Also known as: dipole antenna, dipole
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A dipole antenna is a simple resonant antenna made of two conductors fed at the centre, classically a half wavelength long.1 It is the reference against which other antennas’ gain is often compared — the unit dBd means “decibels relative to a half-wave dipole,” which itself has about 2.15 dBi of gain over an isotropic radiator.
How it works
At resonance a half-wave dipole carries a standing wave of current that is maximum at the centre feedpoint and falls to zero at the two ends. That current distribution radiates most strongly broadside — perpendicular to the wire — and hardly at all off the ends, producing the familiar doughnut-shaped radiation pattern wrapped around the axis. A vertical dipole is therefore omnidirectional in the horizontal plane, which is exactly what a scanner wants when signals can come from any bearing. The wave’s polarization follows the wire’s orientation: a vertical dipole is vertically polarized, a horizontal one horizontally polarized.
The centre feedpoint impedance of a thin half-wave dipole in free space is close to 73 Ω resistive, a good match to common 50–75 Ω coax and a big reason the design is so practical. Because the two halves are balanced with respect to ground while coax is unbalanced, a purist adds a balun (balance-to-unbalance transformer) at the feedpoint to keep current off the outside of the coax shield, which would otherwise distort the pattern and raise noise pickup.2 For receive-only scanning the effect is usually mild, but a balun or a few ferrite chokes on the feedline can noticeably clean up a marginal install.
Variants
- Half-wave dipole — the canonical λ/2 form described above.
- Folded dipole — two parallel conductors joined at the ends, raising the feedpoint impedance to about 300 Ω (useful with twin-lead) and widening the bandwidth.
- Quarter-wave monopole — effectively half a dipole worked against a ground plane; a ground-plane whip is the everyday scanner antenna.
- Driven element of a Yagi — a dipole surrounded by parasitic reflector and director rods to concentrate the pattern into a beam.
In practice
A dipole is easy to build to size: total length in metres is roughly 143 / f(MHz) for the half-wave, trimmed a few percent for the wire’s thickness and end effects. Cut for the centre of a target band, it stays usable across a fair bandwidth on either side. Mounted vertically and up high with a clear path, it is one of the best value receive antennas for land-mobile and trunked monitoring.
Relevance to SDR
A dipole (or its grounded cousin, the quarter-wave whip) cut for the target band is a cheap, effective receive antenna for scanning. Its omnidirectional broadside pattern suits a wideband SDR that watches many sites at once, and its predictable ~73 Ω feedpoint keeps SWR low without tuning. GopherTrunk decodes whatever the front end delivers; a resonant dipole simply hands the ADC a stronger, cleaner signal than a random wire, improving lock on weak control channels.
Where to buy
For SDR and scanner use, the RTL-SDR Blog Multipurpose Dipole Antenna Kit (around $25) is the standard starter antenna: adjustable telescopic elements you extend to length for your target band, plus a mount, tripod, and SMA coax. It is the cheapest worthwhile upgrade from the tiny whip bundled with most dongles.
For wideband discones and band-cut options, see the best SDR antenna guide.
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Sources
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Dipole antenna — Wikipedia, for the construction, feedpoint impedance, and radiation pattern of the half-wave dipole. ↩
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Balun — Wikipedia, on balanced-to-unbalanced transformers and why they are used at a dipole’s feedpoint. ↩
Frequently asked questions
Which dipole should I buy for an SDR?
The RTL-SDR Blog Multipurpose Dipole Antenna Kit (around $25) is the standard SDR starter antenna: two telescopic elements you extend to length for your target band, a mount and tripod, and SMA coax. It is a cheap, effective step up from the tiny whip that ships with most dongles.
How do I set the dipole length for my band?
Each element is about a quarter wavelength: total half-wave length in metres is roughly 143 / f(MHz), so extend each side to half of that. Cut for the centre of your target band and it stays usable across a fair range on either side. The RTL-SDR Blog kit includes a length chart for common bands.
Should I mount the dipole vertically or horizontally?
Vertically for scanning. Land-mobile P25, DMR, and NXDN traffic is vertically polarized, so a vertical dipole matches it and stays omnidirectional in the horizontal plane — exactly what you want when signals can come from any bearing. Mount it up high with a clear path.
Is a dipole better than the whip that came with my dongle?
Almost always. A resonant dipole cut for the band hands the SDR a stronger, cleaner signal than the short, poorly grounded rubber-duck whip, which improves lock on weak control channels. A few ferrite chokes on the feedline can clean up a marginal install further.