Phased and Parasitic Arrays: Measure Currents, Loss and Pattern

Phased and parasitic arrays are established antenna techniques. A two-element system can reinforce radiation in one direction and cancel it in another. The engineering question is not whether arrays work; it is whether the built array achieved the modeled current, loss and field pattern being claimed.

1. Separate the Model, Match, Currents and Pattern

A useful array design may include precise modeled gain, front-to-back ratio, efficiency and SWR values. A physical build may also show a good input match. Those results answer different questions and should be reported separately.

Precise installed gain and rejection require the corresponding measurement chain. A model is a prediction, a match trace describes one port, current phasors describe achieved excitation, and a calibrated field test checks the resulting beam.

2. Cable Phase Is Not Element-Current Phase

A cable with electrical length 0.144 wavelength has about 51.8° of one-way propagation phase under its stated velocity factor. That is useful design information. It is not automatically the phase between currents entering two mutually coupled antennas.

For a two-element driven system:

Z₁₁ and Z₂₂ are the self terms in the assembled array; Z₁₂ and Z₂₁ describe mutual coupling. The terminal currents result from solving the coupled network with the real feedlines, matching devices and terminations. An element that measures 50 Ω alone can present a very different driving-point impedance when its neighbor is excited.

Line attenuation, mismatch and multiple reflections further alter terminal magnitude and phase. A fixed cable’s phase changes with frequency, and velocity factor can be frequency- and construction-dependent. A multiband claim therefore needs band-specific network data rather than one nominal phase label.

The QEX report on G3NPC’s 21 MHz four-square demonstrates a defensible workflow: measure isolated and mutual impedances, characterize cable impedance, length, velocity factor and attenuation, solve the coupled feed network, then measure the actual element-current magnitudes and phases. Even deliberate engineering did not make every measured current equal the ideal target.

3. Terminal Currents Are Necessary, but Not the Whole Field

For identical elements in simple array-factor teaching, the field is often written schematically as a weighted sum. In a real mutually coupled array, a more careful form uses each element’s embedded pattern:

The complex coefficient a_n represents the achieved excitation under the chosen normalization, while F_n,embedded includes geometry, environment and mutual coupling. For wire antennas, the current distribution along each conductor matters—not only one terminal-current sample.

Measured terminal phasors are therefore powerful validation evidence, but they do not replace accurate element geometry, ground, support and feedline modeling. If the physical elements, radial systems or nearby environment differ, identical terminal currents need not produce the ideal embedded patterns.

4. Phase Needs a Convention

“52 degrees” is incomplete unless the documentation states:

• the e^jωt or e^−jωt time convention;
• the reference element and whether positive means lead or lag;
• which physical path contains the delay;
• the voltage/current reference planes;
• the current-reference direction at each terminal; and
• whether the value describes source voltage, line propagation or element current.

Changing the time convention reverses a phase sign without changing the antenna. Moving the delay to the other element reverses the preferred direction. Reversing a probe reference adds 180°. These are ordinary conventions, but missing conventions make a result impossible to reproduce.

5. Parasitic Elements Still Need Current Evidence

A parasitic array removes the separate drive cable from the reflector or director. It does not remove the current requirement. If element 2 is passive and its applied terminal voltage is zero:

NBS/NIST Technical Note 1082 develops this driven/reflected-current relationship and combines the fields to obtain the array response. The passive current depends on mutual impedance and the passive element’s complex self impedance. Length is only one influence.

A slightly longer element often acts as a reflector and a shorter one as a director in a conventional Yagi-like geometry. That is a useful heuristic, not a current measurement. Radial geometry, soil, termination, mast, support, coax exterior, nearby conductors and small dimensional errors can all change the induced current and pattern.

6. Deep Nulls Are the Fragile Part

Forward gain and a deep rear null do not have equal sensitivity. The null depends on near-cancellation. A small amplitude or phase error can leave a residual field far above the ideal prediction while the forward lobe changes only modestly.

Front-to-back and front-to-side figures therefore need:

• current magnitude and phase accuracy;
• element, radial and feed-path symmetry;
• controlled common-mode current;
• defined azimuth, elevation and polarization;
• adequate receiver dynamic range and noise-floor margin; and
• a measured angular sweep rather than only “front” and “back.”

A normalized NEC cut can predict where an ideal null should occur. It cannot establish that the built array achieved the cancellation.

7. The Feed Network Has a Power and Phase Budget

A practical phased-array feed system may include a T or power divider, switching box, feed cables, phase lines and matching networks. Connectors, chokes, relays and terminations can each add attenuation, mismatch, imbalance, phase error and common-mode coupling.

When the factors are separable and have not already been counted, a main-input result can be written:

In decibels, the multiplicative efficiencies become additive loss terms. The active IEEE 145-2025 antenna terminology standard is the current formal reference, but the article must still declare whether its “gain” excludes mismatch, includes the feed network or is merely a relative improvement over another antenna.

Swanson’s four-square is an instructive example, not a prediction for the PERformer. He measured about 2 dB loss from splitter input to element inputs, or roughly 63% feed efficiency. Combined with about 70% element efficiency, he reported about 42% effective array efficiency at the feedpoint. The lesson is that array-network loss can be material and must be measured.

8. Reference Plane First, Gain Second

Possible power reference planes include the transmitter output, main array input, divider outputs, individual element terminals and the sum of accepted element powers. They produce different loss budgets.

• Directivity is pattern concentration relative to total radiated power.
• Gain includes antenna radiation loss relative to accepted power under the declared antenna boundary.
• Realized gain also includes input mismatch at the stated port.
• Complete-system realized gain from the main connector must include feed-network loss between that connector and the elements.

Absolute gain in dBi and relative improvement over a reference antenna in dB are both useful, but they are not the same quantity. Each needs its own label, stated reference plane and equal accepted-power normalization.

9. SWR Does Not Measure a Beam

An analyser measures reflection at its calibration plane. It does not reveal how current divides between elements, whether the required current phase exists, or where the array radiates.

A 50 Ω SWR trace shows the match at its declared calibration plane. It does not, by itself, establish the input to every divider, switch, phase line and matching-network branch in the physical array.

Several systems can present the same 50 Ω input:

• the intended low-loss array with the intended current ratio;
• a low-loss array with the wrong phase and pattern;
• an imbalanced array with radiating feedlines;
• a lossy network that broadens the match; or
• a matched divider feeding unequal coupled loads.

Low SWR is useful operational evidence. It is not a current-phase meter, direction indicator or gain measurement.

10. What Current Measurements Should Show

For each band, mode and beam direction, report:

• complex terminal current normalized to one named element;
• phase sign and probe-current direction;
• coupled driving-point impedance at each element;
• accepted power at the main input and element ports;
• complete feed-network loss and switch state;
• coax-exterior common-mode current;
• probe loading, calibration, de-embedding and uncertainty; and
• repeatability after reconnecting or switching the network.

For a parasitic array, measure or model the induced current relative to the driven element and validate its consequence with a pattern. If the coax exterior carries material current, it has become another radiating array conductor and must be included.

11. What a Credible Pattern Test Looks Like

1. Document frequency, geometry, soil, surroundings, polarization and all conductor heights.
2. Demonstrate adequate range distance or quantify the remaining phase-curvature error.
3. Normalize every reading to equal accepted power at the declared main input.
4. Measure enough azimuths to resolve lobes, sides and nulls.
5. Use repeated A–B–A or rapid beam switching to expose drift and propagation changes.
6. Record receiver dynamic range and noise floor so a “deep null” is not the instrument floor.
7. Publish raw readings, corrections, uncertainty and site notes.
8. Use a calibrated reference antenna or traceable range method for absolute gain.

Swanson used stations every 22.5° on a 38 m radius, a field-strength meter with 90 dB dynamic range and all four switchable headings. He put the measured currents back into the model and obtained good pattern agreement. He still identified the estimated absolute gain as requiring comparison with a standard antenna. Pattern shape and absolute gain were not conflated.

12. Reproducibility Checklist

The ARRL Antenna Book support material includes multielement-array resources, transmission-line tools and phased-array model files. That is a useful reminder that the feed system and coupled antenna are one design—not an ideal pattern plus an afterthought cable.

13. Takeaways You Can Trust

• Phased and parasitic arrays can both produce useful directionality.
• The installed pattern depends on actual current distributions and embedded element patterns.
• Mutual coupling means cable phase is not automatically terminal-current phase.
• A parasitic element’s nominal length influences current but does not measure it.
• An ideal equal split is not a 3 dB heat loss; real excess loss must be measured.
• Deep nulls are especially sensitive to small phase, amplitude and symmetry errors.
• Low SWR validates input match—not current balance, beam direction, rejection or gain.
• A measured pattern validates directionality; a calibrated reference is needed for absolute gain.

In Summary

Nominal geometry and cable arithmetic define a useful design target, but they do not establish the precise current ratio or field pattern of a physical array.

For phased arrays, measure the complex currents, complete feed-network loss and controlled pattern. For parasitic arrays, establish the induced current and measure the pattern. Define the gain reference plane, distinguish dBi from relative dB, and keep model, impedance and field evidence separate.

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