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G4E

AMATEUR RADIO PRACTICES

- Mobile and portable HF stations; alternative energy source operation

G4E011 of 11

What is the purpose of a capacitance hat on a mobile antenna?

Why A mobile whip on HF is far shorter than a quarter wave, so it looks capacitive and needs loading to become resonant. A capacitance hat is a set of spokes, a disc or a wheel added near the top that adds capacitance to ground, which shifts the current distribution up the whip and makes the antenna behave electrically like a longer radiator. That lets you reach resonance with less loading coil inductance, and the lower coil loss usually raises efficiency.

Watch out The choice about reducing radiation resistance has it backwards: by keeping more current flowing higher on the whip, a capacitance hat tends to raise radiation resistance, which improves the ratio of radiated power to loss.

Hat on top makes a short whip 'taller' electrically. Cap hat = electrical length, not power rating.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G4E022 of 11

What is the purpose of a corona ball on an HF mobile antenna?

Why A short HF mobile whip is electrically very short, so the current node at the tip means the RF voltage there is extremely high, often thousands of volts. A sharp point concentrates that electric field and lets it leak away as corona discharge, wasting power and eroding the tip. A corona ball is a smooth metal sphere that spreads the field over a larger radius, raising the voltage needed to start a discharge. As a bonus it adds a little top capacitance, but its job is suppressing the discharge.

Watch out The choice about striking objects sounds plausible because a ball on the end looks like a safety tip, but corona balls are about electric field control, not mechanical protection. The Q and bandwidth choices are backwards in emphasis: the added top capacitance slightly helps loading efficiency, it is not the reason the ball is there.

Corona = sharp points leak RF. Round the tip, blunt the field, keep the watts in the antenna.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G4E033 of 11

Which of the following direct, fused power connections would be the best for a 100-watt HF mobile installation?

Why A 100 W HF transceiver draws roughly 20 to 25 amps on peak, so the feed has to be short, heavy gauge and fused at both ends to keep voltage drop and fire risk down. Connecting directly to the battery gives the lowest impedance source and lets the battery act as a large filter capacitor that smooths out alternator whine and other vehicle electrical noise. Tapping the accessory or fuse-panel wiring instead usually means undersized wire and a sagging supply voltage that distorts your transmit audio.

Watch out Running to the alternator or generator puts you right at the noise source with no battery smoothing, and balanced transmission line is meant for RF feedlines, not DC power runs.

100 W HF pulls ~20 A: heavy wire straight to the battery, fused. Battery = built-in noise filter.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G4E044 of 11

Why should DC power for a 100-watt HF transceiver not be supplied by a vehicle's auxiliary power socket?

Why A 100 watt HF transceiver draws roughly 20 to 25 amps on transmit peaks (about 100 W output plus losses at around 13.8 V). Auxiliary or cigarette-lighter sockets are typically fused at 10 to 15 amps and use light gauge wire with a loose plug contact, so the wiring can overheat and the voltage sags badly under key-down. The recommended practice is a heavy gauge fused cable run directly to the battery terminals.

Watch out The socket polarity is standard (center positive, shell negative), and nothing about the socket makes the engine overheat; RF shielding of the power cable is not what limits a lighter socket.

100 W out means about 20+ A in. Lighter socket fuse is 15 A. Run fused wire straight to the battery.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G4E055 of 11

Which of the following most limits an HF mobile installation?

Why A quarter wave on 75 meters is about 20 meters tall, so a mobile whip of 2-3 meters is a tiny fraction of a wavelength. Electrically short antennas have very low radiation resistance, and the loading coil, ground losses and vehicle body resistance eat most of the power as heat, so efficiency can be a few percent on the low bands. That antenna inefficiency, not the radio or the power feed, is what dominates HF mobile performance.

Watch out "Picket fencing" is the rapid flutter heard on VHF/UHF mobile signals as the vehicle moves through reflections, not an HF problem, and there is no FCC rule capping mobile output power on 75 meters.

Short antenna, short signal: mobile HF lives or dies on antenna efficiency.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G4E066 of 11

What is one disadvantage of using a shortened mobile antenna as opposed to a full-size antenna?

Why A physically shortened HF mobile whip is made resonant with a loading coil, which makes the antenna a high-Q resonant circuit. High Q means a sharp, narrow resonance, so SWR climbs quickly as you tune away from the design frequency and you may need to retune or adjust the coil tap for even a small frequency change. Full-size antennas like a quarter-wave have lower Q and cover much more of a band.

Watch out The claim that the Q will be very low has it backwards: shortening and loading raises Q, which is exactly what causes the narrow bandwidth. Distortion and harmonic radiation are transmitter issues, not a function of antenna length.

Short antenna = high Q = narrow bandwidth. Short whip, short tuning range.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G4E077 of 11

Which of the following may cause receive interference to an HF transceiver installed in a vehicle?

Why Modern vehicles are full of switching electronics, and any of them can radiate broadband hash into an HF receiver. The alternator and its voltage regulator produce a whine that rises and falls with engine speed, the electric fuel pump and injector drivers pulse current at audio rates, and the engine, transmission and body control computers run fast clocks and switch loads. Because all three are real, documented sources of vehicle-generated RFI, the inclusive answer is correct.

Watch out Picking just one source, such as the charging system, is tempting because alternator whine is the most famous automotive noise, but it is only one of several culprits in a modern car.

Charge it, fuel it, compute it: in a car, everything electrical makes HF noise, so pick 'all of these'.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G4E088 of 11

In what configuration are the individual cells in a solar panel connected together?

Why A single silicon photovoltaic cell only develops about 0.5 V regardless of its size, so cells must be stacked in series to reach a useful voltage - roughly 36 cells in series for a nominal 12 V panel. Larger panels then place strings of series cells in parallel to raise the available current. That combination of series strings wired in parallel is a series-parallel configuration.

Watch out Shunt, bypass and full-wave bridge are all terms borrowed from other circuits; bypass diodes are indeed used across cell groups to protect against shading, but they are added components, not the way the cells themselves are interconnected.

Series for volts, parallel for amps - a panel needs both, so series-parallel.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G4E099 of 11

What is the approximate open-circuit voltage from a fully illuminated silicon photovoltaic cell?

Why A photovoltaic cell is just a large silicon PN junction, and the open-circuit voltage it can develop is limited by the junction's barrier potential, roughly 0.5 volt for silicon. That is true regardless of the cell's physical size; area determines current, not voltage. This is why solar panels wire many cells in series, about 36 cells to charge a nominal 12 V battery.

Watch out The 1.38 V figure looks like a battery cell voltage (nickel or alkaline chemistry range), not a silicon junction; the 0.02 V and 0.2 V choices are far too low to be useful for charging anything.

A solar cell is a big silicon diode: about 0.5 V, same ballpark as a diode's forward drop. Current scales with area, voltage does not.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G4E1010 of 11

Why should a series diode be connected between a solar panel and a storage battery that is being charged by the panel?

Why A solar panel only produces voltage when it is lit. In darkness or deep shade the panel becomes a load, and the battery would push current backward through the cells, slowly draining itself. A series 'blocking' diode conducts only in the panel-to-battery direction, so charge current flows in but reverse current cannot flow out. The tradeoff is a forward voltage drop of roughly 0.3 V for Schottky types or about 0.7 V for silicon.

Watch out Regulating charge voltage or limiting current is the job of a charge controller, not a diode; a plain diode has no way to sense or adjust either one.

Blocking diode = one-way valve: charge flows in at sunrise, nothing leaks back at sunset.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G4E1111 of 11

What precaution should be taken when connecting a solar panel to a lithium iron phosphate battery?

Why Solar panels put out an unregulated voltage that rises well above the battery's safe charging range in full sun (a nominal 12 V panel can reach 20 V or more open circuit), and lithium iron phosphate cells are damaged or made hazardous by overcharging. A charge controller regulates the current and holds the battery to the correct charge profile, typically constant current then a constant voltage of about 14.6 V for a 12 V LiFePO4 pack, then stops. Lithium chemistries are less tolerant of overcharge than lead-acid, so the controller must also be set for the lithium profile, not the lead-acid one.

Watch out A series resistor only limits current, it cannot stop the panel voltage from climbing above the battery's safe limit, so it is no substitute for regulation. Grounding the panel frame is good general safety practice but does nothing to protect the battery.

Panel to battery always goes through a charge controller, and set it to the lithium profile.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

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