ORA Field Day 2026: Two Stations, One Controlled RF Site
ORA Field Day 2026: Two Stations, One Controlled RF Site
A 100 W HF station and a wideband public listening site can share one Field Day—provided we stop treating distance as magic and prove every transmit-to-receive combination before visitors put on the headphones.
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.
The ORA plan separates two jobs that are usually forced into one radio position. The main site makes contacts on 160, 80 and 40 metres. A second, receive-only site lets operators and visitors watch several bands, hear propagation change and explore signals without interrupting the active QSO. That public-listening role is not a side attraction; it is the reason for building the second station.
The intended starting point places the receive site roughly 200 metres from the transmitting antenna. That is useful separation, but it is not an isolation specification. Coupling depends on both antenna patterns, height, terrain, polarisation, feedline routes, common-mode current, transmitter spectrum, filter response and receiver large-signal behaviour. The plan therefore begins with a layout and ends with measurements.
The acceptance rule: no receiver is declared safe or usable because the S-meter looks calm. For every intended TX/RX band pair, increase transmit power in controlled steps and verify wanted-signal copy, noise-floor change, blocking, false signals, ADC overload and recovery. Record the configuration that produced the result.
The Main Station Has One Clear Job
The main station uses a non-resonant doublet, open-wire line and balanced matching for low-band SSB. The balanced line is a low-loss way to carry a wide range of impedances when it is installed symmetrically and kept away from conductive clutter. It is not a band-pass filter. Off-band energy can still reach the tuner, feedline and antenna, and an apparently balanced arrangement can still develop common-mode current.
The matching network belongs close enough to its designed reference plane that the operator knows what is being transformed. Any switchable band-pass filter belongs on the nominal 50-ohm side, between the transceiver and tuner, where its insertion loss, rejection and power handling can be measured under the impedance it was designed to see.
Matching and common-mode control remain separate jobs. A balanced output does not prove that station wiring, the tuner enclosure or the coax exterior carries no RF. We will inspect the current path with a repeatable clamp-on RF-current measurement and add or move choking where the map shows it is needed.
Filtering Must Protect Both Directions
A co-site problem has at least two sides. The transmitter can produce wanted-band energy, out-of-band energy and spurious products. The receiver can overload, block or create intermodulation even when the transmitter itself is clean. One good filter does not certify the complete pair.
| Layer | What it is expected to do | What must be measured |
|---|---|---|
| Transmitter low-pass filtering | Reduce harmonic output after the power amplifier | Conducted spectrum at the defined output reference plane and realistic power |
| Transmit band-pass filtering | Reject energy outside the selected operating band | Insertion loss, rejection, heating and power handling in the actual band path |
| Receive preselection | Reduce strong out-of-band energy before nonlinear receiver stages | Wanted-signal sensitivity with the local transmitter off and on |
| Receiver protection or T/R isolation | Limit energy that can reach vulnerable inputs | Peak and average coupled power, switching sequence, recovery time and failure state |
| Physical separation and geometry | Reduce direct and reradiated coupling | Coupled level for every actual antenna, orientation, feedline route and band pair |
ITU-R SM.332 distinguishes receiver selectivity from a complete protection ratio and warns that strong unwanted signals can expose nonlinear behaviour. ITU-R SM.329 separately defines unwanted emissions in the spurious domain. That distinction is exactly what matters here: transmitter cleanliness and receiver headroom are related, but neither one proves the other.
The Receive Site Is a Listening Classroom
The second site uses a wideband receive antenna, a multichannel SDR server and several supervised listening positions. Visitors can compare 160, 80 and 40 metres, see a waterfall, tune a signal and hear how a band changes while the main team keeps operating. Capacity is set by the installed SDR, server, network and client configuration; it should be load-tested rather than advertised from a nominal user count.
An active receive antenna must be treated as an input to a strong-signal system, not as an inherently protected sensor. Its element, amplifier, power feed, common-mode path, filters and receiver front end all see the nearby transmitter through different coupling routes. Magnetic-field response does not make electric-field pickup disappear, and antenna orientation cannot be reduced to one universal null angle at an improvised site.
The receive antenna will therefore be rotated and moved while watching both a weak reference signal and the coupled local carrier. The useful orientation is the one that preserves wanted-signal SNR and receiver linearity in that installation. A small rise, trees or a different feedline route may change the result, but none is counted as attenuation until the measurement shows it.
Spacing is not a protection device: if the receive hardware cannot tolerate the measured coupled power with adequate margin, use a tested hardware disconnect, limiter, preselector or interlock. The safe failure state must protect the receiver when control power, software or a network link is lost.
Two Hundred Metres Is a Starting Geometry
A free-space field calculation is not enough to clear this site. At HF and at the dimensions of temporary antennas, the receive point may not satisfy the assumptions behind a simple far-field formula. Antenna gain and elevation pattern, ground reflection, near conductive objects, feedline radiation and terrain all change the field at the receiver.
ITU-R SM.337 treats frequency and distance separation as coordination variables, while ITU-R SM.575 shows why strong nearby fields can desensitise and block monitoring receivers. Those recommendations do not give this temporary station a universal safe distance. They support the engineering approach: use distance, frequency separation and filtering together, then verify the actual receiver input and behaviour.
Feedlines will be routed deliberately and documented. Laying coax on the ground may change coupling, but it does not guarantee that the shield exterior stops radiating. Ferrite type, turns and position are chosen from the measured exterior-current path; a fixed impedance figure copied from a parts table is not a site result.
Operating Discipline Is Part of the RF Design
The initial plan keeps both roles on different bands and uses SSB at the main station. That reduces some conflicts, but it does not make simultaneous operation automatic. The team needs an operating matrix that says which transmitter band, receive band, filter state and power level have been verified.
- Only use TX/RX combinations that passed the coupled-power and receiver-linearity test.
- Keep filter selection mechanically or electrically interlocked with the active transmitter band.
- Prevent visitors from changing protection, gain or attenuation settings that define the tested state.
- Keep a fast shutdown path at both sites and one operator responsible for the co-site condition.
- Repeat the checks after moving an antenna, feedline, filter, computer, power supply or bonding connection.
Safety bonding, lightning precautions, generator practice and RF-exposure control belong to one event plan and the applicable local rules. “RF isolation” is not a reason to defeat protective bonding. The public boundary, antenna clearances and exposure assessment must be established before transmitting, independently of whether the receive system appears to work.
The Pre-Event Test That Decides the Layout
Record antennas, heights, orientation, feedline routes, filters, attenuation, gain, power supplies and receiver settings.
Check filter S-parameters, transmitter spectrum, exterior-feedline current and coupled power before connecting vulnerable hardware.
Use a weak reference signal, step transmit power upward and log desense, blocking, false signals, ADC overload and recovery.
Run the intended SDR clients, displays, audio and network while transmitting, then verify that the tested protection state cannot be bypassed.
The result may be a clean simultaneous combination, a lower power limit, a different antenna position, more filtering or a rule that certain bands cannot be used together. All are valid engineering outcomes. The failed outcome is allowing a neat sketch to become an untested operating promise.
Bottom line: ORA Field Day 2026 uses two stations because transmitting and public listening are different jobs. Distance, filters, current control and operating discipline create the opportunity; measured receiver headroom decides whether the opportunity is safe and useful.
Primary references
- ITU-R SM.332 — Selectivity of receivers
- ITU-R SM.329 — Unwanted emissions in the spurious domain
- ITU-R SM.337 — Frequency and distance separations
- ITU-R SM.575 — Protection of monitoring receivers from nearby or strong transmitters
- ICNIRP — Guidelines for limiting exposure to radiofrequency electromagnetic fields
Mini-FAQ
- Why use two stations at Field Day? The main station can keep making contacts while the receive-only site lets operators and visitors explore several bands. Separating the roles also makes protection and interference tests easier to define.
- Is roughly 200 metres enough separation? It is the planned starting geometry, not a guarantee. The answer depends on antennas, terrain, feedlines, filters, power and receiver headroom, so every intended band pair must be measured.
- Does a magnetic receive antenna reject the nearby transmitter? Not automatically. The complete installed antenna, feedline, common-mode path and receiver determine coupling. Find the useful location and orientation by measuring wanted-signal SNR and overload behaviour.
- Why put the band-pass filter on the 50-ohm side of the tuner? That is the reference impedance for which a conventional 50-ohm filter is designed and characterised. Its loss, rejection and power handling still have to be verified.
- Can an SDR waterfall prove there is no desense? No. Gain control and display scaling can hide lost sensitivity. Inject or receive a stable weak signal and compare copy, level and distortion with the local transmitter off and on.
- What happens if one band pair fails? Change spacing, routing, filtering, protection or power and retest. If it still fails, that combination stays out of the operating matrix.