Antennas · Volume 12
Discone & Wideband (Discone-Family) Antennas
Discone geometry, sleeve, conical monopole, biconical — the impedance-derived wideband family for scanners and wideband SDR receive (and the surprising fact that they transmit reasonably well too)
Contents
1. About this volume
The discone is the canonical wideband antenna. Where the Yagi-Uda family (Vol 11) achieves gain through carefully-tuned constructive interference at one frequency, the discone family achieves bandwidth through geometric scale invariance — a structure whose shape is defined entirely by angles (no characteristic length) has theoretically infinite bandwidth. In practice the bandwidth is constrained by the antenna’s physical extent (no antenna can radiate efficiently at a wavelength much longer than the antenna itself), but a properly-designed discone routinely covers a 10:1 frequency range (e.g. 30 MHz to 300 MHz, or 100 MHz to 1 GHz) with SWR < 2:1.
For the Hack Tools hub, the discone is the antenna that ties together:
- HackRF One (deep dive) wideband receive from 1 MHz – 6 GHz (the discone handles 25 MHz – 1.3 GHz, the heart of the HackRF’s most-used range)
- RTL-SDR (deep dive) 25 MHz – 1.7 GHz receive
- Flipper Zero (deep dive) sub-GHz receive (a discone designed for low end at 25 MHz also catches the 300–928 MHz Flipper bands)
- Public-safety scanners (Uniden, Whistler, AOR) which monitor 25–1300 MHz across multiple bands
- Aircraft band monitoring (118–137 MHz AM)
- NOAA weather satellites at 137 MHz APT downlink
- Amateur 6 m / 2 m / 70 cm / 23 cm monitoring with one antenna
The “one feedline, all my listening” reality is why every scanner sold over the past 40 years has been packaged with a discone (or its sleeve-antenna miniature variant). Where the EFHW (Vol 10) is the dominant HF amateur antenna, the discone is the dominant scanner/wideband-receive antenna.
This volume covers the discone family at engineer-grade depth. The sibling volume Vol 13 (Log-periodic & structured wideband) covers the directional wideband family (LPDA, horn, spiral, Vivaldi) — same bandwidth-via-geometric-similarity principle, but with directional patterns. The discone family is the omnidirectional wideband family.
2. Geometry & theory — why a cone over a disc is wideband
2.1 The Rumsey frequency-independence principle
Victor Rumsey (1957) proved that any antenna whose geometry is defined entirely by angles (no characteristic length) is frequency-independent. Examples:
- A cone (defined by half-angle + extending to infinity)
- A spiral (defined by winding rate, extending to infinity)
- A biconical structure (two cones tip-to-tip)
- An equiangular spiral (the canonical frequency-independent antenna)
Real antennas are finite, so they’re not truly frequency-independent. But within their finite extent, an antenna whose geometry is angle-defined exhibits a self-similar structure that performs consistently across frequency: at any frequency where the antenna is electrically “big enough” (the lowest dimension is ≥ λ/4), the impedance and pattern are roughly constant.
For the discone specifically: the cone half-angle determines the impedance, and the cone’s slant height determines the lowest frequency. At the lowest frequency where the slant height ≈ λ/4, the antenna starts working; at any higher frequency, it still works (the antenna just looks “smaller in wavelengths,” but the geometric similarity preserves the impedance and pattern).
2.2 The cone-over-disc geometry
Armig Kandoian’s original 1945 discone has the following geometry:
● feedpoint at gap (coax center to disc, shield to cone apex)
╱ ╲
╱ ╲
╱ GAP ╲ ← critical: ~5-10 mm at 144 MHz scale
╱ ╲
●═══════════════● ← horizontal disc, diameter ≈ 0.7 × cone slant
/│\
/ | \
/ | \
/ | \
/ | \
/ | \ ← cone, slant height = λ/4 at lowest frequency
/ | \
/ | \
/ | \
/ | \
● ● ● ← cone base diameter
← cone half-angle determines impedance
(typically 25-60°; 60° gives 50 Ω)Key relationships:
- Slant height (cone) = λ/4 at the lowest design frequency
- Disc diameter = 0.7 × cone slant
- Gap distance (between disc and cone apex) = 0.3–0.5 × cone slant
- Cone half-angle = 25–60° (60° gives ~50 Ω feedpoint Z; smaller half-angles give higher Z)
- Cone base diameter = 2 × slant × sin(half-angle)
A discone designed for 30 MHz – 300 MHz has:
- Slant height: ~2.5 m (λ/4 at 30 MHz)
- Disc diameter: ~1.75 m
- Cone base diameter: ~4.3 m at 60° half-angle (a huge structure)
- Bandwidth: 30 MHz – 300 MHz minimum, often up to 1 GHz
A discone designed for 100 MHz – 1 GHz has:
- Slant height: 75 cm (λ/4 at 100 MHz)
- Disc diameter: ~52 cm
- Cone base diameter: ~130 cm at 60° half-angle
- Bandwidth: 100 MHz – 1 GHz minimum, often up to 2 GHz
The discone scales straightforwardly — smaller for higher frequency, larger for lower frequency. The 30–300 MHz “amateur scanner discone” is the standard size; the 100 MHz – 1 GHz size is the indoor/desktop version.
2.3 The cone as a tapered transmission line
The discone’s wideband behavior comes from the tapered transmission line that the cone forms. The cone’s characteristic impedance varies along its length: high at the tip, low at the base. This impedance taper acts as an impedance-matching transformer that gives a smooth Z presentation across frequency.
The same principle applies to:
- Sleeve antennas (§4): half a discone over a ground plane, with the cone replaced by an actual ground plane and the disc replaced by a coaxial sleeve choke
- Biconical (§5): two cones tip-to-tip, fed at the gap — the symmetric version
- Conical monopole: a cone over a ground plane — the half-version
- Bow-tie: a planar version of the biconical (often seen on TV-broadcast receive antennas)
2.4 Why “feedpoint at the gap, coax to apex”
Coax connection at the discone’s gap: the center conductor connects to the disc (which is the “high-impedance” point of the antenna), and the shield connects to the cone apex (which is the “low-impedance” current point). This connection orientation matters:
- Reversing it (center to cone, shield to disc) typically degrades performance by 1–3 dB and shifts the pattern
- Adding a 1:1 current BALUN at the feedpoint suppresses common-mode current on the coax shield
- The gap distance is electrically small (a few percent of the lowest-frequency wavelength), so the gap behaves essentially as a point feed
3. The discone proper — Kandoian’s original geometry
3.1 The 1945 design
Armig Kandoian published the discone in a 1945 paper in IRE Proceedings. The geometry was developed during WWII at FCC Laboratories as a wideband VHF/UHF receive antenna for monitoring foreign communications. The original Kandoian discone used:
- 60° cone half-angle
- Disc-to-cone-slant ratio of 0.7
- Solid (or near-solid) cone construction
- Solid disc construction (later replaced with mesh / spokes for wind tolerance)
The 1945 paper established the canonical geometric relationships that essentially every subsequent discone has followed. Modern variations refine the cone shape (16 wire spokes instead of solid, varying radial geometry), but the core geometry is unchanged.
3.2 Solid vs spoked construction
Modern commercial discones use spoked construction for both the disc and cone:
- Spoked disc: 8–16 radial spokes from the center outward, no solid surface. Wind transparent (much less wind load than solid), electrically equivalent to solid at frequencies where each spoke is electrically small (~λ/20 or smaller spacing between spokes).
- Spoked cone: 8–16 vertical or tilted spokes from the apex outward, forming the cone shape. Wind transparent, electrically equivalent to a solid cone.
The number of spokes matters: at the high end of the discone’s frequency range, the spoke-to-spoke spacing approaches λ/4 and the antenna starts to look like discrete radiators rather than a continuous cone. 8 spokes are adequate for 1 GHz operation; 16 spokes give cleaner pattern at the high frequency end.
3.3 Disc shape variations
Some discone designs use slight modifications of the simple flat disc:
- Inverted-cup disc: a slight upward cup shape (instead of flat). Improves bandwidth at the high-frequency end.
- Capacitively-loaded disc: a small disc-shape with a top hat. Extends the low-frequency end.
- Eccentric disc (rare): a disc offset from the cone’s vertical axis. Produces a slight directional pattern at one frequency.
For most amateur and scanner applications, the simple flat disc is fine. The disc shape variations matter mostly for EMC-test applications where the bandwidth at the high end is being squeezed for measurement coverage.
3.4 The static-discharge consideration
A discone’s apex point (where the cone tip meets the coax shield connection) is at high voltage and is exposed. Lightning hitting the discone’s apex would conduct directly into the coax shield. Two common protective approaches:
- DC short at the apex: a small ferrite bead with a short conductor across the apex provides a DC return path that drains static accumulation but is invisible at RF (the ferrite blocks RF current). Many commercial discones include this.
- Polyphaser arrestor at the bulkhead: blocks high-voltage transients before they enter the shack. The recommended approach for any permanent installation (covered in Vol 20 §5).
The static-discharge issue is more important for outdoor discones; indoor desktop discones are less exposed to lightning and atmospheric static.
4. The sleeve antenna — half a discone over a ground plane
4.1 The half-discone-on-a-pole
A sleeve antenna takes the lower half of a discone (just the cone) and mounts it inverted over a metal sleeve forming a quarter-wave choke. The result is a vertically-polarized omnidirectional antenna in a much more compact form than the full discone.
● feedpoint (coax center conductor exits at top of sleeve)
│
/ \
/ \
/ \ ← cone (inverted, base at top, apex at sleeve top)
/ \
═══════════════════ ← sleeve (coaxial, λ/4 long at center freq, acts as choke)
///// feedline through interior of sleeveThe sleeve replaces the ground plane that a vertical monopole would need. Inside the sleeve, the coax shield is connected to the upper-end of the sleeve, and the sleeve itself becomes the antenna’s “other half” (mirrored against the upper cone).
4.2 Sleeve antenna performance
- Bandwidth: 3:1 to 5:1 (less than a true discone, but still wideband)
- Pattern: vertically polarized omnidirectional, peak elevation 20–35° (similar to a discone)
- Gain: 1–3 dBi (better than a simple vertical because of the sleeve’s slight collinear effect)
- Form factor: a single cylinder ~1 m tall for a 50 MHz – 200 MHz design — much more compact than a 30 MHz – 300 MHz discone
The sleeve antenna is the canonical “compact all-band vertical” used in commercial scanners and amateur “no-tuner verticals”:
- Comet UHV-10: 6/10/15/20/40 m + 50/144/430 MHz amateur sleeve vertical, $300
- Diamond AZ-510: a compact 5–500 MHz sleeve scanner antenna, $80
- MFJ-1796: a 4-band amateur sleeve antenna, $230
For applications where a full-size discone is too large or too visible, the sleeve antenna is the compact compromise. Performance is slightly worse than a full discone but adequate for most scanning and casual amateur use.
5. Biconical and conical monopole — the EMC-test variants
5.1 Biconical: two cones tip-to-tip
A biconical antenna is two cones tip-to-tip, fed at the gap between them. Symmetric geometry, balanced feedpoint. The biconical’s properties:
- Vertical polarization when the cones’ common axis is vertical
- Omnidirectional in azimuth (rotationally symmetric)
- Bandwidth: similar to a discone (~10:1 typical)
- Smaller than a discone at the same lowest frequency (no disc means more vertical extent and less horizontal extent)
The biconical’s most prominent application is EMC (electromagnetic compatibility) test antennas. FCC EMC compliance testing uses a standardized biconical antenna for 30–300 MHz emissions measurements. The standardized model is the Bilog (a hybrid biconical + log-periodic), but pure biconicals are used for the 30–200 MHz range.
Examples:
- A.H. Systems SAS-540: EMC-test biconical, 30–300 MHz, $2500
- R&S HK116: 30–300 MHz biconical, $3000+
- EMCO 3104C: calibrated biconical, $5000+
For amateur use, biconicals are rare — the discone’s flat horizontal disc provides a slightly cleaner “horizontal” orientation reference, and the disc gives a smaller cone half-angle option (60° is impractically wide for a biconical at low frequencies).
5.2 Conical monopole
A conical monopole is a cone over a ground plane — half of a biconical, electrically equivalent to a full biconical (the ground plane acts as the image of the bottom cone).
● feedpoint (coax)
│
│
│
│ ← cone
│
│
/ │ \
/ │ \
/ │ \
═════════════════════════════════════ ← ground plane (radials or sheet metal)Properties:
- Vertical polarization
- Omnidirectional
- Bandwidth: 5:1 to 10:1 (similar to a discone, but the ground-plane requirement adds installation complexity)
- Compact: ~half the height of a biconical at the same lowest frequency
The conical monopole is sometimes used as the wideband element on a vehicle mast where the vehicle roof acts as the ground plane. For amateur use, the discone is usually preferred (no ground plane needed).
5.3 Bow-tie — the planar version
A bow-tie antenna is the 2D planar version of a biconical: two triangles tip-to-tip on a flat plane. Used in:
- TV-broadcast indoor receive antennas (the “rabbit ears” replacement — better aesthetics, wider bandwidth)
- PCB-printed wideband antennas (UWB receive on a circuit board)
- 2.4 GHz / 5 GHz Wi-Fi enclosures (a small PCB bow-tie inside a router)
The bow-tie is not an omnidirectional antenna in the same sense as the conical/biconical/discone — its pattern is figure-8 (similar to a dipole) with the polarization in the bow-tie’s plane. Useful for fixed-orientation receive where the source direction is known.
6. Feedpoint impedance and matching
6.1 The natural 50 Ω impedance
A properly-proportioned discone (60° cone half-angle, 0.7 disc-to-slant ratio) presents 50–70 Ω impedance across its operating bandwidth — directly compatible with 50 Ω coax. No external matching network is needed.
The impedance varies smoothly across the band:
| Frequency (relative to f_min) | Typical Z |
|---|---|
| 1.0× (lowest design) | 50–55 Ω |
| 1.5× | 55–65 Ω |
| 2× | 60–70 Ω |
| 3× | 60–70 Ω |
| 5× | 55–70 Ω |
| 10× (highest usable) | 60–75 Ω |
SWR on 50 Ω coax stays below 2:1 across the entire 10:1 frequency range for a well-built discone. No matching network is required.
6.2 Variations from the natural impedance
If the cone half-angle is wrong, the impedance drifts:
| Cone half-angle | Typical Z | SWR on 50 Ω coax |
|---|---|---|
| 25° (narrow cone) | 100–150 Ω | 2:1 to 3:1 |
| 30° | 80–100 Ω | 1.6:1 to 2:1 |
| 45° | 60–80 Ω | 1.2:1 to 1.6:1 |
| 60° (the standard) | 50–70 Ω | 1:1 to 1.4:1 |
| 75° | 35–50 Ω | 1:1 to 1.4:1 |
| 90° (flat disc + flat ground plane) | 30–40 Ω | 1.25:1 to 1.7:1 |
The 60° standard is the universal choice for amateur discones. Commercial discone designs sometimes use 45° (slightly less wide impedance, marginally better gain) but 60° dominates.
6.3 Common-mode current control
A 1:1 ferrite choke at the feedpoint suppresses common-mode current on the coax shield. The choke is typically a string of Mix-43 beads slid over the coax just below the connector, or a 6–10-turn coil of coax through a 4″ form. Common-mode suppression is more important on discones than on dipoles because the discone’s “ground side” (the cone) is electrically large and capable of carrying significant current.
A discone without common-mode choking will:
- Have a degraded pattern (asymmetric, slight skew)
- Pick up shack-side RF noise on the shield (the operator’s body and shack equipment become receive elements)
- Have a slightly elevated SWR (variable, frequency-dependent)
The choke is $10–20 worth of parts and improves discone performance materially.
7. Radiation pattern — omnidirectional with low-angle null
7.1 The pattern is omnidirectional in azimuth but not peak-at-zenith
A discone’s elevation pattern is the inverse of intuition — it doesn’t peak straight up. Instead, the pattern has:
- Peak elevation at ~25–35° above horizon (similar to a quarter-wave vertical)
- Null at zenith (no radiation straight up)
- Reduced gain near horizon (the cone’s apex points down at the horizon)
- Omnidirectional in azimuth (slight ripple, ±1 dB)
Elevation pattern (cross-section, side view of discone):
↑ zenith null (no radiation)
╱─╲ ╱─╲
╱ ╲ ╲
╱ ╲ ╲ ← peak at ~25-35° elevation
╱ peak ╲ ╲
╱ ╲ ╲
●═════════════════════════● horizon (~0° elevation)
feedpoint at antenna's vertical center
↑ disc
↓ cone7.2 The “good for scanning, bad for handhelds” tradeoff
The discone’s elevation pattern is excellent for scanning (signals come in from elevated angles — repeaters on mountaintops, aircraft overhead, satellites passing) but suboptimal for horizon-line-of-sight receive of nearby vertically-polarized handhelds.
A 2 m handheld at horizon distance is essentially a horizon signal — the discone’s null at horizon means the handheld’s signal is reduced by 3–6 dB compared to a vertical or J-pole. For “monitor my local handheld friend,” a J-pole or vertical outperforms a discone.
For monitoring everything else (repeaters elevated above the horizon, aircraft, satellites, atmospheric beacons), the discone’s pattern is optimal.
7.3 Azimuth uniformity
A discone with 8 spokes has ~±0.5 dB azimuth ripple; with 16 spokes the ripple drops to ~±0.2 dB. For typical scanner / SDR use, even 8-spoke discones have azimuth ripple low enough that “azimuth dependence” isn’t noticeable.
Asymmetric installations (a discone close to a building wall or large metal object) produce significant azimuth pattern distortion — 3–6 dB or more. Mount discones in clear space, away from buildings.
8. Frequency response — the 10:1 bandwidth claim, examined
8.1 The theoretical 10:1 bandwidth
A frequency-independent antenna (Rumsey’s principle) should have infinite bandwidth. A real discone with a finite cone slant height L has:
- Lower bound: f_low ≈ c / (4L), where L is the cone slant
- Upper bound: limited by the finite-size effects (the disc has a fixed size, the cone has a fixed angle)
In practice, the SWR < 2:1 bandwidth is about 10:1 for a well-built discone. The 10:1 ratio (e.g. 30 MHz to 300 MHz) is the standard amateur discone specification.
8.2 The 40:1 “useful range” claim
Vendor specifications sometimes claim 30 MHz – 1300 MHz (a 40:1 range) for a single discone. The reality:
- 30 MHz – 300 MHz: SWR < 2:1, full performance
- 300 MHz – 600 MHz: SWR 2:1 to 3:1, mild degradation
- 600 MHz – 1000 MHz: SWR 2.5:1 to 4:1, noticeable degradation
- 1000 MHz – 1300 MHz: SWR 3:1 to 5:1, significant degradation
For receive applications, SWR up to 4:1 is acceptable (mismatch loss < 1.5 dB, no impact on noise figure). For transmit applications, the SWR must be < 2:1 — meaning the actual transmit-usable bandwidth is the 10:1 range, not the 40:1 “receive-everything” range.
The vendor spec’s 40:1 claim is honest for receive; the transmit usable range is 10:1.
8.3 Bandwidth degradation modes
Three things limit the discone’s bandwidth at the edges:
- Lower bound: the slant height becomes too short relative to wavelength (the antenna is electrically too small)
- Upper bound: the disc-to-cone spacing becomes too large relative to wavelength (the gap region becomes a transmission line of significant length)
- Internal resonances: at certain frequencies, the cone spokes or disc structure can support resonant modes that distort the pattern and impedance
A 30 MHz – 1300 MHz discone has:
- Lower bound: cone slant ~2.5 m, which is λ/4 at 30 MHz
- Upper bound: disc-cone gap ~25 cm, which is λ/4 at 300 MHz (where the impedance starts to drift) and λ at 1200 MHz (the upper limit of clean operation)
Above the upper bound, the discone still receives but with significant pattern degradation and impedance variation.
9. Best-case use
The discone family is at its best when:
- Wideband SDR receive across HF/VHF/UHF: 25 MHz – 1.3 GHz on a single feedline. HackRF One, RTL-SDR, AirSpy, Flipper Zero sub-GHz — all benefit from one antenna covering everything.
- Aircraft band monitoring: 118–137 MHz AM. A discone has clean pattern in this range and adequate gain.
- NOAA weather satellite receive at 137 MHz: APT or HRPT downlink. A discone catches this naturally.
- Public-safety / FM-broadcast / amateur VHF/UHF reception in one antenna. Single feedline covers it all.
- Trunked-radio monitoring: trunking systems use multiple control frequencies across multiple bands; a discone covers them all without retuning.
- Scanner hobbyist: the canonical scanner antenna. Uniden, Whistler, AOR — all designed assuming a discone outside.
- Wideband EMC measurement in research labs.
- Wireless intrusion detection at moderate range — a discone hears WiFi 2.4 GHz at the low end of its design range, plus all the sub-GHz / VHF / UHF wireless traffic.
- Wide-band signal-of-interest hunting: a discone + SDR + a spectrum-analysis script catches everything in the air, then humans look at the captures.
10. Worst-case use
The discone is wrong for:
- HF receive (below 25 MHz): a discone designed for 30 MHz + will tolerate HF receive but with poor sensitivity. For HF, use an EFHW (Vol 10) or a random wire (Vol 10 §3).
- High-gain DX: omnidirectional is the antithesis of high-gain. For long-distance VHF/UHF point-to-point, use a Yagi (Vol 11).
- Transmit on amateur amplifier power: most discones are 100–200 W rated. For 500 W + operation, the matching at the discone’s edges of frequency range becomes lossy (heating the gap-region insulator) and the SWR variability is a problem.
- Indoor confined-space receive: a 30 MHz – 300 MHz discone is 2.5 m tall and 1.75 m wide — physically too large for indoor use. Use a smaller sleeve antenna or a desktop-sized discone (75 cm tall for 100 MHz – 1 GHz).
- Local low-angle receive of vertically-polarized handhelds: the horizon null reduces receive sensitivity for nearby horizon signals. Use a J-pole or vertical for that case.
- Direction-finding receive: omnidirectional pattern provides no nulls. Use a small loop or a Yagi instead.
- High-power AM/SSB transmit on a single VHF band: a band-specific Yagi or vertical is more efficient than a wideband discone for transmit on a single frequency.
11. Power handling
| Limit | Typical max continuous |
|---|---|
| Disc-cone gap dielectric | 100–500 W (depending on construction) |
| Cone spoke heating | 1 kW+ (spokes are well-cooled) |
| Coax connector / mounting | 500 W typical |
| Matching at edge of band | Reduced at SWR edges |
Commercial discone power ratings (mid-2026):
| Model | Power rating |
|---|---|
| MFJ-1868 | 100 W |
| Diamond D-130J | 200 W |
| Comet DS-150S | 250 W |
| Sirio SD-1300U | 250 W |
| AOR DA-3200 | 250 W |
| A.H. Systems wideband discones | 500 W+ (commercial-grade) |
| Watson WD-130 | 200 W |
| Custom DIY all-aluminum discones | 1 kW+ |
For amateur transmit at typical 100 W levels, a $80–150 commercial discone is fine. For amplifier (500W+) operation, look for a commercial-grade unit (A.H. Systems, R&S) or a DIY discone with robust construction.
The DC short at the cone tip (sometimes included for static drain) reduces lightning vulnerability but doesn’t change the power rating.
12. DIY build — a 100 MHz – 1 GHz discone from sheet aluminum
This is a desktop / mid-size discone for VHF/UHF wideband receive and modest transmit. About 6 hours of work plus tuning. Total parts cost ~$45 USD.
12.1 Geometry
| Parameter | Value |
|---|---|
| Cone slant height | 75 cm (λ/4 at 100 MHz) |
| Disc diameter | 52 cm (0.7 × cone slant) |
| Cone base diameter | 130 cm (60° half-angle) |
| Cone-to-disc gap | 8 mm (typical small gap) |
| Cone half-angle | 60° (50 Ω natural impedance) |
| Spokes per cone | 12 |
| Spokes per disc | 12 |
12.2 Bill of materials
| Part | Specification | Source | Mid-2026 price |
|---|---|---|---|
| Aluminum sheet (for disc and small cone parts) | 0.040” 6061 aluminum, ~30 cm × 30 cm | Online Metals or local | $15 |
| Aluminum welding rod (for cone spokes) | 3/16” diameter × 75 cm each, 12 pieces | Local hardware | $8 |
| Threaded coupler / brass fitting | 1/2” NPT brass tee + bushing (for apex assembly) | Local hardware | $5 |
| SO-239 chassis connector | Standard amateur coax connector | DigiKey | $4 |
| Insulating ring (between disc and cone apex) | PTFE / nylon washer, ~5 mm thick | DigiKey | $3 |
| Mounting hub for disc | Aluminum plate, 5 cm × 5 cm | Local | $2 |
| Hardware (screws, washers, nuts) | Stainless | $5 | |
| Coax pigtail | RG-8X, 30 cm with PL-259 | Times Microwave | $8 |
| Weatherproofing | 3M tape + Coax-Seal | $5 | |
| Total | ~$55 |
12.3 Step-by-step construction
Cut the disc. From the 0.040” aluminum sheet, cut a 52 cm diameter circle. Use tin snips or a jigsaw. Drill 12 evenly-spaced holes around the perimeter for spoke attachment, plus a center hole for the apex-to-coax connection.
Cut and bend the disc spokes. Cut 12 lengths of aluminum welding rod, each 26 cm (the radius of the disc). At one end of each, bend a 90° tab for attaching to the rim of the disc. The other end attaches to the central hub. This gives you a 12-spoke disc.
Assemble the disc. Bolt each disc spoke at one end to the central hub (aluminum plate); the other end is loose at this stage. Connect spoke tips to a continuous wire perimeter ring (12-gauge bare copper, ~52 cm × π = 1.63 m around the perimeter).
Cut the cone spokes. Cut 12 lengths of welding rod, each 75 cm (the cone slant height). At one end, bend a 90° tab for attaching to the apex hub.
Assemble the cone. Attach 12 cone spokes radially at the apex hub. The spokes splay outward at 30° from vertical (60° half-angle). The cone base is open (no perimeter ring needed — wind transparent).
Assemble the apex. The cone apex hub is a small aluminum block (~3 cm × 3 cm × 3 cm) with the 12 cone spokes attached at one end and a 1/2” NPT brass tee at the other end. The disc’s central hub bolts to the top of the cone apex hub via a PTFE insulating washer.
Mount the SO-239. The SO-239 connector mounts in the cone apex hub. The center conductor of the SO-239 connects to the disc’s central hub (passing through the PTFE insulator). The shield of the SO-239 connects to the cone apex hub.
Mount the whole assembly on a PVC support. A 1.5” PVC stub pipe (1 m long) with a UV cap acts as a mast adapter. The cone apex hub bolts to the top of the PVC stub; the disc sits ~8 mm above the cone apex via the insulating washer.
Connect the coax. A 30 cm coax pigtail with a PL-259 on one end connects to the SO-239. The other end of the pigtail goes to your main feedline.
Test with the NanoVNA. Connect the NanoVNA to the coax. Sweep 80–1000 MHz. SWR should be < 2:1 across 100 MHz – 800 MHz; slightly higher at 800 MHz – 1 GHz; degraded beyond 1 GHz (the design limit).
12.4 Tuning verification
A successful 100 MHz – 1 GHz discone build shows:
- SWR < 1.5:1 across 100–500 MHz
- SWR < 2:1 across 500–800 MHz
- SWR < 2.5:1 at 800 MHz – 1 GHz
- SWR < 3:1 at 1 GHz – 1.3 GHz (acceptable for receive)
- No anomalous SWR spikes within the design band (a spike at, e.g., 400 MHz indicates a parasitic resonance from the spoke geometry; the antenna is still usable but the dimension is wrong)
- Pattern omnidirectional in azimuth (verify by rotating and observing constant signal level on a distant fixed source)
If the lower edge (100 MHz) shows SWR > 3:1, the cone slant is too short. If the upper edge (1 GHz) shows SWR > 4:1, the disc-cone gap is too large.
13. Commercial buys
Sorted by tier and use case (USD, mid-2026):
| Tier | Model | Frequency range | Price | Notes |
|---|---|---|---|---|
| Budget | MFJ-1868 | 25 MHz – 1.3 GHz | $80 | Entry-level scanner discone |
| Budget | Diamond D-130J | 25 MHz – 1.3 GHz | $95 | The classic Diamond scanner discone |
| Budget | Comet DS-150S | 25 MHz – 1.5 GHz | $110 | Comet’s scanner discone |
| Budget | Tram 1410 | 25 MHz – 1.3 GHz | $90 | Tram (Decatur) commercial |
| Budget | Workman 102 | 25 MHz – 1.3 GHz | $75 | Budget option |
| Mid | Sirio SD-1300U | 25 MHz – 1.5 GHz | $180 | Italian scanner discone; well-built |
| Mid | AOR DA-3200 | 25 MHz – 3 GHz | $230 | Higher upper-end coverage |
| Mid | Watson WD-130 | 25 MHz – 1.3 GHz | $150 | UK/EU brand, solid build |
| Mid | Diamond X-300A | 25 MHz – 1.5 GHz | $220 | Diamond’s heavier-duty model |
| Mid | Yaesu YDS-101 (older) | 25 MHz – 1 GHz | $200 (used) | Discontinued but excellent |
| Premium | A.H. Systems wideband discone | 30 MHz – 6 GHz | $700–1500 | EMC-grade build |
| Premium | A.H. Systems SAS-540 | 30 MHz – 300 MHz biconical | $2500 | EMC-test biconical |
| Premium | R&S HK116 | 30 MHz – 300 MHz biconical | $3000 | Calibrated EMC |
| Premium | Eikos Discone-Pro | 25 MHz – 6 GHz | $1500 | Custom-fabricated |
| Premium | Bonito MegaDiscone | 25 MHz – 3 GHz | $800 | Active discone (with LNA) |
What to avoid:
- “Ultra-cheap 10 GHz discones” on Amazon — the geometric scale at 10 GHz produces a 5 cm cone, which is not a 30 MHz – 1 GHz antenna. These are usually mislabeled higher-frequency-only antennas.
- “5G discones” — 5G operates at 5 GHz and 28 GHz; a discone designed for these frequencies is tiny. “5G discone” usually means a small higher-frequency antenna repackaged with 5G branding.
- “Multi-band miracle discones” with implausibly low prices ($30 for 25 MHz – 6 GHz) — the cone size required for 25 MHz operation alone is ~2.5 m slant; physics doesn’t allow it to be cheap.
14. Companion gear
A discone’s supporting gear:
- Mast — 1.5” to 2” PVC stub or aluminum pole, for indoor / desktop use. For outdoor permanent installation, a proper mast at the height that puts the discone above local obstructions (Vol 21).
- Coax — LMR-400 minimum for any frequency above 200 MHz. For 1 GHz operation across long runs, use LDF4-50 (Heliax) or equivalent low-loss cable.
- Common-mode choke — string of Mix-43 ferrite beads slid over the coax, or 6–10 turns of coax through a 4” form. Critical for clean pattern.
- Polyphaser arrestor — at the bulkhead for lightning protection (Vol 20 §5).
- DC short at the apex — many commercial discones include this; for DIY builds, add a small ferrite bead with a short conductor across the apex.
- Filter (optional) — for SDR use, a pre-amplifier and/or bandpass filter at the antenna can improve sensitivity. The Bonito MegaActiv combines a wideband LNA with a discone-style antenna.
15. Common gotchas and myths
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“Discone covers DC to daylight” — false. A discone is a ~10:1 bandwidth design (e.g. 25 MHz – 250 MHz, or 30 MHz – 300 MHz). Vendors quote a 40:1 “useful range” but the transmit-usable bandwidth is the 10:1 range.
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“Discone has 6 dBi gain” — false. ~0 dBi to +2 dBi omnidirectional. Gain claims confuse discones with conical-monopole “stacked” arrays (which can have 8–12 dBi but aren’t discones).
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“Discone is broken when SWR jumps” — usually false. Discones can degrade gracefully across their range; SWR jumps at the band edges are normal. The antenna still receives; transmit may be problematic on those edges.
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“Discone is direction-finding” — false. Discones are omnidirectional. Use a Yagi or small loop for DF work.
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“Bigger discone = better” — only up to a point. A discone designed for 30 MHz – 300 MHz is ~2.5 m × 4.3 m — huge. For receive coverage of only 100 MHz – 1 GHz, a smaller 75 cm discone is sufficient and more practical.
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“More spokes = better” — diminishing returns after 16 spokes. 12 spokes is fine for 1 GHz operation; 16 spokes for 2 GHz; beyond that, the spoke geometry approaches a solid disc/cone equivalent.
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“My discone gets no signal below 50 MHz” — expected if the discone is designed for 100 MHz – 1 GHz. The lower bound is set by physics — a 75 cm slant antenna can’t be resonant at 25 MHz.
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“Discone gain is below a dipole” — true on a band-specific basis. A 144 MHz dipole has 2.15 dBi gain at its design frequency; a 30–300 MHz discone has ~0 dBi gain on 144 MHz. For single-band transmit, a band-specific antenna outperforms a wideband discone.
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“The discone has a peak at the lowest frequency” — false. The discone has roughly flat performance across its design band; it doesn’t peak at a single frequency.
16. Resources
- Kandoian 1945 paper (Armig Kandoian, “Discone — A Wide Bandwidth Antenna,” Proceedings of the IRE) — the foundational discone publication.
- ARRL Antenna Book Ch. 9 (broadband antennas) — discone, biconical, and conical-monopole coverage.
- Rumsey 1957 paper (“Frequency Independent Antennas,” IRE National Convention Record) — the theoretical basis for the discone family.
- A.H. Systems datasheets — published characteristics of EMC-grade biconicals and discones.
- AOR / MFJ / Diamond discone tech notes — published manufacturer information on common scanner discones.
- ARRL Wideband Antennas Handbook — broader coverage of the wideband antenna family.
- Balanis Ch. 11 (broadband antennas) — academic treatment.
- DG0SA discone build articles — community-published construction notes.