Active E-Dipole Height Above Ground vs Performance
(Updated 2026-01-18) — Field results always depend on local noise sources, nearby conductors, and feedline common-mode control.
In this article I’ll compare two physical configurations of the RF.Guru SkyTracer2 wideband active E-dipole and explain what changes as you raise it higher above ground. The SkyTracer2 is a wideband active (receive) “shorted active loaded dipole” specified for roughly 500 kHz–30 MHz, with versions that mainly differ in element length and capacitive end loading.
The two configurations
- Standard — 2 × 0.5 m arms + 15 cm capacitive end hats (tip-to-tip ≈ 1 m)
- Ultra — 2 × 1.0 m arms + 15 cm capacitive end hats (tip-to-tip ≈ 2 m)
Why “bigger” changes the result on a short active dipole
Effective height drives received voltage
On the low bands, both versions are electrically short. In that regime, you can think of the antenna as a voltage sensor: the induced open-circuit voltage is proportional to the incident electric field and the antenna’s effective height. Practically, that means that increasing the physical size tends to increase the available signal voltage.
If both antennas are still “short” on a band, doubling the physical dipole length often approaches ~2× voltage available from the field (≈ +6 dB in voltage), assuming the environment and noise coupling remain the same.
In real installations, the environment is rarely “the same” — so you may see less (or occasionally more) than this on SNR, even if the raw output voltage rises.
End hats help twice: effective height and capacitance
Capacitive end hats (end disks) are not decoration — they help a short E-dipole behave “longer” electrically. They also increase the antenna’s capacitance, which matters a lot when the antenna is connected to a high-impedance active front end.
The hidden limiter in many active antennas: a capacitive divider
With a high-impedance amplifier, the antenna capacitance and the amplifier’s input capacitance form a capacitive divider. If the antenna capacitance is too small, the amplifier “loads” the sensor and you lose signal voltage before any gain happens. Increasing the physical size and adding hats generally increases antenna capacitance and reduces that divider loss.
- Standard already uses hats to push the small-capacitance problem in the right direction.
- Ultra pushes further: longer arms + hats generally means more low-band output voltage, especially on 600–160 m and 160–80 m.
- On higher HF, the difference becomes less about “more signal” and more about height, pattern, and local noise geometry.
Height above ground: why it often matters more than element length
Low HF / MF: height is mostly about noise coupling
Below a few MHz, the antenna is extremely small compared to a wavelength, so “classic gain thinking” is rarely the limiter. Most of the time, your reception is limited by external noise (atmospheric + man-made), and by how strongly your antenna couples to near-field noise sources such as house wiring, chargers, Ethernet gear, solar inverters, and neighboring installations.
In that region, raising the antenna from 2 m to 6 m often improves real-world readability because the antenna “sees” a cleaner field environment — sometimes a bigger win than switching from Standard to Ultra on a noisy property.
Mid/upper HF: height shapes arrival angles and null behavior
As frequency rises, your mounting height becomes a meaningful fraction of a wavelength. Then, ground reflection begins to shape the elevation response: at some heights you get more high-angle response (NVIS-ish), at others you get more low-angle response (DX-ish), and at higher fractions you can get multiple lobes and deeper/shallower nulls.
A simple wavelength cheat sheet
Height in wavelengths is roughly: h(λ) ≈ height(m) ÷ (300 / fMHz). Here’s what 6 m looks like on common HF bands:
| Band (approx.) | Frequency | 6 m height is about | What you typically notice |
|---|---|---|---|
| 40 m | 7 MHz | ~0.14λ | More high-angle response; environment/noise still dominates |
| 20 m | 14 MHz | ~0.28λ | Mixed angles; useful selectivity begins to show |
| 10 m | 28 MHz | ~0.56λ | Lower angles improve, but lobes/nulls become more complex |
What to expect at real-world heights (Standard vs Ultra)
~1–2 m above ground (very low)
- Expectation: local noise coupling often dominates; both versions can become “everything got louder” rather than “everything got clearer.”
- Standard vs Ultra: Ultra may show more raw output voltage, but SNR may not improve if the noise floor rises with it.
- Use case: temporary/portable mounting, quick tests, or unavoidable low placement.
~3–4 m (low-but-usable)
- Expectation: often a meaningful SNR improvement vs 1–2 m because you’re farther from near-field noise sources.
- Standard vs Ultra: Ultra’s advantage becomes clearer on 160/80 m and below; Standard stays very competitive when noise-limited above ~3–5 MHz.
~5–6 m (practical sweet spot for Standard)
- Expectation: the antenna starts behaving like a serious HF receiving system instead of a near-house noise sensor.
- Standard vs Ultra: Standard often feels “complete” here for general coverage; Ultra still pulls ahead on the low bands if your site is already reasonably quiet.
~6–8 m (strong compromise height; Ultra shines on low bands)
- Expectation: better arrival-angle selectivity on higher HF; more consistent null behavior when the installation is clean.
- Standard vs Ultra: Ultra is the stronger pick if you want maximum low-band output (600–160 m / 160–80 m) and more voltage headroom into an SDR.
>8–10 m (when you can do it)
- Expectation: on 20–10 m, low-angle response can improve, but lobes/nulls can shift noticeably with small height changes.
- Standard vs Ultra: on the highest HF bands, height and environment usually matter more than the extra physical length.
- Tip: if a band “dies” at one height, moving only a couple of meters can change the lobe/null interaction dramatically.
Installation tips that make height “count”
- Keep it away from conductors: gutters, metal roofs, downspouts, fences, balcony railings, and large wiring bundles can dominate coupling and spoil null symmetry.
- Control common-mode on the coax: place a good RX line isolator near the antenna and again before the coax enters the shack if you want the antenna (not the feedline) to be the sensor.
- Maintain separation from TX antennas: even with limiters, don’t treat receive antennas as “immune.” Separation and good station grounding are still your friends.
- Test SNR, not only signal level: if noise rises with signal, you didn’t win. Use an SDR waterfall or narrowband measurements to compare before/after changes.
Bottom-line recommendations
- If you can mount around 5–6 m: the Standard configuration is an excellent “general HF RX” solution and performs very well in typical installations.
- If you can mount around 6–8 m and you care about low bands: the Ultra configuration is the better pick for more low-band output voltage and headroom into modern SDRs.
- If you’re forced below ~2–3 m: prioritize noise mitigation and feedline common-mode control; height usually buys more real improvement than swapping versions.
Mini-FAQ
- Will Ultra always beat Standard? Not always on SNR. Ultra usually delivers more raw output voltage on the low bands, but SNR depends on whether your noise floor rises too.
- Why do the 15 cm hats matter? They increase effective behavior on a short dipole and raise antenna capacitance, reducing capacitive divider loss into the amplifier input.
- What’s the “minimum usable” height? Around 3–4 m is often where things start getting noticeably cleaner than near-house placements, but it depends heavily on local noise sources.
- What’s the best height if I want one compromise for 0.5–30 MHz? For many installs, 5–8 m is a strong compromise: low enough to be practical, high enough to reduce near-field noise coupling and improve HF selectivity.
- Why does 10–20 m sometimes get weird at higher heights? As height becomes a larger fraction of a wavelength, lobes and nulls multiply. A small height change can move a desired path into (or out of) a null.
- Should I add attenuation at the SDR? Sometimes yes. If the band looks “busy but messy,” attenuation can improve dynamic range and readability. See the related reading on SNR tuning with attenuators.
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