Antenna Calculator

The 468/f formula is a starting point, not an exact answer. Real-world factors affect the actual resonant length:

• Height above ground — Lower antennas need to be shorter (ground capacitance)
• Wire insulation — Insulated wire needs to be ~3% shorter than bare wire
• Nearby objects — Metal gutters, towers, and trees detune the antenna
• Wire diameter — Thicker wire = slightly shorter antenna
• End effects — How you terminate the ends affects length

Always cut 3-5% long and trim to resonance. An antenna analyzer or NanoVNA makes this much easier.

Short answer: Yes, it's highly recommended.

A dipole is a balanced antenna but coax is unbalanced. Without a balun, RF current flows on the outside of your coax shield (common mode current), causing:

• RF in your shack — hot mic, computer interference, RFI complaints
• Distorted radiation pattern — your feedline becomes part of the antenna
• Inconsistent SWR — readings change when you touch or move the coax
• Increased noise pickup on receive

A simple 1:1 choke balun at the feedpoint solves this. Many hams run dipoles without baluns and "it works," but adding one typically improves performance and eliminates problems. See our Choke Calculator to build one.

It depends on what you want to do:

For DX (long distance): Get it as high as possible — ideally 1/2 wavelength or higher. At this height, the radiation angle is low, favoring skip propagation. For 40m, that's about 66 feet; for 20m, about 33 feet.

For NVIS (regional, 0-500 miles): Lower is actually better! A dipole at 1/8 to 1/4 wavelength high radiates straight up, hitting the ionosphere and coming back down for reliable regional coverage. This is ideal for emergency communications and nets.

Practical advice: Most hams can't get a dipole to optimal DX height. Don't worry — a dipole at 30-40 feet will work both DX and regional. "Best antenna is the one that's up."

Neither is universally "better" — they have different characteristics:

Inverted-V advantages:

• Only needs ONE high support point (center)
• Slightly more omnidirectional pattern
• Takes up less horizontal space
• Often easier to install

Flat dipole advantages:

• More gain broadside to the wire (~2 dB)
• Better for working specific directions
• Slightly more bandwidth

Inverted-V tips: Keep the included angle above 90° (120° is ideal). Steeper angles reduce efficiency and bandwidth. The ends should be at least 10 feet off the ground.

More is better, but there's a point of diminishing returns:

• 4 radials: Absolute minimum. Works, but you're losing 3+ dB
• 16-20 radials: Good performance for most stations
• 24-32 radials: Excellent — near maximum practical efficiency
• 32 vs 64 radials: The difference is negligible according to real-world testing

Key principles from DX Commander (M0MCX) research:

• More shorter radials beat fewer longer radials — this is critical
• Total radial wire = 2× wavelength of your band is good; 4× wavelength is the practical maximum benefit
• For ground-mounted radials, exact length doesn't matter much — just get wire on the ground
• Elevated radials are different — these should be tuned to frequency, and 4 elevated radials can work well

Practical example: For a 20m vertical, 24 radials at 10ft (3m) each gives you 4× wavelength of total wire — near optimal performance without excessive cost.

Further reading: Callum (M0MCX) at DX Commander has done extensive real-world testing on radial systems. His research paper "How many radials do I need for a vertical antenna?" provides practical measurements and recommendations.

Elevated radials are an entirely different ground system from buried/ground-mounted radials. Instead of dozens of wires laid on or under the soil acting as an imperfect mirror, you use a small number of resonant wires lifted off the ground. The currents flow in the radials themselves rather than through lossy soil, so a few elevated radials can match the efficiency of a much larger buried field.

Key numbers (from published research):

• How many: 2 works (with a slightly skewed pattern), 3 is acceptable, 4 is the practical standard. Going beyond 4 yields only small additional gains.
• Length: Each radial is a resonant 1/4 wavelength — start at 234/f(MHz) feet (71.5/f in meters) and trim each one to resonance once installed. Surrounding objects detune them, so in-place adjustment matters.
• Minimum height: Roughly 0.04 λ above ground (≈ 2.8 ft on 20 m, ≈ 5.5 ft on 40 m, ≈ 11 ft on 80 m). Below that, ground losses start to dominate again. Higher is better; gains taper above ~0.1 λ.
• Feed impedance: Horizontal elevated radials give roughly 30–36 Ω. Drooping them at ~45° raises the impedance to ~50 Ω — a direct match to coax without a transformer.
• Equivalence: Christman's NEC modeling (QST, Aug 1988) showed that 4 elevated radials ≈ 60 buried radials in terms of low-angle gain efficiency.

When elevated wins:

• Portable / POTA / SOTA operation — no shovels, no lawn destruction
• Rocky, sandy, or poor-conductivity soil where buried radials work poorly
• Roof, balcony, or hilltop installs where you can't lay a ground field at all
• Anywhere you want fewer wires and a near-50 Ω match

When buried/ground-mounted wins:

• Permanent backyard install with grass over the wires
• Multi-band verticals — buried radials are non-resonant and broadband; elevated radials must be cut for each band
• Salt-water or marsh sites where ground conductivity is already excellent

Practical tips:

• Space radials symmetrically (90° apart for 4 radials, 120° for 3) — asymmetric layouts skew the radiation pattern.
• Use a common-mode choke at the feedpoint — elevated radial systems tend to put RF on the coax shield more than buried ones.
• If you must run only 1 or 2 radials (e.g., space constraints), expect a few dB of loss and an asymmetric pattern.

References:

• Christman, A.D. (KB8I, now K3LC), "Elevated Vertical Antenna Systems", QST, August 1988, pp. 35–42 — the canonical NEC modeling study comparing buried vs. elevated radial counts.
• Severns, R. (N6LF), "Verticals, Ground Systems and Some History" — multi-part QEX series and follow-up measurement papers (real over-the-air field tests confirming Christman's modeling).
• ARRL Antenna Book, current edition — chapter on vertical antennas and ground systems summarizes both regimes.

Common choices and when to use them:

Stranded copper wire (most popular):

• 14 AWG for permanent installations up to 1500W
• 16-18 AWG for portable antennas — lighter and flexible
• Insulated (THHN) is fine — just cut 3% shorter

Copperweld (copper-clad steel):

• Much stronger for long spans
• Good for 80/160m dipoles that sag
• Harder to work with (stiff, hard to solder)

Avoid:

• Aluminum wire — hard to connect, corrodes
• Steel wire — lossy at RF unless copper-clad
• Speaker wire — too thin for transmitting

Yes, but you'll need a tuner and accept some compromises:

Dipoles have harmonics: A 40m dipole works reasonably well on 15m (3rd harmonic). A 80m dipole works on 40m, 20m, and 10m. The pattern changes on harmonic bands.

Using a tuner: An antenna tuner can match almost any antenna to your radio, but it doesn't change the antenna's efficiency. A 40m dipole tuned to 80m will "work" but most of your power becomes heat in losses.

Rule of thumb: Antennas work best within about 2:1 of their resonant frequency. A 20m dipole can be tuned 15m-30m reasonably well, but don't expect good results on 80m.

Better solutions: Fan dipole, trap dipole, or OCF dipole if you need multi-band from one antenna.

It's derived from the speed of light with a correction factor:

In free space, a half-wave is: 492 / f(MHz) feet

But real antennas have "end effect" — the wire's ends have capacitance to ground, making the antenna electrically longer than its physical length. This shortens the required physical length by about 5%.

492 × 0.95 = 467.4 ≈ 468

Why it's approximate: The 5% correction assumes a thin wire in free space. Your actual correction factor depends on wire diameter, height, and surroundings. That's why we always cut long and trim.

They look similar but work very differently:

End-Fed Half-Wave (EFHW):

• Cut to a specific length (half-wave of your frequency)
• Uses a 49:1 transformer to match high impedance (~2500Ω) at the feed end
• Works on fundamental and even harmonics (40m EFHW works on 20m, 15m, 10m)
• Efficient — most power goes into radiation

Random wire:

• Any length wire (though some lengths work better)
• Requires a tuner at the radio to match varying impedance
• Works on many bands but less efficiently
• More RF on the feedline and in the shack

Bottom line: EFHW is a resonant antenna; random wire is a compromise antenna. Both have their place.

Low SWR doesn't mean good antenna! Common issues:

• Lossy feedline: Old or cheap coax can have high loss, especially on VHF/UHF. RG-58 at 100ft on 2m loses over 5dB!
• Antenna too low: A dipole at 10ft works, but poorly. Height is gain.
• Bad ground system: Verticals need radials. No radials = most power heats the ground.
• Common mode currents: RF on the feedline shield radiates inefficiently. Add a choke balun.
• Nearby obstructions: Metal buildings, power lines, and trees absorb RF.
• The "dummy load effect": A perfect 50Ω match that radiates nothing is useless. Resistive losses show as "good" SWR.

A 2:1 SWR antenna that's high and clear often outperforms a 1.1:1 antenna that's low and obstructed.