Crossed Field Antenna (CFA)
The Crossed Field Antenna combines a small vertical E-field element with a separate H-field element (typically a plate or loop), fed in a specific phase relationship its proponents say lets a physically tiny structure radiate as efficiently as a much larger conventional antenna. This is one of the more actively disputed designs in amateur antenna circles — the claims made for it are not accepted by mainstream antenna engineering, and independent testing has not confirmed the extraordinary efficiency figures sometimes published for it. This guide describes the design and the claims made for it honestly, alongside what independent analysis has actually found, so you can decide for yourself whether it's worth building as an experiment.
The Basic Structure
A CFA typically consists of a short vertical cylinder or rod (intended as the E-field radiating element) mounted near a horizontal plate or loop (intended as the H-field element), fed through a network designed to establish a specific amplitude and phase relationship between the current in each element. The stated goal is to produce, from this small combined structure, a radiating field comparable to a conventional quarter-wave or larger antenna.
The Original Claims
Proponents of the CFA — most notably work associated with Andrew Underhill dating from the 1990s — have claimed the design achieves radiation efficiency and pattern characteristics that would ordinarily require an antenna many times its physical size, describing the mechanism in terms that some critics argue depart from standard treatments of Maxwell's equations and conventional antenna theory. These claims have been published in patents and some promotional and hobbyist literature, and have generated genuine, sustained interest and construction attempts within the amateur and broadcast engineering communities.
What Independent Analysis Has Found
Multiple independent antenna engineers and researchers have modeled and measured CFA designs over the years, and — as with the EH antenna elsewhere on this site — have generally not confirmed the extraordinary efficiency claims made for the design. The mainstream antenna engineering community's position is that a CFA, once properly analyzed, behaves consistently with standard antenna theory for a small radiator of its physical size — meaning it is subject to the same fundamental small-antenna efficiency limits as any other electrically small structure, not exempt from them.
- What's real: a CFA structure can be built, fed, and matched to a transmitter, and it does radiate some signal.
- What's disputed: whether that radiated signal represents anything beyond what standard small-antenna theory would already predict for a structure of that size — mainstream analysis says no, and no widely accepted independent measurement has demonstrated otherwise.
Setting Honest Expectations Before You Build
If you build a CFA, treat it as an experimental or educational project — a chance to test a genuinely controversial claim yourself against a reference antenna — rather than as a way to get full-size antenna performance from a small structure. Expect signal reports consistent with an electrically small, loaded radiator of similar physical size, not the extraordinary results sometimes described in promotional material. This is precisely the kind of claim that rewards building it yourself and measuring against a known-good reference, rather than taking either side's word for it.
| Component | Typical size | Notes |
|---|---|---|
| E-field element (vertical cylinder) | 2-4 ft tall, 4-8 in diameter | Aluminum tube or sheet rolled into a cylinder |
| H-field element (plate or loop) | 2-3 ft diameter | Mounted horizontally near the base of the E-field element |
| Phasing/matching network | Small coil and capacitor network | Establishes the amplitude/phase relationship between elements |
CFA Element Sizing Calculator
This calculator gives a starting-point structure size based on standard small-antenna scaling — it makes no claim about the disputed efficiency figures published for this design, only a reasonable physical size to build and test against.
Materials for a small experimental CFA build
Building an Experimental CFA
Build this as a test-and-measure project, with a known-good reference antenna on hand for genuine comparison.
Build the E-Field Cylinder
Form the aluminum sheet or tube into the vertical cylinder shape from the dimensions table, mounted on a non-conductive support.
Build the H-Field Element
Construct the horizontal plate or loop element and mount it near the base of the E-field cylinder, maintaining the intended spacing between the two elements.
Build the Phasing/Matching Network
Assemble the variable coil and capacitor network intended to establish the amplitude and phase relationship between the two elements, with test points accessible for measurement.
Connect the Feedline
Connect your coax to the matching network's input.
Sweep SWR and Adjust the Network
Using the NanoVNA, sweep SWR and adjust the variable network components to find a low-SWR condition at your target frequency.
Test Against a Known-Good Reference Antenna
Using a field-strength meter, a WSPR comparison, or on-air signal reports, compare this antenna's actual performance against your known-good reference antenna on the same band and power level.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Can't find a low SWR anywhere | Matching network component values outside a workable range, or a wiring fault | Check continuity and component values against the network design | Adjust the variable inductor/capacitor range; re-check all connections |
| Low SWR achieved but signal reports are weak | Consistent with mainstream small-antenna theory for a structure this size | Compare directly against your reference antenna at the same power | Not necessarily a fault — this is the expected outcome per independent analysis of this design |
| Results vary significantly between test sessions | Ground conditions, nearby objects, or network drift affecting the measurement | Repeat comparisons under consistent conditions | Control test variables (time of day, power level, propagation) as tightly as possible for a fair comparison |
| Network components heat up or drift under power | Component values or power handling inadequate for your operating power | Check component temperature and power ratings | Use higher-power-rated components, or reduce test power |
Does the CFA really work as originally claimed?
The claims of full-size antenna performance from a small combined E-field/H-field structure have not been confirmed by independent antenna engineers who have modeled and measured CFA designs, and the mainstream antenna engineering community does not accept the theoretical mechanism as described by proponents. Expect performance consistent with a small radiator of similar physical size, not the extraordinary results sometimes claimed.
Is it worth building at all?
As an experimental or educational project — genuinely testing a disputed claim yourself against a reference antenna — yes, it's a legitimate and interesting build. As a way to get full-size HF performance from a small footprint, the honest answer based on available independent analysis is no.
Why do some people report good results with a CFA?
Anecdotal on-air reports are hard to control for propagation, power differences, and confirmation bias. Rigorous independent testing — controlled comparisons against a known reference antenna — is what actually settles a disputed antenna claim, and that kind of testing has not supported the original extraordinary claims for this design.
How does this compare to the EH antenna elsewhere on this site?
Both are electrically small radiators with disputed efficiency claims that independent analysis hasn't confirmed, and both are presented on this site the same way — described honestly, with the controversy stated plainly, rather than as settled, proven designs.
What should I actually expect if I build one?
Signal performance roughly consistent with any other electrically small, loaded antenna of similar physical size — workable for casual use or as a curiosity, but not a substitute for a full-size antenna where one will fit.
Is this a scam or is it a legitimate area of ongoing research?
The people who have built and promoted CFAs generally appear to believe in the design, and this isn't presented here as a deliberate deception — it's presented as a disputed claim that mainstream antenna theory and independent measurement have not supported, which is a meaningfully different (and more accurate) framing than either "proven" or "fraud."