Antennas
Calculator and build guide for the most popular home-built antennas. You get resonant and cutting lengths with velocity factor, matching and a dimensioned drawing — ready to take out into the garden.
λ = 42.22 m
Insulated PVC wire slows the wave by about 3 %.
= 0.24 λ
Half-wave dipole
7.1 MHz (40m) · λ = 42.22 m- Antenna wire — the radiating part
- Mast
- Guy / support rope (non-conductive)
- Coax cable to the radio
- Dimension line
- Balun
- Insulator at the wire end
Cutting list
| Part | Resonant | Cut |
|---|---|---|
| Leg (× 2) | 9.73 m | 10.17 m |
| Total length | 19.45 m | 20.34 m |
- Feed impedance
- ≈ 70 Ω
- Matching
- 1:1 current balun / choke
- Trimming
- 1 cm off each leg ≈ +7.3 kHz
Cutting length = resonant length + 3 % trimming reserve + 15 cm to fold through the insulator.
Build tips
- Cut both legs to the cutting length, fold the extra 15 cm through the insulator and wrap it back on the wire — then you can unwrap if you trim too much.
- Sweep SWR across the band. If the dip is below your frequency the antenna is too long: shorten both legs equally.
- A 1:1 current balun (or 6–8 turns of coax on a ferrite core) at the feed point keeps common-mode current off the coax and out of the shack.
How to build the 1:1 current balun
The dipole is about 50–75 Ω, so no transformation is needed. The balun is a choke that stops current flowing down the outside of the coax shield and into the shack.
Materials
- Option A — air-wound (costs almost nothing): a piece of 110 mm PVC drain pipe, about 20 cm, and the feed coax itself.
- Option B — ferrite (broadband and small): 1 × FT240-31 (best on 160–40 m) or FT240-43 (best on 30–10 m) and about 1.5 m of RG-316 or RG-58.
- Centre insulator with SO-239 connector, or a weatherproof box and two stainless bolts for the legs.
Connections
- Coax centre conductor (after the balun) → one dipole leg
- Coax shield (after the balun) → the other dipole leg
- Option B only: the other end of the balun coax → the SO-239 / feed cable to the radio
Steps
- 1A: Wind 8 tight turns of the feed coax on the PVC pipe right below the feed point (for 80 m: 12 turns). Hold the turns with cable ties.
- 2B: Pass 10–12 turns of RG-316 (or 6–8 turns of RG-58) through the toroid. Spread the turns so the start and end do not touch.
- 3Mount the balun right at the feed point — it only works where the coax meets the antenna.
- 4Seal the coax ends with self-amalgamating tape so no water gets into the cable.
Test before it goes up: If the balun works, SWR barely changes when you touch or move the coax. If you get RF in the microphone or "bites" from the radio, add more turns.
Height and radiation
10.0 m = 0.24 λ · main lobe straight up
High-angle radiation (NVIS): good for nearby countries (0–600 km), weaker for DX.
Simplified pattern for a horizontal antenna over good ground, broadside to the wire. Real ground, houses and trees change the picture.
Lengths are starting values from standard formulas. Surroundings, wire and mounting always shift resonance a little — cut long, measure SWR and trim. Keep antennas well away from power lines.