Vertical E-Probes for DX and NVIS: Choose the Useful Pattern
Vertical E-Probes for DX and NVIS: Choose the Useful Pattern
A receive antenna can be a good choice for distant low-angle signals without being the first choice for a regional net. For low-angle DX, a well-installed vertical E-probe is a useful compact candidate. For high-angle NVIS, I would start with a horizontal dipole whose height gives useful overhead coverage. The reason is the field each antenna responds to—not the size of the signal on an S-meter.
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.
Listening across an ocean and following a regional traffic net are different jobs. An antenna that gives useful sensitivity towards the horizon may give much less sensitivity towards the sky above it. That can be an advantage for one job and a disadvantage for the other. I choose the receive pattern for the communication path; I do not ask one antenna to be the best at everything.
The vertical-versus-horizontal comparison gives us a sensible starting choice. Then the real reference structure, cables, ground and local noise decide how well that choice works at the site. Verify the installed pattern and SNR with the intended receiving role in mind.
What a Vertical E-Probe Actually Senses
An E-field probe uses a conducting sensing element and an input circuit to respond to electric field. When the element is electrically short, its open-circuit signal is related to the component of the local electric field projected onto its effective-height vector. That is a vector relationship: element orientation matters, but so do the direction and polarisation of the arriving wave.
The active front end does not create a new law of radiation pattern. It loads the sensing element, sets the transfer function presented to the feed system and contributes noise, non-linearity and overload limits. Input resistance, capacitance, protection devices, feedback, enclosure, mounting and cable coupling all affect the realised response. A broad frequency-response curve is not an elevation-pattern measurement.
An ideal short vertical electric dipole in free space has a null along its axis and maximum response broadside to the element. A vertical monopole above an ideal conducting plane is a different model: the ground image and boundary conditions shape the upper-hemisphere pattern. A real active probe near imperfect soil, a roof, a mast and bonded building metal is neither ideal model by itself.
That ideal picture still explains the useful preference. A wave arriving near the horizon can have a substantial vertical electric-field component for a vertical element to sense. As its arrival direction approaches overhead, the transverse electric field lies increasingly in the horizontal plane; the ideal vertical element's response falls towards its axial null. A horizontal dipole, by contrast, is broadside to an overhead arrival and can respond to the horizontal field component along its wire. Its orientation is a better starting point when high-angle skywave is the wanted signal.
The Missing Half Is the Reference and Return Structure
A one-terminal sensing rod is not electrically alone. Displacement current returns through the front-end reference, enclosure, mounting capacitance, feed-line shield, power or control wiring, nearby structures and the environment. If the outside of the coax or another cable carries significant current, that cable becomes part of the receiving antenna.
This is why moving a choke, changing feed-line route or touching a support can change the result. The change may be impedance transformation, common-mode conversion or a genuine pattern change. It is not automatically a property of the short vertical element.
Ground is also not a universal mirror. Conductivity and permittivity vary with material, moisture, temperature and frequency. Roof membranes, reinforcing steel, solar-panel frames, rainwater systems and utility cables can dominate the local geometry. ITU-R BS.705 treats ground and surrounding structures as explicit influences on practical HF antenna patterns; ITU-R P.527 provides frequency-dependent ground electrical characteristics for propagation work.
DX Does Not Have One Arrival Angle
Long-distance HF propagation can use different ionospheric regions, hop counts and paths at the same time. Frequency, time, season, solar and geomagnetic state, path length and ionospheric structure determine which modes are supported. Terrain and local clutter then modify the field at the antenna. “DX” therefore does not mean a fixed 5–20-degree arrival window.
Low elevation can be valuable for many long paths, especially when the supported mode and local horizon favour it. Other DX paths arrive in a higher lobe, by a different hop count, through scatter or after a disturbed mode change. Within its 2–30 MHz prediction range, ITU-R P.533 treats circuit performance through path-specific modes and elevation angles rather than assigning one angle to all distant communication.
For a low-band DX listening station, that gives a vertical E-probe a clear potential role: obtain useful response towards the wanted horizon without needing a full-size horizontal span. If the wanted signal arrives at low elevation while a competing regional signal arrives much higher, a pattern favouring the first direction can improve their ratio. That is useful spatial discrimination. It is not a promise to reject noise generated by nearby wiring, and it disappears as an advantage when wanted and unwanted fields couple in the same way.
A receiving antenna that is weak at one elevation may still work a different lobe. Conversely, an antenna with strong low-elevation response in one azimuth can have an installed null in the wanted bearing. “Vertical” and “low angle” are not enough to predict that three-dimensional result.
NVIS Is High-Angle Propagation, Not a Single Zenith Ray
Near Vertical Incidence Skywave is used for regional HF coverage by launching and receiving relatively high-angle modes. It is not confined to exactly 90 degrees, and its useful distance is not fixed at 500 kilometres. The supported range depends on frequency relative to the ionosphere, virtual height, absorption, ground distance and which high-angle modes exist at that time.
At exact zenith, a propagating plane wave's electric field is transverse to its direction of travel, so a perfectly vertical electric element has no vertical field component to sense in the simplest free-space picture. Real NVIS paths usually arrive away from exact zenith and with ionospheric mode, Faraday rotation, ground reflection and local scattering in the chain. A vertical probe may receive part of that field. How much is an installed polarisation-and-pattern question, not a binary NVIS rejection switch.
For a regional net, I would therefore start with a horizontal dipole at a height that provides useful high-angle response, rather than depend on a vertical probe's residual response near its ideal axial null. This is a positive design choice: place useful antenna response where the wanted regional skywave arrives. It does not mean laying the dipole as close to the soil as possible.
Ben Witvliet and colleagues' NVIS elevation-angle and dipole-height study combines ray tracing, arrival-angle measurements, antenna simulations and experiments. It distinguishes the height that maximises transmitted signal from the height that gives the best received SNR. That supports choosing a useful high-angle installation rather than assuming that the lowest wire must be the best NVIS antenna. The study's particular soil and height results are not universal settings for every site.
At a noisy site, a weaker wanted signal can still have better SNR if the antenna rejects more noise in the relevant directions or keeps the receiver out of overload. Keep that qualification attached to the choice: a horizontal dipole is a sound NVIS starting point, and the installed signal-to-noise ratio decides whether it is the best branch to use.
Choose for the Job You Actually Want to Do
| Receiving job | My starting choice | Why it makes sense |
|---|---|---|
| Low-angle DX listening, especially where a long horizontal span is impractical | A vertical E-probe with a deliberate reference and controlled cable pickup | Useful response to the vertical field component of low-elevation arrivals, in a compact installation. The site and front end must preserve that advantage. |
| Regional high-angle skywave and NVIS nets | A horizontal dipole with suitable high-angle coverage | Broadside overhead response puts sensitivity towards the wanted path instead of relying on the residual high-angle response of a vertical element. |
| Both distant listening and regional coverage | Separate selectable receive branches when space permits | Different patterns provide useful choices as propagation and interference change. Choose by intelligibility and SNR, not by which branch produces the largest reading. |
Hearing a regional station on the vertical does not invalidate the comparison; hearing DX on the horizontal dipole does not invalidate it either. The question is which pattern is better suited to the wanted path under the present conditions. Keeping both options is often more useful than trying to make one antenna win every argument.
Local Noise Has No Mandatory Polarisation
House noise is not automatically vertically polarised. Switch-mode supplies, solar inverters, LED drivers, network wiring and appliances launch differential and common-mode currents onto conductors with many orientations. Buildings, wiring and the ground scatter and convert the resulting fields.
A vertical probe may be very sensitive to a vertical conductor near the house, or it may reject a source that couples more strongly to a horizontal loop. Move either antenna and the comparison can reverse. ITU-R P.372 supplies statistical external-noise models for planning, but it does not predict a particular neighbour's cable-current geometry. That requires measurement at the site.
Do not confuse lower output with lower noise. A small probe can produce less signal and less noise while leaving SNR unchanged. It can also improve SNR through spatial or polarisation discrimination—or degrade it through common-mode pickup or front-end overload. Calibrated level and receiver-headroom controls are essential.
Receive Performance Is Pattern Plus Electronics
| Question | What must be established | What does not prove it |
|---|---|---|
| Does it favour low-angle DX? | Installed elevation and azimuth response in the wanted bearing, plus a supported propagation mode | Vertical element orientation alone |
| Does it reject NVIS? | Installed high-angle response across relevant polarisations and bearings | An ideal axial null copied from a free-space pattern |
| Is it low noise? | Wanted-signal and noise levels through a calibrated, linear receive chain | A lower S-meter reading |
| Is the probe controlling the pattern? | Low exterior-cable current and repeatable response when routing and bonding are restored | Low SWR or galvanic continuity |
| Does the front end cover the band? | Transfer response, noise contribution, compression and intermodulation with the real source and load | A small-signal gain trace by itself |
On the lower HF bands, atmospheric and man-made external noise can exceed receiver internal noise. In that regime, more preamplifier gain is rarely the main objective. The active chain needs enough sensitivity, but also the headroom to preserve signal and noise without compression or intermodulation. A nearby broadcast transmitter can make a sensitive probe look noisy when the real fault is overload.
A Test That Separates Pattern from Level
After choosing the intended DX and NVIS roles, check that the installed antennas deliver them. A useful comparison needs more discipline than alternating two antenna connectors and watching the S-meter. Make wiring or mounting changes only with equipment de-energised and nearby transmission prevented:
- Define the paths. Select several DX bearings and regional paths, not one favourite station. Record band, time and a plausible propagation mode.
- Document geometry. Record probe height, orientation, ground or roof construction, mast, cable routes, bonding, nearby metal and every powered lead.
- Calibrate the receive branches. Measure cable and filter transfer. Use fixed receiver gain, bandwidth and detector settings. Confirm that neither branch overloads.
- Map common-mode current. Use a suitable clamp-current probe on the complete feed and control cables where practical. Re-route or choke only at a measured RF boundary, then restore the baseline.
- Measure signal and noise separately. Compare wanted-signal level, adjacent clear-channel noise and receiver artefacts. SNR is the result that matters.
- Use simultaneous or rapid A/B/B/A observations. HF paths fade. A restored baseline helps distinguish an antenna change from propagation drift.
- Repeat across angle and bearing. Use multiple regional and distant signals, or a calibrated field source when available. One contact cannot establish a three-dimensional pattern.
Modelling can help explain the result, but it must include the reference structure, feed line, mast, building and ground that carry current. A model of only the short rod answers a different question from the installed antenna.
Primary Technical Sources
- ITU-R BS.705-2 — HF transmitting and receiving antenna characteristics and diagrams: antenna patterns, ground effects, surrounding structures and practical pattern variation.
- ITU-R P.533-14 — Method for prediction of HF-circuit performance: path-specific ionospheric modes and elevation-angle treatment.
- ITU-R P.372-17 — Radio noise: atmospheric, galactic and man-made radio-noise planning data and variability.
- ITU-R P.527-6 — Electrical characteristics of the surface of the Earth: frequency-, material-, moisture- and temperature-dependent ground parameters.
- ITU-R P.368-10 — Ground-wave propagation from 10 kHz to 30 MHz: the distinct ground-wave boundary and its dependence on frequency and ground.
- Witvliet et al. — NVIS elevation angles and optimum horizontal-dipole height: measured and modelled high-angle propagation, with separate transmit-signal and receive-SNR criteria.
Practical Conclusion
For low-angle DX, I would consider a properly installed vertical E-probe a useful compact receive option. For a regional NVIS net, I would start with a horizontal dipole that gives useful high-angle response. Those choices follow the wanted field and antenna orientation; they are not opposing loyalties.
Control the reference and cable currents, keep the front end linear and use the branch with the better wanted-signal SNR. Where both jobs matter, keep both patterns available if practical. An antenna does not have to be the best NVIS receiver to earn its place at a DX station—and a regional-net antenna does not have to win every DX comparison to be the right tool.
Mini-FAQ
- Is every vertical receive antenna best for low-angle DX? No. The ideal element suggests a tendency, but ground, structures, the reference and return path, cable current, frequency and arriving polarisation determine the installed response.
- Does a vertical E-probe completely reject NVIS? No. An ideal vertical element has an axial null, but real NVIS paths, polarisation, ground and installation geometry are more complex. Measure the high-angle response and regional SNR.
- Is all long-distance HF energy low angle? No. HF DX can use different ionospheric regions, hop counts and arrival angles. Frequency, time, path, terrain and ionospheric state decide which modes are present.
- Is house noise normally vertically polarised? No. Local interference follows the geometry and modes of its source wiring, cables, structures and ground. It can contain several field components and change across a site.
- Does a lower noise-floor reading prove better reception? No. The antenna may simply deliver less of everything. Compare wanted-signal SNR with calibrated branch loss, fixed receiver settings and confirmed linear operation.
- What is a useful starting choice for DX versus NVIS? Consider a well-installed vertical E-probe for low-angle DX and a horizontal dipole with useful high-angle response for NVIS. Verify the installed SNR and use separate receive branches when both jobs matter.