ORA Field Day 2026 – Two Stations, Smarter RF Management
ORA Field Day 2026 – Two Stations, Smarter RF Management
Updated October 2025
For Field Day 2026, we’re taking a new engineering approach: running a 100 W HF transmit/receive station right beside a dedicated wide-band receive site — and making them coexist perfectly. The goal: zero desense, zero lock-up, and clean spectra, even with visitors listening and operators transmitting on 160, 80, and 40 meters simultaneously.
• Inter-station isolation ≥ 60 dB during TX
• RX noise rise ≤ 3 dB during local transmission
• No preamplifier compression or spurious creation at 100 W
Why Two Stations?
Running a separate receive-only site lets operators and visitors explore the entire band landscape while the main station transmits. It’s not just for contest agility — it’s also a public outreach experiment. By offering live listening positions, newcomers can hear propagation in real time and discover what amateur radio is really about, without interrupting ongoing QSOs.
Main TX/RX Station Architecture
The main station uses a non-resonant doublet fed with 600 Ω open-wire line into a balanced tuner. This long, balanced line already acts as a broad, low-Q preselector: off-band signals encounter poor impedance transformation, reducing unwanted drive to the receiver. The result — a cleaner input spectrum even before filtering begins.
A 1:1 current balun with ≥ 5 kΩ common-mode impedance between tuner and filter keeps everything symmetrical. Equal leg lengths and a straight, balanced feedline prevent common-mode current from wandering back toward the shack.
Layered Selectivity – The Band-Pass Filter
After the tuner, a three-band 50 Ω BPF module provides sharp, switchable preselection for 160, 80, and 40 m. Each section is a 7-pole Chebyshev or quasi-elliptic network with ≤ 0.7 dB insertion loss and ≥ 50 dB rejection an octave away. Filtering here not only protects the TS-850 from overload but also keeps nearby receivers linear.
At this point the impedance is stable (≈ 50 Ω) and voltage levels are moderate — ideal conditions for precise filtering without overstressing components, especially on 160 m.
Transmission Cleanliness
The TS-850’s internal low-pass filters remain active, ensuring harmonics stay below −43 dBc. Inside the filter box, positive-make/positive-break relays guarantee only one bandpath is active at a time, maintaining isolation between sections.
The RX-Only Station – Quiet by Design
Our RX-only site is centered around the TerraBooster Maxi active magnetic-field antenna. The head unit is fully shielded, and the shield includes an integrated RC filter to ground. Because it couples primarily to the magnetic component of the wave, the TerraBooster is naturally less sensitive to electric near-fields from the nearby transmitter — a critical advantage at close range.
Placed about 200 m from the transmit antenna field and aimed so the loop’s null faces 90° away from the doublet’s main direction, the TerraBooster gains additional isolation. Its low-pass limiting around 30 MHz and strong LW/MW filtering keep broadcast energy out without using narrow band-pass selection — perfect for wide-coverage monitoring across multiple bands.
At 200 m from a 100 W transmitter, the field is about 0.27 V/m — only a few millivolts on a 1 m loop, well below the compression point of the active preamplifier. Tests confirm ample headroom.
Front-End Protection
If AM broadcast energy is present, a broad high-pass filter (> 1.6 MHz) keeps intermodulation low. A PTT-triggered limiter can momentarily short the input to 50 Ω during TX bursts — though practical tests showed it is rarely needed once spacing and orientation are correct.
Field Layout & Cabling
The RX loop sits roughly 200 m from the TX antenna field — ideally behind trees or a small rise for extra attenuation. Feedlines are simply laid on the ground during the event, which naturally reduces re-radiation in temporary setups.
All coax lines include mix-31 ferrite chokes at both ends (5–10 kΩ common-mode impedance). With no rotators or Ethernet links, each station remains stand-alone. Grounds are bonded externally for safety yet kept isolated to avoid loops.
Operating Practice
Even the best filtering can’t replace good habits. During Field Day, both stations will operate SSB only, each on a different band to ensure clean separation. The main station handles the primary contest band, while the RX site listens on the others — providing continuous coverage of band openings and DX activity across 160, 80, and 40 m.
The TerraBooster loop is mounted close to the ground, so its null is shallow rather than deep. Instead of aiming it directly at the transmit antenna, it is oriented 90° away from the doublet’s main radiation direction. This reduces coupling to the dominant E-field polarization of the TX antenna and improves sensitivity to cross-field components from distant signals — a simple and effective way to minimize local pickup without deep nulls.
The RX site will feature a Web-888 SDR system capable of hosting 13 simultaneous users. Its Zynq 7010 FPGA and optimized server software enable multi-channel monitoring with real-time waterfall displays for every channel. Two Raspberry Pi 500 stations with displays and headphones will be available for visitors to experience how a modern SDR receiver works. In addition, a Raspberry Pi 5 connected to a large screen with speakers and a Bluetooth keyboard and mouse will serve as the main live demo console for interactive band and signal exploration throughout the day.
Visitors are encouraged to try different listening positions and compare how each band behaves as conditions change — a hands-on demonstration of geometry, polarization, and propagation in action.
Verification Before Field Day
- VNA-measure each BPF for insertion loss and stop-band depth.
- Confirm choke performance by probing common-mode current under TX.
- Transmit 100 W on all bands and verify RX noise floor rises ≤ 3 dB.
- Inject a weak test tone at the RX site and check for compression or spurs.
- Note loop orientation for quick alignment on site.
- Listen for residual clicks or birdies and add ferrites until quiet.
Why It Works
This layout uses layered selectivity and geometry rather than brute force. The balanced open-wire feedline acts as a broad preselector, the band-pass module adds sharp filtering, the TerraBooster’s shielded H-field design minimizes coupling, and 200 m of open field provides physical isolation. Together they keep both sites linear and quiet, even with multiple receivers and visitors listening live.
Bottom line: One transmit station, multiple listening posts, and no interference — a clean, educational showcase of amateur radio at its best.
Mini-FAQ
- Why two sites? — It lets operators and visitors monitor real propagation while transmitting without overload or intermod issues — ideal for demonstrations and recruitment.
- Does the RX site need power isolation? — Yes. Running on a battery or isolated DC source avoids ground-loop pickup and keeps it silent during transmission.
- Can the TerraBooster handle strong nearby transmitters? — Yes. Its shielded, RC-grounded enclosure and low-pass filtering keep it linear even 200 m from a 100 W station.
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