Vertical E-Probes for DX and NVIS: Measure the Installed Pattern

The attractive shortcut is simple: a vertical has a low-angle lobe and an overhead null, so a vertical receive probe must be excellent for DX and poor for NVIS. That picture is useful as a first ideal model. It becomes unreliable when it is promoted into a verdict about an installed active probe.

My practical position is more demanding. First establish the installed directional and polarisation response. Then establish the arrival angles and polarisations that matter on the wanted path. Finally measure wanted-signal SNR with the receiver kept inside its linear range. Only that complete chain tells us whether a vertical probe helps DX, NVIS, both or neither at a particular site.

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.

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. ITU-R P.533 treats circuit performance through path-specific modes and elevation angles rather than assigning one angle to all distant communication.

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.

A low horizontal dipole often provides useful high-angle response on the lower HF bands. That makes it a strong NVIS candidate, not a universal receive winner. 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.

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

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

A useful comparison needs more discipline than alternating two antenna connectors and watching the S-meter:

1. Define the paths. Select several DX bearings and regional paths, not one favourite station. Record band, time and a plausible propagation mode.
2. Document geometry. Record probe height, orientation, ground or roof construction, mast, cable routes, bonding, nearby metal and every powered lead.
3. Calibrate the receive branches. Measure cable and filter transfer. Use fixed receiver gain, bandwidth and detector settings. Confirm that neither branch overloads.
4. 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.
5. Measure signal and noise separately. Compare wanted-signal level, adjacent clear-channel noise and receiver artefacts. SNR is the result that matters.
6. Use simultaneous or rapid A/B/B/A observations. HF paths fade. A restored baseline helps distinguish an antenna change from propagation drift.
7. 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.

Joeri's Bottom Line

A vertical active E-probe can be a very useful DX receiving tool. It can also receive regional high-angle signals. Neither result follows from the word “vertical.” The ideal-element sketch tells us where to start looking; the installed current system tells us what is actually happening.

For DX and NVIS alike, follow three things: the arrival field, the installed pattern and the receive-chain headroom. Measure wanted-signal SNR while controlling common mode and fading. That is how a useful antenna choice replaces a polar-plot slogan.

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