Full-Wave Antennas Aren’t Bad
Full-Wave Antennas Aren’t Bad
A centre-fed full-wave dipole can be awkward on 50 Ω coax. That does not make a delta loop, a quad element or a horizontal sky loop a bad antenna.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
Someone sent me a YouTube video comparing half-wave and full-wave antennas. The demonstration was reasonable, and the observations made sense for the antennas being compared. The problem came with the conclusion.
Both antennas were straight, centre-fed dipoles, yet the result was used to suggest that full-wave antennas in general are bad. That is a much broader claim than the comparison supports. A difficult centre feed does not condemn a full-wave loop, a delta loop, a quad element or a horizontal sky loop.
There is a useful lesson in that video: a centre-fed, one-wavelength dipole is usually an awkward direct load for a system built around 50 Ω coax. I have no argument with that. My argument is with turning a particular feedpoint problem into a verdict on an entire family of antennas.
“Full Wave” Is a Length, Not a Topology
Electrical length describes phase change along a conductor or current path. It does not by itself specify the conductor shape, feed position, current distribution, impedance or radiation pattern. Expecting everything containing a wavelength of wire to behave alike is rather like expecting a bicycle wheel and a clothesline to behave alike because both contain wire.
| Structure | Feed condition | Basic consequence |
|---|---|---|
| Straight dipole, centre fed | Feed is near a current node | Very high and length-sensitive impedance |
| Straight dipole, fed near a current maximum | Feed moved roughly a quarter wavelength from an end | More manageable impedance with essentially the same ideal current pattern |
| Closed loop at its fundamental mode | Two-terminal feed in a continuous conductor | Usually moderate feed resistance and broadside radiation |
| Folded half-wave dipole | Two closely coupled conductors connected at their ends | Half-wave span and pattern despite roughly one wavelength of total wire |
| Long wire, V-beam or rhombic, often several wavelengths long | Standing-wave or travelling-wave operation | Directional lobes set by length, angle, height and termination |
That is why “it uses one wavelength of wire” cannot predict whether an antenna is easy to match, efficient, directional, quiet on receive or suitable for a particular path.
Why the Half-Wave Dipole Is Convenient
A thin half-wave dipole in free space has a current maximum near its centre and current nodes at its ends. The ideal feed resistance is about 73 Ω; practical trimming to remove reactance and the influence of height, diameter and surroundings commonly move the input elsewhere in the broad 50–75 Ω region.
Its ideal free-space directivity is about 2.15 dBi. The broadside figure-of-eight pattern is easy to understand, and a balanced feedpoint can be connected to coax through suitable common-mode control. These properties make it convenient—not uniquely capable of efficient radiation.
The Rohde & Schwarz Antenna Basics guide separates directivity from gain and emphasizes that radiation resistance must be associated with a stated location, commonly the feedpoint or a current maximum.
The Centre-Fed Full-Wave Dipole
This is the difficult antenna in the reader’s comparison. Its feedpoint is not in the same electrical situation as the centre of the half-wave dipole, even though both wires are connected in the middle.
For an ideal thin straight dipole of exactly one wavelength, the standing-wave current has:
- current nodes at both open ends;
- another current node at the centre; and
- current maxima approximately one quarter wavelength in from each end.
Putting the feed terminals at the centre therefore puts them at a voltage maximum and current minimum. In the infinitesimally thin, exact-length model, terminal current tends to zero and the centre-feed radiation resistance tends to infinity. A real wire has finite diameter, a feed gap, losses, supports and imperfect electrical length, so the measured impedance is finite—but it can still be thousands of ohms and can change rapidly with frequency and surroundings.
The feedpoint reference matters: the often-quoted roughly 200 Ω radiation resistance for a full-wave straight dipole is associated with a current-maximum reference or a practical off-centre feed. It is not the limiting impedance of an exactly one-wavelength, ideal centre feed.
Virginia Tech’s Radio Systems Engineering text explicitly distinguishes the centre terminals from terminals placed at a current maximum. Its full-wave dipole example makes the practical point: changing the feed position can make the impedance manageable without throwing away the useful ideal radiation pattern.
Connecting this high-impedance point directly to 50 Ω coax creates severe mismatch. The transmitter may reduce power, a mismatched lossy feedline can dissipate more of the available power, and high RF voltage stresses insulation and matching components. These are real disadvantages. They explain why the half-wave dipole is the easier choice in this comparison; they do not establish poor radiation efficiency for every full-wave design.
A high feed resistance does not automatically mean high loss. Radiation resistance and loss resistance must be referred to the same feed current. The useful comparison is accepted power and realised gain after matching, feedline, conductor and environmental losses are included.
The Full-Wave Dipole Pattern
A straight full-wave dipole still has broadside maxima in the ideal free-space model. Its beam is narrower than that of a half-wave dipole, and its ideal directivity is about 3.8 dBi rather than 2.15 dBi—roughly 1.65 dB more directivity.
That number is not guaranteed installed gain. Directivity describes how radiated power is redistributed in angle. Gain includes radiation efficiency, and realised gain also includes mismatch at the stated reference plane:
G = ηradD
Grealised = ηmismatchηradD
The full-wave dipole concentrates somewhat more power broadside and less elsewhere; it creates no free power. The analytical pattern for a finite centre-fed wire is developed in this University of Texas treatment of linear antennas.
Moving the feedpoint near one of the current maxima makes impedance transformation more practical without fundamentally changing the ideal standing-wave distribution. In a real installation, however, the asymmetric feed, transformer, feedline exterior and nearby conductors must be included because unwanted common-mode current can alter pattern and impedance.
Close the Wire into a Loop and the Circuit Changes
Here is where the video’s generalisation breaks down. Bend roughly a wavelength of conductor into a closed loop and you have a different antenna: the boundary conditions and current distribution change. The difficult centre node of the straight dipole no longer describes its feed.
Near the fundamental loop resonance, common square, circular and triangular loops often have feed resistance somewhere around 100–150 Ω. The exact impedance depends on shape, conductor diameter, feed gap, feed position, height, ground and nearby objects. A geometric perimeter of exactly one free-space wavelength is not a universal cutting length; trim or model the loop in its installed environment.
A full-wave loop normally has two broadside lobes perpendicular to its plane. A Virginia Tech resonant-loop study calculates approximately 3.4 dBi directivity for its one-wavelength circular loop, compared with 2.15 dBi for a half-wave dipole. That is a modest, shape-dependent difference—not an installed-gain promise. It is nevertheless a perfectly useful radiation pattern, not evidence of a defective antenna.
Horizontal Full-Wave Loops
A horizontal loop well below half a wavelength over real ground often favours high elevation angles. That can suit near-vertical-incidence skywave and regional HF work when the ionosphere supports those paths.
“Low loop equals NVIS” is still too simple. Very low height can increase ground interaction and loss; the exact elevation pattern depends on height in wavelengths, soil, shape and feedline current. NVIS coverage additionally depends on frequency relative to ionospheric critical frequency, absorption, time and path—not antenna angle alone.
A Naval Postgraduate School study modelled a one-wavelength horizontal quad loop for NVIS and medium-range HF communication. It supports the use case, not a universal height or coverage promise.
Raise the loop and lower-angle lobes become more important. Use it on higher bands, where its perimeter is multiple wavelengths, and the azimuth and elevation patterns develop additional lobes and nulls. Such a loop is not omnidirectional across all bands.
Multiband use can work well with low-loss balanced line and a suitably rated balanced matching system. A shack tuner can provide a comfortable transmitter impedance, but it cannot recover power already dissipated in a long, high-SWR coax run.
A horizontal loop may receive less local noise in one installation because of pattern, polarisation, balance or reduced feedline pickup. It is not inherently “quiet.” A loop with common-mode feedline current can collect plenty of household noise.
Vertical Delta and Quad Loops
Turn the loop vertically and its broadside lobes can support useful DX directions. The loop plane sets the two main azimuth headings, while feed position strongly influences polarisation.
- Feeding the centre of a horizontal side of a square or rectangular loop commonly favours horizontal polarisation.
- Feeding the centre of a vertical side commonly favours vertical polarisation.
- A bottom-centre-fed delta commonly favours horizontal polarisation.
- A lower-corner or appropriate quarter-wave-offset delta feed commonly favours vertical polarisation.
These are design starting points, not guarantees. Shape, height, feed gap, unequal current, ground and feedline common mode can create mixed polarisation and distorted patterns.
A closed loop has a two-terminal conductive antenna path and does not need a radial system in the way a ground-mounted monopole does. That does not make it immune to common-mode current. If the feed transition is unbalanced or the environment is asymmetric, the coax exterior can still become part of the antenna. Use suitable common-mode impedance and verify outside-coax current.
The earth remains part of the boundary. A vertical loop near lossy soil can experience pattern, feed-impedance and efficiency changes even though no radials are required.
Full-Wave Loops as Beam Elements
A cubical quad is an especially concrete answer to the claim that full-wave antennas are bad: its driven element is a full-wave loop by design. A somewhat larger loop can serve as a reflector and smaller loops as directors. Delta-loop arrays use the same broad principle with triangular conductors.
Forward gain and front-to-back ratio come from the coupled element currents and their phases, not from the word “full wave.” Element spacing, circumference, conductor diameter, boom, feed and surrounding structure determine the array.
The ARRL General Class study guide introduces quad and delta-loop arrays and the effect of feed position on polarisation. ARRL’s HF loop antenna collection provides practical examples.
The trade-off is often mechanical: more conductor, larger spreaders, greater wind and ice load, more support points and a larger turning radius. Electrical merit does not solve an unsafe structure.
The Folded Dipole Proves That Wire Counting Fails
A conventional folded dipole uses two closely spaced half-wave conductors joined at their ends. Its total conductor length is roughly one wavelength, but its end-to-end span and pattern remain those of a half-wave dipole.
For equal conductor diameters and close spacing, the feed impedance is approximately four times that of the corresponding ordinary dipole—often near 300 Ω in the ideal free-space case. Unequal diameters, spacing, height and nearby structures change the transformation ratio.
The same MIT antenna material explains the coupled-conductor relationship. The folded dipole is not a one-wavelength straight dipole; that is precisely the point. Counting the copper cannot tell us which current mode we have built.
Long Wires, V-Beams and Rhombics
Antennas several wavelengths long deliberately use phase progression along their conductors to create directional lobes. Standing-wave long wires and unterminated V-beams are frequency sensitive; terminated travelling-wave designs can provide broader impedance and pattern behaviour but dissipate some accepted power in the termination.
That resistor loss is intentional and must be included in efficiency. The directional pattern may still provide useful realised gain in the wanted direction, but “low SWR” does not make termination power radiated power.
The U.S. Army HF antenna manual documents field-expedient long-wire, V and half-rhombic systems. Their successful use makes the broader point: passing half a wavelength does not make a conductor defective.
Matching Does Not Erase Feedline Loss
Any of these antennas can be efficient and still present an inconvenient port impedance. Matching and loss must be separated:
- Antenna impedance is the complex terminal ratio at the defined feed plane.
- Mismatch describes power not accepted at that plane for the connected source or line.
- Feedline attenuation dissipates power as waves travel in both directions on a mismatched lossy line.
- Matching-network loss dissipates power in conductors, capacitors, cores and contacts.
- Radiation efficiency compares radiated power with accepted power after antenna-system losses at the chosen boundary.
Place the impedance transformation where it yields the lowest safe total loss. That may mean a feedpoint network, an intentional transmission-line transformer, or low-loss balanced line to a remote tuner. It does not mean every high impedance is harmless: voltage rating, current, insulation, arcing, common mode and bandwidth still matter.
Which Would I Actually Put Up?
If I want a light, simple antenna with a broad useful pattern and an easy coax feed, a half-wave dipole is an excellent answer. Its convenience is an engineering advantage. There is no need to pretend that a longer wire must be better.
If the supports suit a loop and the job is regional coverage, a horizontal full-wave loop can be a very sensible answer. For two preferred DX headings, a suitably installed vertical loop may fit the job. For a directional array, a driven full-wave quad or delta element is a legitimate design choice. These advantages come from the current distribution, feed and orientation—not from collecting more copper.
A fair comparison includes the matching system and feedline, height, ground, polarisation and the directions that matter. Compare equal accepted antenna power to study the radiators themselves; include all upstream losses when comparing complete stations. Do not judge a high-angle regional antenna solely by a low-angle DX result, or call a difficult 50 Ω connection proof that the wire cannot radiate.
The Lesson Is About the Feed, Not a Bad Wavelength
The reader-sent video is useful when its conclusion stays with the antennas it compared. A centre-fed, one-wavelength straight dipole is usually a poor direct match for 50 Ω coax. That is true and worth knowing. It is not evidence that full-wave antennas are inherently bad.
Put that wavelength of conductor into a suitable shape, feed it correctly and install it for the paths you want, and it can be an excellent HF antenna. A loop does not need to win every comparison to disprove a blanket dismissal; it needs to do its intended job well.
The wavelength is not the problem. The design—and the conclusion we draw from it—is what matters.
Technical references
- Virginia Tech — Radio Systems Engineering, dipoles and current-maximum feeding
- Rohde & Schwarz — Antenna Basics
- MIT OpenCourseWare — Receivers, Antennas and Signals
- Virginia Tech — resonant loop geometry and performance study
- Naval Postgraduate School — horizontal full-wave loop for NVIS and medium-range HF
- U.S. Army ATP 6-02.53 — long-wire, V and half-rhombic HF antennas
Mini-FAQ
- Are full-wave antennas inefficient? No. Electrical length alone does not set efficiency; conductor, ground, matching, termination and feedline losses do.
- Why is a centre-fed full-wave dipole difficult? At exactly one wavelength its centre approaches a current node and voltage maximum, so feed impedance becomes extremely high.
- Is a full-wave dipole 200 Ω? Roughly that scale can apply when referenced to a current maximum. It is not the ideal exact centre-feed impedance.
- Does a full-wave loop have much more gain than a dipole? No. The cited circular-loop model has about 3.4 dBi directivity versus 2.15 dBi for a half-wave dipole. Installed gain also depends on loss and surroundings.
- Does a vertical full-wave loop need radials? It does not need a monopole radial system, but ground and feedline common mode can still affect loss and pattern.
- Can a full-wave loop work on several bands? Yes, often with low-loss balanced line and a suitable tuner, but higher-band operation produces multiple lobes and nulls.