The 15 cm “Cheat Disk”: Why We Prefer a Capacitive Hat on E-Probes
and Why 4-Wire Spiders Are Often Overkill
If you’re building an E-probe / active whip / active short dipole, you’ll see endless arguments about “bigger top hats” and “more wires” as if that automatically means more receive performance. Here’s the RF.Guru version: for these antennas, a capacitive hat is usually about coupling efficiency, not directional gain.
In this article, “gain” means received voltage level increase (what looks like an S-meter rise), not directional antenna gain.
RF.Guru products that use this exact idea:
SkyTracer ... uses a 15 cm diameter capacitive disk as part of the element to boost coupling into the front-end:
• Configuration A: 2 × 1 m elements + 15 cm disk
• Configuration B: 1 × 0.5 m element + 15 cm disk
Product link: SkyTracer active receive antenna system
EchoTracer ... offers an option for a 15 cm diameter disk for the same reason: better capacitance ratio / better recovered RF.
Product link: EchoTracer E-probe active antenna
The disk doesn’t “create gain” in the directional sense ... it changes the probe capacitance so the amplifier sees more of the available RF.
The nuance most people miss: the capacitive divider inside many E-probe designs
In many E-field probe antennas, the probe capacitance and the amplifier’s input/stray capacitance behave like a capacitive divider. The practical consequence is brutal and simple: the larger your probe capacitance is relative to the amplifier’s effective input capacitance, the more RF you recover.
First-order model (voltage-probe style E-probe):
V_in ≈ V_oc · (C_ant / (C_ant + C_in))
So the hat’s “job” is to increase C_ant. How much it helps depends heavily on C_in (amp input + stray + layout).
Important reality check: more level doesn’t always mean better SNR
On LF/MF, many installations are already external-noise limited (atmospheric + man-made noise dominates). In that situation, raising antenna output level often raises both signal and noise ... so SNR may not improve. What can improve (or worsen) is overload behavior: too much level can push a front-end into IMD/overload.
On HF (especially above ~10 MHz), the capacitance-ratio loss can become more relevant (external noise is often lower and receiver/front-end limits matter more).
A hat doesn’t become more directional or “stronger” with frequency; it simply increases C_ant, recovering a small amount of RF voltage that can be easier to notice on HF
(or traded for more attenuation/preselection headroom).
“Most noticeable” depends on what you mean:
- Most predictable level bump: MF and low HF (electrically very short probe; changes can be easier to repeat).
- On high HF: the expected change is still only a small dB, but it’s often masked by propagation, local geometry, and feedline/common-mode effects ... so A/B results can be harder to attribute.
- On VHF: environment/geometry dominate; hats may introduce “quirks” and results are less repeatable.
Disk vs 4 wires: how much smaller can the disk be?
If the goal is “same result” = similar capacitance / similar electrical loading effect, a solid disk generally provides more capacitance for the same span than four thin radial wires.
Useful rule-of-thumb (similar capacitance/loading):
-
Equivalent disk diameter ≈
0.65–0.80 ×the 4-wire hat’s tip-to-tip diameter - Meaning: the disk can often be ~20–35% smaller in diameter than the wire span for about the same capacitance effect.
Example: 4 wires sticking out 15 cm from the hub ... tip-to-tip span is 30 cm ... comparable disk is often about 20–24 cm diameter.
This lines up with the classic E-probe / mini-whip reality: at LF the whip behaves largely as a capacitively coupled sensor, and “shape” matters mainly because it changes capacitance.
So what does a 15 cm disk typically add? (receive voltage increase)
A 15 cm diameter disk has a free-space self-capacitance on the order of a few pF. Once installed near a mast, bracketry, coax, and real-world objects, the incremental capacitance increase you actually “get” is commonly in the ballpark of ~3–5 pF.
These numbers are intentionally presented as ranges, because mounting geometry and effective amplifier input capacitance dominate the outcome.
A) 1 m E-probe + one 15 cm disk hat
A typical 1 m thin whip is often roughly ~8–12 pF. Adding a 15 cm disk might bring it to roughly ~11–17 pF.
- Expected recovered RF voltage increase: ~+0.5 to +2.8 dB
- Most likely expectation: ~+1 to +2 dB
B) “Short” active dipole, 50 cm legs (total ~1 m), with a 15 cm disk at each end
Two hats (one per end) typically create a bigger total capacitance increase than the monopole case.
- Expected recovered RF voltage increase: ~+0.7 to +3.5 dB
- Typical: ~+1.5 to +2.5 dB
C) Active dipole, 1 m legs (total ~2 m), with a 15 cm disk at each end
Because the element already has more capacitance, the same hats become a smaller percentage change.
- Expected recovered RF voltage increase: ~+0.2 to +1.6 dB
- Typical: ~+0.5 to +1 dB
If your “shorted dipole active” is a true current-probe (virtual-ground) design
Some “shorted dipole” active antennas are actually current-sensing designs (feedpoint held near virtual ground; output proportional to short-circuit current).
In that topology, improvement can track more directly with C_ant, so the perceived level change can be larger.
Ballpark (same capacitance assumptions):
- 1 m E-probe + one 15 cm disk: ~+2 to +4 dB
- 50 cm/leg dipole + two disks: ~+3 to +7 dB
- 1 m/leg dipole + two disks: ~+2 to +3.5 dB
If you’re already external-noise limited, this may look like “more bars” without real SNR improvement ... and it can increase overload risk. Use attenuation as a tool, not as a defeat.
The part most people miss: SNR vs “more bars”
On LF/MF, it’s common to see higher level but no meaningful SNR improvement if you’re dominated by atmospheric/man-made noise. On HF, the same small capacitance change can sometimes feel more important because the noise budget shifts and receiver/headroom considerations matter more. But the expected level change is still only a small dB and can be masked by propagation, local geometry, and common-mode ... so don’t oversell it.
This is why we keep repeating the same advice at RF.Guru: if you want real performance, you must manage SNR and overload margin ... not just chase level.
Mini-FAQ
- Does a capacitive hat add directional gain? Usually not for E-probes/active whips/active short dipoles. It mainly increases element capacitance so the probe couples more strongly into the preamp input.
- Why does capacitance matter so much? Many E-probe front-ends behave like a capacitive divider: the larger the probe capacitance relative to amplifier input/stray capacitance, the more RF you recover.
- Will a 15 cm disk improve SNR? Not always. On LF/MF you’re often external-noise limited, so level may rise without SNR improvement. It can also increase overload/IMD risk.
-
Disk vs 4 wires: how much smaller can the disk be? For similar capacitance/loading, a disk is often about
0.65–0.80×the 4-wire tip-to-tip diameter (about 20–35% smaller). - How much “level gain” should I expect from a 15 cm disk? In many voltage-probe E-probes, a typical expectation is roughly ~+1 to +2 dB on a 1 m probe, with wider ranges depending on topology and amplifier input capacitance.
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