Yuriy Fuchs, AC6A
The Nonresonant Doublet and PA0FRI S-Match on 80 Meters
Our ARRL Field Day 2026 site included 11 co-located transmitters, creating a demanding RF environment for antenna testing. This project used a nonresonant 80-meter wire doublet, low-loss open-wire feed line, a homebrew PA0FRI S-match tuner, a 1:1 coaxial choke balun, and a multi-pole band-pass filter.
The purpose was to challenge a common assumption: an antenna does not have to be cut to a resonant half-wave length to be effective. Resonance can simplify a coax-fed, single-band installation, but it is not the same as radiation efficiency, radiation pattern, or field performance. A nonresonant antenna can work well when the radiator, feed line, matching network, common-mode control, filtering, and installation are engineered as one system.
The Antenna System
The antenna was a wire doublet about 50 m long, installed as a slight inverted-V with an apex approximately 12 m above ground. The installation was neither perfectly horizontal nor symmetrical.
At 3.59 MHz, the wire was approximately 0.60λ long—modestly longer than a half-wave dipole. It was fed with approximately 15 m of open-wire ladder line.
The S-match was installed at the station-side end of the ladder line. It transformed the impedance of the antenna-and-feed-line system to approximately 50 Ω. Its matched output passed through a 1:1 coaxial choke balun, then through approximately 30 m of coaxial cable to the band-pass filter and transceiver.
The ladder line was central to the design. A nonresonant antenna can produce high SWR on its feed line. In coaxial cable, high SWR can substantially increase loss; low-loss open-wire line is more tolerant of high SWR. The S-match at the station-side end of the ladder line transformed the antenna-and-feed-line impedance to approximately 50 Ω.
FIELD-DEPLOYMENT ADVANTAGE
A conventional resonant dipole may require precise measurement, raising the antenna, checking resonance, lowering it for trimming, and repeating the cycle. We selected a mechanically convenient wire length that fit the available supports, installed it once, and completed final matching from the operating position. |
Measurement still matters. Wire length, feed-line length, installation height, and station-end impedance should be documented. But exact trimming to a resonant frequency was not a prerequisite for placing the antenna in service.
The same wire-and-ladder-line system can be used on multiple amateur bands when paired with a tuner having sufficient matching range. Each band presents a different impedance, efficiency, and radiation pattern, so multiband operation must be evaluated band by band.
Before deployment, these tradeoffs can be evaluated with antenna-modeling software such as MMANA-GAL or EZNEC. Model the actual wire lengths, antenna geometry, installed height, and representative ground conditions. The software can estimate feed-point impedance, SWR, gain, and radiation pattern on each intended band. Modeling does not replace field measurements, especially where local conditions differ from the model. It is an efficient way to identify extreme impedances and unacceptable compromises before the antenna is raised.
Resonance Is Not PerformanceTable 1—Resonance, impedance match, and radiation efficiency describe different properties of an antenna system.
Property | Meaning |
Resonance | Little or no net reactance at a specified point and frequency |
Match | The transmitter sees an acceptable load, typically near 50 Ω |
Efficiency | The fraction of accepted power radiated rather than lost in wire, ground, feed line, or matching components |
A tuner does not make the wire radiator resonant. It transforms the impedance presented by the complete antenna-and-feed-line system.
A doublet shorter than 0.5λ becomes increasingly capacitive as it is shortened. Its radiation resistance falls, matching-network current rises, and system losses become more important. A moderately shortened wire can nevertheless be practical when low-loss balanced line and a capable tuner are used.
A longer doublet has a different tradeoff. As length increases beyond 0.5λ, its current distribution and radiation pattern change. Directivity may increase in selected directions, but the pattern develops narrower lobes and deeper nulls.
The PA0FRI S-Match
The S-match transformer provides galvanic isolation between the transceiver-side circuit and the balanced antenna-side circuit. The added coaxial choke serves a different purpose: it presents high impedance to common-mode RF current flowing on the outside of the coax shield. With approximately 30 m of coax between the tuner and radio, the choke reduced the likelihood that the outer conductor, station equipment, and attached wiring would become unintended parts of the antenna system.
The matching network was a homebrew S-match based on the design by Frits Geerligs, PA0FRI. The S-match is a balanced tuner using a variable capacitor, variable inductor, and transformer arrangement. It is intended for balanced feed systems and provides galvanic isolation between the transceiver-side and antenna-side circuits.
Our implementation retained the basic PA0FRI S-match topology and added a 1:1 coaxial common-mode choke at its 50 Ω output.
The tuner used:
- A roller inductor for continuous adjustment.
- A T-300A-2 powdered-iron toroidal core.
- A 1:1 coaxial choke balun at the 50 Ω output.
The S-match was selected for its broad practical matching range. We have also used the same topology with nonresonant verticals and loop antennas. This does not mean that all antennas have equal efficiency or the same radiation pattern. It means that many practical antenna-and-feed-line impedances can be matched successfully within the tuner’s component voltage, current, and power limits.
Selectivity and Filtering
The S-match had a high loaded Q and a sharp tuning peak. When adjusted precisely to the operating frequency, it acted as a tuned preselector and attenuated some off-frequency energy before it reached the receiver.
The 1:1 choke balun at the S-match’s 50 Ω coaxial output reduced common-mode RF current entering the approximately 30 m coaxial run to the operating position. A separate multi-pole 80-meter band-pass filter at the transceiver end provided stronger band-selective rejection.
Table 2—The S-match, common-mode choke, and band-pass filter serve separate functions in the 80-meter station.
Device | Function |
PA0FRI S-match | Matches the ladder-line system to 50 Ω and adds narrow, load-dependent tuned-circuit selectivity |
1:1 coaxial choke balun | Adds common-mode impedance at the S-match coaxial output and reduces unintended RF current on the transceiver-side coaxial path |
Multi-pole band-pass filter | Provides dedicated 80-meter filtering and greater rejection outside the intended band |
Together, the tuned S-match, choke balun, and band-pass filter supported operation in the crowded Field Day environment. The narrow S-match tuning was useful on the selected digital frequency, but moving to the phone portion of 80 meters required retuning the roller inductor and capacitor.
The digital station operated alongside 10 nearby transmitters without observed interference that prevented normal operation.
Field Results
The system was used primarily for 80-meter digital operation. The 50 m doublet and ladder line were matched smoothly by the S-match at the operating frequency. Despite its nonresonant physical length and imperfect field geometry, the system provided reliable communication in the active Field Day RF environment.
KEY ENGINEERING POINT
A nonresonant doublet can be an effective antenna when paired with open-wire line, a wide-range balanced tuner, common-mode control, and appropriate station filtering. |
Conclusion
A resonant antenna remains a simple solution when the site permits it. But temporary installations rarely offer ideal supports, ideal height, perfect geometry, or unlimited time.
This project showed that resonance should not become a deployment barrier:
- Use a wire length and geometry that fit the site safely; a wide-range balanced tuner avoids repeated lowering and trimming for one resonant frequency.
- When substantial mismatch is expected, use low-loss balanced line for the high-SWR section, or place the matching network at or near the antenna feedpoint so the long coaxial feed line operates close to 50 Ω.
- Model the actual installation with MMANA-GAL, EZNEC, or comparable software before deployment, then verify key assumptions with an antenna analyzer and on-air operation.
- Check each intended band separately for impedance, gain, and radiation pattern.
- Use a properly characterized 1:1 choke balun to reduce common-mode current on the coaxial transceiver-side path.
- Use dedicated band-pass filtering where required at dense multi-transmitter sites.
The 50 m doublet, open-wire line, S-match, choke balun, approximately 30 m of coaxial cable, and multi-pole band-pass filter formed a practical 80-meter Field Day system without requiring an exact resonant wire length or repeated trimming at height. The same antenna can be evaluated for additional bands, provided its impedance, matching range, radiation pattern, and component stresses are considered separately.
A nonresonant antenna is not automatically a poor antenna. It is an antenna whose complete system must be engineered deliberately.
Suggested Figures
Figure 1. The 50 m doublet installed as a slight inverted-V. Field constraints determined the final wire geometry.
Figure 2. System block diagram: 50 m doublet, 15 m open-wire ladder line, PA0FRI S-match, 1:1 coaxial choke balun, approximately 30 m coaxial cable, multi-pole 80-meter band-pass filter, and transceiver.
Figure 3. Interior view of the S-match, showing the roller inductor, variable capacitor, transformer, and balanced feed-line terminals.
Figure 4. Simplified S-match schematic, including the transformer, variable capacitor, roller inductor, transceiver input, and balanced output.
References
1. F. Geerligs, PA0FRI, “S-Match Universal Antenna Tuner.” http://www.pa0fri.com/ATU/Smatch/smatcheng.htm
2. F. Geerligs, PA0FRI, “Travel S-Match Antenna Tuner.” https://www.pa0fri.com/ATU/TravelS-match/TravelS-matcheng.htm
3. American Radio Relay League, “Transmatch/Antenna Tuner.” http://www.arrl.org/transmatch-antenna-tuner
4. American Radio Relay League, “Understanding SWR by Example.” https://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf
5. American Radio Relay League, “Antenna Modeling.” http://www.arrl.org/antenna-modeling
6. W7EL, “EZNEC Antenna Software.” https://eznec.com/
7. MMANA-GAL, “MMANA-GAL Basic.” http://gal-ana.de/basicmm/en/
8. DJ0IP, “Open-Wire-Fed Antenna.” https://www.dj0ip.de/open-wire-fed-ant/
9. OnAllBands, “What Does a Common-Mode Choke Do?” https://www.onallbands.com/ham-radio-tech-what-does-a-common-mode-choke-do/
The tuner used:
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