or even just across shrubs or dry ground.

As well as linear loading provided by helical winding along the antenna, loading can be placed in the base, or in the middle, and a plain rod or tube of metal used for the element. If it is stiff, and possibly guyed with non-conductive line, with or without loading coils, stiff wires radiating to a ring can be added; this, or a similar arrangement; being called a capacitive hat. This increases current flow in the lower parts of the antenna, having it appear to be electrically longer. Centre-mounted coils are termed "Bug-catchers". Antennas with central coils, and many with capacitive hats, would be made from tubing, rather than wire.

Any antenna with an open end (that is, not a loop) can suffer from corona discharge. This increases with both voltage, related to the power level; and to the thinness of the air, increasing with altitude. Closed loop antennas are one answer, often not practical on vehicles. The alternative is to ensure that the end of the antenna is either a metallic ball, or has a non-conductive bead on the end. This is not an absolute preventative, but does limit the likelihood at sensible power levels and normal altitudes.

As you move down in frequency an antenna of a certain, restricted length becomes both less efficient, and narrower in bandwidth. I have been told that if the blank a helical antenna is would on is thicker, then the bandwidth is greater.

Certain ex-military antennas can be used, such as tank antennas. This is to introduce a story: A member of a club I am in was driving through an inner western Sydney suburb, with one on his Mini, or similar car, in the days of (real) trams. While 600 volts DC on the antenna sliding along the wire was not a problem, contacting the grounded over-road railway bridge and the wire at the same time resulted in a section of antenna being welded between the two, and no doubt a loud bang!

If your antenna obstructs your number-plate, in this age of camera based revenue raising, with additional revenue raising applying for even a fine whip which which causes no lack of readability, you need to either move the plate and light to the mount or the side, or fit a small bicycle rack plate. (Many areas without the 5th Amendment right to not incriminate yourself (or your spouse), even despite similar Common Law rights, allow revenue to be raised based only on a photo of the rear plate, with no evidence as to who the driver was.)

There is also the option of using the vehicle as power source and shelter while operating using a larger antenna while stationary, just don't flatten your battery.

Power Connections

One of the things which needs attention is the connection to the vehicle's battery. 100 watt Amateur transceivers can draw up to 22 amps, depending on the mode used. This can cause a high voltage drop, causing the radio to operate poorly, and this can apply to field operations too.

To reduce the voltage drop, heavy cable should be used. For this current, cable of at least 6 mm² should be used for the normal front battery to dash-board distances. This just fits in the yellow crimp terminals. You need to avoid both scuffing and high temperature points. You should use an existing penetration of the firewall. If you wish to place the radio in the boot / trunk of a car with a front-mounted battery, or in the cabin of one with a rear-mounted battery (some BMWs do this), then heavier cable should be used. The best option in the latter case would be to mount the radio in the rear too (but not where battery fumes can damage the radio), and use a "separation kit" to place the control head, a speaker, and the microphone in the front.

The connections should be to the two terminals of the battery. Fusing must be used in the positive line, as close as practical to the battery. Many auto shops sell holders crimped onto a short loop of heavy wire, the options being for standard sized ATO / ATC automotive plastic "blade" fuses, or glass or ceramic 3AG barrel fuses. Replacement auto fuses are available at any petrol station for very little. The battery end might have a eye to go under a stud on the clamp around post, and the other end crimped to the cable to the cabin, using a yellow crimp butt joiner. The first fuse is for the cable, perhaps 35 amps, and a second one is placed close to the radio, rated for the radio itself. For older BMWs a thick lead tab was used through which a bolt was passed, and there are other screw-down arrangements; in these cases an appropriate sized eye is used.

The lighter socket generally has inadequate wiring to operate a 100 watt Amateur transceiver. While lighters may draw high current, they are designed to function at something like 8 volts, due the voltage drop, although if a higher voltage is available they pop out sooner. With the decline in smoking, these are being replaced with outlets which are designed to supply things like in-car fridges, and these may supply a radio with adequate voltage, or the cabling to these can be tapped.

Anderson Powerpoles

Going beyond the exam for a while, these are a range of handy connectors for 12 volts.

The smallest size for Powerpoles houses 15, 30, or 45 amp crimp terminals, named for the size of the wire they can be crimped onto. 25 amp PCB mounted pins are also sold. Ultrasonically welded or glued red and black pairs, made up in the standard format for 12 volts are also available.

One way to step from cable larger than 6mm to the wire to the radio is to use an Anderson SB connector, or a 75 or 120 amp Anderson Powerpole (PP) connector, with appropriate terminals inserted in to each housing. Note SB connectors are colour coded and keyed, so only blue mates with blue, for example. Terminals of various wire sizes are available for SB and PP housings.


Modern cars use high pressure injection systems, and the high current pulses in the electrically operated injectors can generate interference to HF radios used in the car. High pressure gives finer droplets in the fuel spray, giving cleaner emissions, and greater power, including in diesel vehicles. The vehicle's alternator, which charges the battery can also generate whine, usually conducted into the audio stages, and thus audible in the speaker. The question also mentions the vehicle's computer as a source of interference. I have noticed that LED tail-lights and brake-lights flicker, and perhaps these pulsed current also cause either RF interference, or impulses on the power input. I am unsure of this flickering is just brightness control, or directly related to the CAN-Bus system, there instead of a wire going to each lamp to control it, just a lower line and a signal line runs to all lamp clusters, and they are controlled by serial data.

Off the exam, current pulses in spark plug leads in spark-ignition systems, including petrol, ethanol, LPG (propane), and CNG (compressed natural gas) also cause interference, as any systems which include arcing lead to RF interference. If parked by the road with a radio with the squelch open, as you tend to with SSB, you can also hear the ignition of passing cars, even listening on 2 metres. Given many mains voltage generators are spark ignition, they also can generate RF noise.

Field Operations

For various reasons, be they emergency communications, "Field Day" competitions, JOTA (Scout and Guide events), publicity events, SOTA / IOTA / National Parks / DXpedition events, or to operate in a park to avoid (surely unconstitutional) "Home-Owner Associations" rules against antennas (and thus free speech via radio), Amateurs set up stations, often away from sources of mains power. While a battery of adequate size might be lugged along for a short operation, there are options to charge batteries on site.

Solar Panels

The process of converting solar (light) energy to electricity is called photovoltaic conversion, and the cells, photovoltaic cells. Photon striking the cell causes electrons to be released. Each cell typically generates just 0.5 volts, so significant numbers must be placed in series within the panel. For simple charging of lead-acid battery, a diode is placed in series, between the panel and the battery. This prevents the battery discharging through the panel with it falls into darkness. In this case you need to manually watch the voltage.

Off the exam, a controller is often used, to prevent the battery being overcharged. These often use FETs with a low RDS, and therefore often a voltage drop less than the diode, which is typically then not needed.

Wind Generators

Another option is to use a pole-mounted wind generator, charging batteries. The downside is that you may need a large battery, or "energy storage system", as there may be extended periods without wind. The benefit is that they may well work at night.

Other sources

Generators, powered by petrol (gasoline), bottled gas (Propane / LPG / CNG), or diesel motors can be used. As these are often noisy, a long lead can be used, and ideally an earth bank or a wall helps mitigate these. Note that they generate carbon monoxide (CO), a deadly yet odourless gas, and also large amounts of particulates and other nasty emissions (and yes, smoking causes CO poisoning). More expensive "Inverter" models generate more stable voltages. It is generally better to use the 120 or 240 volt output to run a power supply to generate the 13.8 volts needed by most radios, than to use any 12 volt "battery charging" output on the unit. These are often mechanically commutated generators with two nasty glitches, with "bonus" spikes, on each rotation, similar to the "dynamo" in an old car.

A benefit of these is that an appropriately sized unit can power a large amplifier, although in this case an exclusion zone around antennas will be needed. Fluctuating loads caused by some modes can upset some generators, and/or cause significant fluctuations of the output voltage.

About a decade ago there was some excitement about using hydrogen fuel cells for the US Field Day event, these directly generating DC power from hydrocarbon fuels or alcohol, both containing hydrogen. There was also a plan to replace the vehicle's alternator with a fuel cell, the size of a folding umbrella, with the benefit that loads such as air-conditioning could operate with the engine off, as presumably could Ham gear, without the risk of depleting the cranking battery.

Both 4WDs, and some cars support dual batteries, with added charging control circuitry. A deep-cycle battery (sometimes sold for marine uses) is used for this, and if putting it in the boot, a marine battery housing, vented outside the vehicle is needed. Depending on the noisiness of the circuitry, an inverter output from a electric or hybrid car or pick-up could be use to power both power supplies for transceivers, and potentially, amplifiers. Some commercial electric vehicle provice power for trade tools. Note that adding such a system to an existing EV or hybrid requires great care, and the batteries can deliver lethal voltages, and have the ability to supply extremely high fault currents.

Relevant Questions

These are actual questions from the General exam pool.

What is the purpose of a capacitance hat on a mobile antenna?
A. To increase the power handling capacity of a whip antenna
B. To allow automatic band changing
C. To electrically lengthen a physically short antenna
D. To allow remote tuning

This is to electrically lengthen the short antenna, answer C. This arrangement is also used on MF and perhaps LF broadcast towers where there is a limitation to the height of the antenna.

What is the purpose of a corona ball on a HF mobile antenna?
A. To narrow the operating bandwidth of the antenna
B. To increase the "Q" of the antenna
C. To reduce the chance of damage if the antenna should strike an object
D. To reduce high voltage discharge from the tip of the antenna

A plain or sharp wire end has a tendency to have corona discharge off the end, and a ball prevents this, answer D.

Which of the following direct, fused power connections would be the best for a 100 watt HF mobile installation?
A. To the battery using heavy gauge wire
B. To the alternator or generator using heavy gauge wire
C. To the battery using resistor wire
D. To the alternator or generator using resistor wire

Heavy cable to the battery, answer A.

Why is it best NOT to draw the DC power for a 100 watt HF transceiver from a vehicle’s auxiliary power socket?
A. The socket is not wired with an RF-shielded power cable
B. The socket's wiring may be inadequate for the current drawn by the transceiver
C. The DC polarity of the socket is reversed from the polarity of modern HF transceivers
D. Drawing more than 50 watts from this socket could cause the engine to overheat

The wiring for the socket may be inadequate, meaning they could overheat, or more usually, the voltage drop would be excessive, meaning the transmitted signal would be distorted, answer B.

Modern power sockets, designed to actually power accessories MAY be acceptable, but the old ones designed to ignite cancer-sticks have inadequate wiring.

Which of the following most limits an HF mobile installation?
A. "Picket Fencing" signal variation
B. The wire gauge of the DC power line to the transceiver
C. Efficiency of the electrically short antenna
D. FCC rules limiting mobile output power on the 75-meter band

An antenna which can be safely mounted on a moving vehicle can only be a small fraction (around one twenty-fifth, at best, (for the 75 metre band in the previous version of this question) of a wavelength long. Even a helically wound antenna is wound on a short rod. In these cases the efficiency of the antenna is only a few percent. Thus the antenna is the limiting factor, answer C.

If it is necessary to communicate, stopping and rolling out 20 metres of wire, or thereabouts, is probably a better idea.

What is one disadvantage of using a shortened mobile antenna as opposed to a full size antenna?
A. Short antennas are more likely to cause distortion of transmitted signals
B. Short antennas can only receive circularly polarized signals
C. Operating bandwidth may be very limited
D. Harmonic radiation may increase

These antennas typically have a very narrow bandwidth, answer C.

Which of the following may cause receive interference in a radio installed in a vehicle?
A. The battery charging system
B. The fuel delivery system
C. The vehicle control computer
D. All of these choices are correct

All of these may apply, answer D.

What is the name of the process by which sunlight is changed directly into electricity?
A. Photovoltaic conversion
B. Photon emission
C. Photosynthesis
D. Photon decomposition

The cells or panels are called "photovoltaic", taken from the process, so answer A.

What is the approximate open-circuit voltage from a fully illuminated silicon photovoltaic cell?
A. 0.02 VDC
B. 0.5 VDC
C. 0.2 VDC
D. 1.38 VDC

It is 0.5 volts, answer B.

Thus tens or hundreds of cells are included in a panel.

What is the reason that a series diode is connected between a solar panel and a storage battery that is being charged by the panel?
A. The diode serves to regulate the charging voltage to prevent overcharge
B. The diode prevents self-discharge of the battery through the panel during times of low or no illumination
C. The diode limits the current flowing from the panel to a safe value
D. The diode greatly increases the efficiency during times of high illumination

Hmmm, the sought answer is B, but the text should be "B. The diode prevents discharge of the battery though the panel during times of low or no illumination", self discharge being a process within the battery. Also, if using a proper charge controller, the diode is likely unnecessary; consult its manual.

Which of the following is a disadvantage of using wind as the primary source of power for an emergency station?
A. The conversion efficiency from mechanical energy to electrical energy is less than 2 percent
B. The voltage and current ratings of such systems are not compatible with amateur equipment
C. A large energy storage system is needed to supply power when the wind is not blowing
D. All of these choices are correct

A large battery (or other system) is needed to store the energy, for periods during which the wind is not blowing, answer C.

This is the biggest section done, and also about half of this question pool completed.

On to: Electrical Principles 1 - Impedance and Calculations

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, March 2022.

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