Radial system calculator
There is no exact formula for ground loss: it depends on the soil, the moisture and whatever else is buried nearby. What you get here is an interpolation of the classic Brown, Lewis & Epstein measurements of 1937, later confirmed by ON4UN and N6LF. The numbers are indicative, but the order of magnitude is right and the conclusion they lead to is unambiguous.
| Frequency (MHz) | |
| Number of radials | |
| Radial length (in λ) | |
| Placement | |
| Antenna radiation resistance (Ω) | |
| Result | |
| Length of one radial | |
| Total wire length | |
| Estimated ground loss resistance | |
| System efficiency | |
| Shortfall against 120 radials | |
📐 The data behind it
Loss resistance for a quarter-wave vertical, λ/4 radials laid on the ground:
| Radials | R_ground | Loss |
|---|---|---|
| 2 | ≈ 33 Ω | ≈ 4.5 dB |
| 4 | ≈ 18 Ω | ≈ 3 dB |
| 8 | ≈ 12 Ω | ≈ 2 dB |
| 16 | ≈ 6 Ω | ≈ 1 dB |
| 36 | ≈ 3 Ω | ≈ 0.5 dB |
| 120 | ≈ 1.2 Ω | reference |
Efficiency:
η = Rr / (Rr + R_ground)
Elevated radials (at least 0.05 λ above ground): four tuned radials do about as well as several dozen lying on the ground.
- This is an estimate from an empirical table, not a calculation. The real figures depend on your soil
- The table is for a quarter-wave vertical; on a short loaded antenna the losses weigh relatively more
- For radials shorter than λ/4 a simple scaling is applied, and it understates the difference on very short ones
- Elevated radials have to be tuned: “roughly λ/4” does not work here
🎯 In practice
- How much wire to buy. The calculator gives the total length, which is usually the real constraint.
- 16 short or 4 long. For the same amount of wire, the first is almost always better.
- Sizing up someone else's vertical. An antenna with two radials is giving away about 4 dB, and that shows on the air.
- On the ground radials are not resonant — their length matters less than their number. Start with 16.
- You can simply lay them on the soil and peg them down; burying is optional, the grass will see to it.
- All radials must meet at one point with a good connection: one bad twist undoes the whole system.
- Elevated radials are fussy about length and number, but you only need four.
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How to cite this calculator
UR3PKI. «Radial system calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/antennas/radials (licence CC BY-NC-SA 4.0).The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.