G1 · Published 7 September 2026 · By The Ham Desk

HF Antennas: Height, Traps, Feed Lines and Ground

On HF the antenna is the whole game. This unit covers the choices that make the difference between a station that works everything and one that works nothing: how high the wire is, what the feed line is made of, how a multiband antenna switches bands, and what the ground under a vertical actually does.

Band 40 m
Study time 18
Level Core
questions 10

Waveform and antenna diagram for HF Antenna Systems — HF Antenna Systems — waveform overlay and radiation pattern

Pre-flight briefing

Height sets the takeoff angle

A horizontal dipole radiates mostly straight up when it is low - a quarter wavelength or less above ground - and progressively flattens as it goes higher. A low 80 metre dipole at 10 metres height sends most of its energy at high angles, which is ideal for regional contacts out to a few hundred kilometres; the same antenna at 20 metres puts more energy at low angles and reaches further. This is the mechanism behind NVIS, near vertical incidence skywave, which deliberately uses a low horizontal antenna on 40 or 80 metres to fill in the short-skip gap that a low-angle antenna skips over.

In feet, a half-wave dipole for 7.15 MHz is 468 / 7.15 = 65.5 feet, and a quarter-wave vertical for the same band is 234 / 7.15 = 32.7 feet. Height also changes the feed point impedance: a dipole less than a quarter wavelength above ground shows a lower impedance than the 73 ohms of free space, which is why the same antenna may present 30 ohms in one installation and 70 in another.

Traps, feed lines and matching

A trap is a parallel resonant circuit inserted in a dipole element. On its resonant band the trap looks like a high impedance and effectively cuts the element short, so the outer sections take no part; on lower frequencies it behaves as a loading coil and the whole element radiates. That is how one 40/20/10 metre dipole is three antennas in one.

Feed line loss is where HF stations quietly lose their signal. Open-wire or ladder line has the lowest loss of all and tolerates high SWR, coaxial cable is convenient but lossier, and the loss gets worse as frequency rises. An antenna tuner does not repair any of this: it transforms whatever impedance the line presents into something near 50 ohms so the transmitter is happy, but it does not reduce the loss the line has already imposed, and it does not make a mismatched antenna radiate better.

Beams, chokes and ground systems

A Yagi gains by adding elements and by spreading them along a longer boom: gain rises roughly with boom length, and the front-to-back ratio - how much stronger the pattern is in the forward direction than directly behind - is what keeps a nearby station from being walked on. A rotary beam's advantage over a dipole on 20 metres is typically 4 to 6 dB forward, worth far more than any legal power increase.

Two support systems deserve attention. A 1:1 choke balun at the feed point stops RF from travelling back down the outside of the coax shield, which would otherwise distort the pattern and put RF in the shack. And a quarter-wave vertical needs radials, because the missing half of the dipole has to be replaced by something: a good radial field of 16 to 32 quarter-wave wires is what turns a mediocre vertical into an effective one.

Terms you must be able to define
Term	What the examiner wants to hear
Takeoff angle	The elevation angle at which most of an antenna's energy leaves; lower angles favour long-distance contacts.
NVIS	Near vertical incidence skywave: high-angle radiation from a low antenna, used for regional coverage out to a few hundred kilometres.
Trap	A parallel resonant circuit in an element that isolates the outer section on the higher band.
Front-to-back ratio	The difference in dB between an antenna's forward gain and its response directly behind.
Radial field	Buried or elevated wires that supply the missing half of a quarter-wave vertical's ground return.

Question deck 10 questions

G4A01 Core

What is the approximate length of a half-wave dipole for 7.15 MHz?

G4A02 Core

What happens to a horizontal dipole's radiation pattern as it is raised higher above ground?

G4A03 Recall

What is NVIS propagation?

G4A04 Core

What does a trap do in a multiband dipole?

G4A05 Core

Which type of feed line has the lowest loss on the HF bands?

G4A06 Core

What does an antenna tuner actually do?

G4A07 Core

How can the forward gain of a Yagi be increased?

G4A08 Core

What does the front-to-back ratio of a directive antenna describe?

G4A09 Stretch

Why is a 1:1 choke balun fitted at the feed point of a coax-fed dipole?

G4A10 Stretch

Why does a quarter-wave vertical antenna need a radial field?

Reference sheet

The explanations for all ten questions sit on this sheet. Pick an answer on the left and the matching card lights up.

G4A01 Explanation 1

Key D — 65.5 feet (about 20 m)

468 / 7.15 = 65.5 feet overall, or about 20 metres, which is why a 40 metre dipole fits across a modest suburban lot. The 32.7 foot figure is the quarter-wave vertical for the same band.

Rule Fundamentals - antenna length

G4A02 Explanation 2

Key C — Energy shifts toward lower takeoff angles, favouring long-distance contacts

Ground reflection cancels the downward radiation, so the main lobe tilts upward when the antenna is low and flattens as height increases. Low dipoles are excellent regional antennas and poor DX antennas, and vice versa.

Rule Fundamentals - antenna height and elevation angle

G4A03 Explanation 3

Key B — Near vertical incidence skywave: high-angle radiation that covers a region out to a few hundred kilometres

NVIS relies on a low horizontal antenna on 40 or 80 metres, which radiates steeply and comes back down within a few hundred kilometres - filling the skip zone that low-angle signals leave empty. It is a standard emergency communications technique.

Rule 47 CFR §97.1 - emergency communications capability

G4A04 Explanation 4

Key A — It acts as a parallel resonant circuit that isolates the outer element section on the higher band

On the trap's resonant frequency it presents a high impedance and the outer wire is effectively disconnected, leaving a shorter, higher-frequency antenna; on lower bands it is just a loading inductance. Traps introduce some loss, which is the price of multiband operation from one feed point.

Rule Fundamentals - trapped antennas

G4A05 Explanation 5

Key D — Open-wire or ladder line

Open-wire line has almost no dielectric loss and very little conductor loss, so it outperforms every coax type and tolerates high SWR far better. Its drawback is that it must be kept away from metal and from wet structures.

Rule Fundamentals - feed line loss

G4A06 Explanation 6

Key C — It transforms the impedance presented by the feed line so the transmitter sees a load it can drive

A tuner matches impedances at one point in the system. It does not reduce loss already incurred in the feed line, does not change the antenna's own resonance, and does not add gain - it simply keeps the transmitter happy so that power leaves it instead of folding back.

Rule Fundamentals - matching networks

G4A07 Explanation 7

Key B — By adding elements and lengthening the boom

Gain comes from the physical aperture of the antenna, so more directors spaced along a longer boom mean more gain. Transmitter power is a separate variable and does not change the antenna's gain at all.

Rule Fundamentals - directive antennas

G4A08 Explanation 8

Key A — The difference in dB between the strongest forward response and the response directly behind the antenna

Front-to-back describes rejection of signals behind the antenna, which is what protects you from interference in the opposite direction. It is a separate specification from gain, and a beam can have good gain with a mediocre front-to-back ratio.

Rule Fundamentals - antenna specifications

G4A09 Explanation 9

Key D — To stop RF current from flowing on the outside of the coax shield

Current on the shield is common-mode RF: it distorts the radiation pattern, makes the feed line part of the antenna and brings RF into the shack. A choke balun presents a high impedance to that current without affecting the signal travelling inside the coax.

Rule 47 CFR §97.13(c) - RF exposure in the station

G4A10 Explanation 10

Key D — The radials replace the missing half of the dipole and provide the return path for current

A vertical is half an antenna; the ground system is the other half. With too few radials, ground losses rise, efficiency drops and the feed point impedance becomes hard to match - which is why 16 to 32 quarter-wave radials is the usual practical target.

Rule Fundamentals - vertical antenna systems

How to use this unit

Work the ten questions in one sitting with the reference sheet covered, then uncover it and read every explanation - including the ones you got right. The wording of the real exam is different, so learn the reasoning rather than the answer letters.