Where Should the Antenna Resonance Dip Go?
Where Should the Antenna Resonance Dip Go?
The useful answer begins with the frequencies you operate, the equipment limits you must respect and the reference plane at which you measure.
Where should the dip go: below the band, in the centre or on the frequency you use most? My practical answer is direct: first say what you are optimizing and where the impedance is being observed. Without those two declarations, “put the dip here” is folklore.
Joeri’s short version: measure R + jX, not SWR alone. Choose a target that covers the operating frequencies in proportion to how you use them, respects transmitter, tuner, feedline and component limits, and is defined at a stated reference plane. Then leave enough margin for the installed antenna to move with its environment.
Minimum SWR and Resonance Are Different Questions
At one declared antenna terminal or measurement plane, write the input impedance as:
Z(f) = R(f) + jX(f)
Γ(f) = [Z(f) − Z0] / [Z(f) + Z0]
SWR(f) = [1 + |Γ(f)|] / [1 − |Γ(f)|]
An electrical resonance at that port is normally identified by X = 0. Minimum SWR is the frequency where the complete complex impedance is closest to the declared reference impedance Z0 in reflection-magnitude terms. Those frequencies coincide only in a suitable case. A purely resistive 90 Ω load is resonant at that plane but is not a 1:1 match to 50 Ω; a nearby complex impedance can produce the lower SWR.
That is why an SWR dip must not be labelled “the resonance” without inspecting the complex trace. It is also why low SWR does not prove radiation efficiency. Conductor, loading-coil, transformer, ground and feedline loss can all absorb power while making the impedance look less extreme.
The Best Frequency Depends on the Objective
There is no general requirement that X = 0, minimum SWR or any other single marker sit at the geometric centre of an amateur band. The target follows the service you want from the system.
| Declared objective | Useful tuning criterion | Boundary that still matters |
|---|---|---|
| One narrow operating segment | Place the best combined impedance, tuner and transmitter condition near the frequencies actually used. | Leave margin for environmental movement and confirm the full occupied signal bandwidth. |
| Frequent operation across a band | Minimize the worst relevant constraint or use a frequency-weighted score, rather than chasing the deepest single dip. | Check every band edge and any frequency where the tuner, amplifier or feedline sees high voltage, current or loss. |
| Operation without a tuner | Keep the load within the transmitter manufacturer’s permitted impedance or SWR region over the required frequencies. | Foldback thresholds and continuous-duty capability are radio-specific. |
| Operation through a tuner | Evaluate tuner loss, repeatability, component voltage/current and temperature at the tuner’s actual input and output planes. | “The tuner found a match” proves neither low internal loss nor safe stress elsewhere. |
| Multiband operation | Use a compromise across all required bands, with separate weights and limits for each. | One physical adjustment can move several resonances differently or change coupling between elements. |
If the complete system already satisfies the declared limits over every required frequency, moving a dip to make the graph look tidier has no engineering benefit. Conversely, a beautiful centre-frequency dip is not enough when the band edge triggers transmitter foldback, excessive tuner loss, component heating or an unacceptable common-mode current.
“Slightly Below the Band” Is Not a Universal Rule
A simple isolated wire near one resonance may pass from capacitive through zero reactance to inductive as frequency rises. That local behaviour cannot be applied blindly to trapped, loaded, folded, coupled, off-centre-fed or multiband structures. Nearby resonances and matching networks can reverse slopes or create several crossings.
Nor is a positive reactance automatically easier or safer than a negative one. A matching network transforms the complete complex load. Depending on topology, component values and reference plane, either case can produce high circulating current, capacitor voltage, inductor loss or a boundary outside the tuner’s range. Use the tuner and transmitter manuals, and measure temperature and stability under the intended waveform and duty cycle.
If a local sweep shows that moving one antenna’s resonance slightly below one operating segment gives the best bounded result, use it—and record the case. Do not promote that result into a trimming rule for every antenna.
The Reference Plane Changes What the Trace Means
A shack measurement reports the impedance of the installed antenna through the feedline, connectors, chokes and any intervening network. When the load is mismatched, the transmission line transforms the complex impedance with electrical length. Loss changes the magnitude as well as the phase and can make the shack SWR look closer to 1:1 while power is being dissipated in the line.
For feedpoint impedance, calibrate the analyzer at the feedpoint connector when practical. If the calibration plane must remain elsewhere, de-embed a characterized cable or fixture with a method appropriate to its loss, mismatch and delay. A simple port extension corrects delay only under its stated line model; it is not permission to pretend that an unknown lossy feedline has disappeared.
Keep both traces when they answer different questions. A calibrated feedpoint trace describes the antenna terminal. A shack trace describes the load presented to equipment through the installed line. Neither should be relabelled as the other.
Rohde & Schwarz and RigExpert both place calibration standards at the intended device reference plane for accurate antenna impedance measurements. Keysight’s calibration and fixture guidance makes the same boundary explicit: de-embedding requires a known model or measured network, while simple reference-plane extension has assumptions about delay, loss and characteristic impedance.
Common Mode Can Move the Apparent Answer
An antenna analyzer is part of the connected structure. Its case, test lead, operator and feedline exterior can provide an RF-current path. If the trace changes when the coax route, mast bond, analyzer position or choke location changes, the measurement is telling you that the system boundary changed—not merely that the radiator became a different length.
Before final trimming, map net current on the outside of coax and other accessible branches with a characterized clamp probe at low safe test power. Stabilize the intended return path, cable route and choke boundary, then repeat the complex-impedance sweep. A choke-induced move in SWR is evidence that the exterior path was coupled; lower SWR alone does not rank pattern, loss or radiation efficiency.
Trim the Geometry You Actually Have
For a simple symmetric centre-fed wire, folding or trimming equal amounts at both ends is a useful way to preserve the intended symmetry while moving the resonance. Make small equal changes, record the total electrical change and measure again. Folded-back wire remains electromagnetically present, so verify the final result rather than assuming a fold is identical to a cut.
That method is not universal. A fan dipole can couple between branches. A trapped or loaded antenna may require adjustment in a prescribed order. An off-centre-fed antenna has intentional geometric asymmetry. A loop, matching stub or multi-element array can respond to position and spacing as strongly as to total length. Follow the known topology, change one controlled variable and preserve an A/B/A record.
Environment Is Part of the Final Impedance
Height, ground properties, wet foliage, ice, rain, supports, nearby metal and cable routing can move resistance, reactance and common-mode current. Commission the antenna in its final geometry, record the conditions and revisit the sweep after representative environmental changes. A target placed on an equipment limit in dry weather has no useful margin.
Do not hold conductors or stand inside the antenna’s near field to obtain a convenient trace. De-energize and isolate every transmitter before changing wiring. Protect the analyzer from other transmitters, static charge and developing storms in accordance with its manual and the station safety plan.
A Concise Commissioning Workflow
- Write the operating requirement. List the actual frequencies, modes, occupied bandwidths, power and duty cycle; weight them by real use.
- Declare the planes and limits. Record Z0, calibration plane, feedpoint and shack planes, transmitter foldback boundary, tuner range and component thermal/voltage/current constraints.
- Stabilize the installation. Put the antenna, supports, feedline, choke, bonding and intended return path in their final geometry.
- Calibrate and verify. Perform the analyzer’s open-short-load procedure at the chosen plane with suitable standards, then check a known load before connecting the antenna.
- Save complex data. Record R, X, |Γ| or return loss and SWR over a span wider than the operating segment; note resolution, cable state and weather.
- Check the exterior-current path. Repeat current and impedance observations after one controlled cable or choke A/B/A change.
- Adjust one variable. For a symmetric simple wire, trim or fold both sides equally; use the antenna-specific procedure for coupled, loaded or asymmetric structures.
- Verify the complete system. Recheck feedpoint and shack traces, tuner solution, transmitter behaviour, common-mode current and temperature at the intended power and duty cycle.
- Repeat after drift. Confirm representative wet, dry and seasonal conditions and keep margin to every limit.
Primary and Authoritative References
- IEEE 145-2025 — Standard definitions for antennas and antenna systems
- Rohde & Schwarz — Vector-network-analyzer antenna measurement, complex impedance and reference-plane calibration
- Rohde & Schwarz — VNA calibration methods and standards
- Keysight — Measurement calibration and fixed reference planes
- Keysight — Calibration, port extension and network de-embedding boundaries
- RigExpert AA-650 ZOOM manual — Open-short-load calibration at the intended reference plane
- ARRL — Understanding Your Antenna Analyzer notes and feedpoint-versus-feedline examples
- ARRL — Transmission-line transformation, loss and SWR references
- Roy Lewallen, W7EL — Intended and exterior coax-current paths
Joeri’s Bottom Line
Do not begin by deciding that the dip belongs below the band or in its centre. Begin with the operating frequencies, required bandwidth, system limits and measurement plane. Inspect R + jX, control common mode, account for the feedline, and adjust the actual antenna topology in small documented steps.
The right answer may be a dip near your busiest frequency, a deliberately broader compromise, or no further trimming at all. What makes it right is not its position on a pretty SWR plot; it is that the complete installed system meets the declared requirement with measurable margin.
Mini-FAQ
- Is the lowest-SWR frequency always the antenna resonance? No. Resonance is normally where X equals zero at a declared port. Minimum SWR is where the complete impedance produces minimum reflection relative to the declared Z0.
- Should the resonance always be at band centre? No. Choose a target from the frequencies and bandwidth you use, then respect transmitter, tuner, feedline and component limits across that range.
- Should I place the SWR dip slightly below the band? Only if measurements on that specific installed system show it best satisfies the declared objective. Reactance slope and matching-network stress are not universal.
- Does a shack measurement show feedpoint impedance? Not unless the intervening feedline and network have been accurately removed. A shack trace normally shows their transformed, lossy combination with the antenna.
- Should I trim both sides of a dipole equally? For a simple symmetric centre-fed wire, equal small changes help preserve symmetry. Loaded, coupled, trapped or asymmetric antennas need their topology-specific procedure.
- Why did the trace move when I rerouted the coax? The coax exterior or analyzer setup may have been part of the RF current path. Stabilize that boundary and check common-mode current before final trimming.