Rhombic Antenna
A rhombic is four long wires joined into a diamond shape, suspended between four supports and terminated at the far end with a resistor so the traveling wave running down the wires never reflects back — the result is a genuinely broadband, no-tuner HF antenna with real gain and a clean unidirectional pattern toward the open end of the diamond. It's also one of the largest wire antennas a ham is likely to build: legs run several wavelengths long, so this is a big-property or contest-station design, not a suburban-lot project.
Terminated vs. resonant — two very different antennas
The classic rhombic most hams mean is the terminated version: a resistor at the far apex absorbs the wave that would otherwise reflect back down the wires, so current flows in one direction only (a traveling wave) and the pattern fires unidirectionally out the far end across a wide range of frequencies with no tuner needed. Leave off the termination resistor and you get a resonant rhombic instead — a standing-wave antenna with a bidirectional figure-eight-style pattern, somewhat more gain for the same size, but narrower bandwidth and a feedpoint that needs a tuner or matching network.
Resonant (unterminated): standing wave, bidirectional, narrower bandwidth, needs a tuner
Why leg length and tilt angle trade off against each other
Longer legs concentrate more of the antenna's radiation into a tighter forward beam and add real gain, but the tilt angle (how sharply each leg angles away from the antenna's centerline) that produces the cleanest pattern gets narrower as legs get longer. Classic rhombic design charts (originating from wartime and broadcast-era antenna engineering, carried into the ARRL Antenna Book) pair leg length, tilt angle, and height above ground for a given target radiation angle — there isn't one universal tilt angle that works for every leg length.
- Shorter legs (2-3λ): wider tilt angle, less gain, more forgiving of an imperfect site.
- Longer legs (6-8λ): narrower tilt angle, more gain, and a real commitment of land and supports.
The termination resistor is a real, honest power loss
Whatever power the traveling wave still carries when it reaches the far end gets dissipated in that resistor as heat instead of radiating — commonly cited figures put this loss at roughly a third or more of the transmitter's power on a well-designed terminated rhombic. That's the honest tradeoff for the broadband, no-tuner, cleanly unidirectional behavior; the unterminated resonant version avoids this specific loss but gives up bandwidth and directionality to do it.
Why height matters as much as leg length
Like any horizontal HF wire antenna, the rhombic's vertical radiation angle depends heavily on how high above ground it's strung, not just its horizontal dimensions. A rhombic sized for a low DX takeoff angle but hung too low won't deliver the low-angle performance its size promises — height and leg geometry have to be planned together, not independently.
Installation options
- Four-mast/tower diamond: the classic layout — one support at each corner of the diamond, wires strung between them at consistent height.
- Tree-supported diamond: workable on large rural properties with well-placed trees, trading some height consistency for lower cost than four dedicated masts.
- Switchable multi-direction array: some large stations build two or more rhombics aimed at different DX headings, switched at the shack, since a single rhombic only fires well toward its far end.
| Leg length | Approx. tilt angle | Typical gain | Notes |
|---|---|---|---|
| 2λ per leg | ~70° | ~7 dBi | Smallest practical size, most forgiving of site limitations |
| 3λ per leg | ~65° | ~8-9 dBi | — |
| 4λ per leg | ~60° | ~9-10 dBi | Common size cited in general design references |
| 6λ per leg | ~55° | ~11-12 dBi | Serious real-estate commitment |
| 8λ per leg | ~52° | ~12-13 dBi | Large broadcast/contest-station scale |
These tilt-angle and gain figures are commonly published design-chart approximations, not a precise formula — the true optimum for your specific height and target takeoff angle should be checked against a full rhombic design chart or NEC model before you commit to masts and that much wire.
Rhombic Antenna Dimension Calculator
Materials for Rhombic Antenna
Building the Rhombic Antenna
Site layout and support height come first here — get the geometry planned on paper before any wire goes up.
Choose leg length and lay out the diamond on your property
Use the calculator above to get leg length and tilt angle for your chosen size, then mark the four corner positions on your property, confirming you have the clearance and support points needed.
Erect the four corner supports
Install masts or confirm suitable trees at all four corners, aiming for consistent height at each — an uneven diamond skews the pattern and takeoff angle from what the design predicts.
Cut and run all four legs
Cut four equal lengths of wire to the calculated leg length and run them between the corner supports, forming the diamond with insulators at the near apex, far apex, and two side corners.
Install the termination resistor at the far apex
Connect the non-inductive termination resistor across the two legs at the far end of the diamond, weatherproofed against the elements.
Install the feedpoint matching transformer at the near apex
Connect the matching transformer/balun at the feed end of the diamond to bring the rhombic's naturally high impedance down toward 50 ohms for coax feed.
Route the coax feedline to the station
Run the coax from the feedpoint transformer down to your shack, securing it along a support mast rather than leaving it to sag across the antenna's aperture.
Sweep across bands and verify the pattern
Sweep SWR across your intended HF range — the terminated design should show a reasonably flat, broadband match — then verify the forward pattern favors your intended DX direction on the air.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Pattern seems bidirectional instead of unidirectional | Termination resistor is open, disconnected, or the wrong value | Check continuity and value of the termination resistor | Repair or replace the termination resistor with the correct non-inductive value |
| SWR is not as broadband as expected | Feedpoint matching transformer mismatched to the rhombic's actual impedance, or an uneven diamond geometry | Check the four corner heights and the matching transformer ratio | Correct corner height evenness first, then re-check the matching transformer |
| Gain seems lower than the design chart figure | Support height too low for the leg length chosen, or a distorted diamond shape | Compare actual support height and leg symmetry against the design assumptions | Raise supports or correct geometry to match the intended design height |
Do I need the termination resistor?
Only if you want the broadband, no-tuner, unidirectional traveling-wave behavior. Skip it and you get a resonant, bidirectional rhombic instead, with somewhat more gain but a narrower bandwidth and a feedpoint that needs a tuner.
How much power does the termination resistor waste?
Commonly cited figures put the loss at roughly a third or more of transmitter power on a well-designed terminated rhombic — a real, honest tradeoff for the broadband unidirectional behavior, not a minor rounding error.
How much land does this really need?
A 4-wavelength-leg rhombic on 20m needs roughly 260+ feet per leg times two legs of usable straight-line space plus support structures at all four corners — genuinely large-property or club/contest-station territory, not a typical suburban lot.
Can this cover multiple bands?
Yes, that's one of the terminated design's real strengths — the traveling-wave behavior gives usable performance across a wide frequency range without retuning, unlike most resonant wire antennas.
Which direction does it favor?
The terminated version fires toward the far apex (the end with the termination resistor) — point that end toward your target DX region when laying out the diamond.
Is a smaller rhombic still worth building?
Yes, a 2-3 wavelength-leg version is more forgiving of a smaller site and still gives real gain and the broadband, no-tuner advantage — you trade away some of the gain and pattern sharpness a much larger rhombic offers.