The 15 cm “Cheat Disk”: Wideband Linearity for an E-Probe
The 15 cm “Cheat Disk”: Wideband Linearity for an E-Probe
The 15 cm capacitive hat widens the useful response of the one-metre EchoTracer E-probe and improves its frequency-response linearity across the lower 0–150 MHz design window. Other sensing-element geometries optimise other portions of the platform's coverage.
I call it a “cheat disk” because a compact disk gives us the top capacitance of a much more cumbersome structure. That capacitance lets us design the short sensing element and its amplifier as one system, widen the useful bandwidth and deliberately place the smoothest part of the response where we want it.
The design intent: the 15 cm hat broadens and shapes the one-metre E-probe configuration through the lower 0–150 MHz design span. It is a frequency-response component designed together with the active input.
The Hat and Front End Form One Broadband System
An electrically short E-field probe can be treated, as a useful first approximation, as a field-induced voltage source with a predominantly capacitive source impedance. Its open-circuit voltage follows the incident electric field and the effective height of the element:
Voc = E · heffProbe capacitance, amplifier input capacitance, finite input resistance, protection parts, PCB layout and enclosure form one transfer network. In the simplest high-impedance voltage-probe model, the capacitive relationship begins with:
Vin ≈ Voc · Cant / (Cant + Cin)The hat increases Cant. That gives the sensing element more authority in this divider and shifts the combined response of the element and active input. With the real resistive and parasitic terms included, the design can produce a wider and more linear useful response than the same one-metre element without the selected top capacitance.
One Geometry Optimises One Wideband Portion
A short probe with too little capacitance places more of the response shaping inside the electronics and makes the lower part of the transfer harder to keep smooth. The 15 cm hat moves part of that work into the sensing geometry. The element and amplifier can then be optimised together instead of compensating for an undersized electrical source after the fact.
For the one-metre EchoTracer configuration, the 15 cm disk was selected to optimise the lower wideband section from the low-frequency edge through roughly 150 MHz. The practical RF limit remains above DC, and the complete configuration can remain useful beyond 150 MHz; 0–150 MHz identifies the response-design portion that the hat is intended to improve.
Wideband is a set of deliberate compromises: the one-metre whip and 15 cm hat favour the lower wideband range. Shorter sensing elements move the optimised portion upward. EchoTracer spans a very broad spectrum by matching the element geometry to the intended job.
Frequency-Response Linearity Is the Design Goal
Here, linearity means a smoother and more predictable relationship between incident field and output as frequency changes across the intended span. The hat changes the probe capacitance and source impedance so the broadband transfer can be shaped around that goal.
The active input, protection network, PCB, enclosure and installation capacitance still influence the final curve. The 15 cm geometry is the deliberate front-end component that widens and optimises the lower portion of that complete response. It is not a claim of a mathematically flat line or static-field reception at DC.
Why Fifteen Centimetres
The disk diameter is an engineering compromise. Increasing the hat changes probe capacitance and therefore the response-shaping range, but it also changes wind loading, mechanical leverage, sensitivity to nearby conductors and the distributed behaviour at the upper end. A very large hat can improve one portion of the spectrum while making another portion less controlled.
Fifteen centimetres gives the one-metre probe enough capacitive loading to make the lower wideband response useful without turning the top of the antenna into an awkward structure. It is a repeatable geometry that can be designed together with the high-impedance input. That repeatability matters more than a folklore conversion between a disk and a four-wire spider.
A Compact Disk Instead of a Large Wire Spider
A solid disk provides a compact and repeatable top-capacitance geometry. It avoids the large span and variable geometry of a wire spider, keeps wind loading manageable and gives the electrical design a known element that can be reproduced from one probe to the next.
Its role is to set effective probe capacitance, source impedance and the field-to-output transfer for the lower wideband portion. The disk therefore belongs to the RF design itself: change the disk, and the frequency-response compromise changes with it.
The EchoTracer Configuration
The EchoTracer3 active E-probe combines a high-impedance wideband front end with selectable whip geometries. The one-metre whip plus 15 cm capacitive hat is the lower-frequency wideband configuration. The hat widens and linearises that configuration's lower response; shorter whips move the practical optimisation toward VHF, UHF and the upper end of the platform's coverage.
The EchoTracer3 Technical Overview describes the current configurations and their intended bands. The platform is wideband because the sensing element and front end are configured for the part of the spectrum being monitored.
Bottom line: the 15 cm cheat disk is a compact wideband response-shaping element. It raises probe-side capacitance so the one-metre E-probe and its active input can be optimised as one system. The result is a wider, more linear useful response across the lower 0–150 MHz design window.
Engineering foundations and current configuration
Mini-FAQ
- Why is the 15 cm hat on the E-probe? It increases probe-side capacitance so the one-metre sensing element and high-impedance input can be optimised as one broadband system through the lower 0–150 MHz design span.
- How does the hat widen the useful bandwidth? It changes the capacitive relationship between the sensing element and active input, letting the combined frequency response remain smoother over a broader lower-frequency span.
- What does “better linearity” mean here? A smoother and more predictable response versus frequency across the intended design window.
- Does 0–150 MHz include static fields at DC? No. Zero names the lower-edge direction of the design window; the practical RF low-frequency limit of the complete configuration is above DC.
- Why use other whips at higher frequencies? The one-metre whip and hat favour the lower wideband range; shorter whip configurations shift the optimised portion upward.
- Why is the disk 15 cm? It provides the required capacitance in a compact, repeatable geometry while keeping wind loading, mechanical leverage and upper-frequency distributed effects within the design compromise.