DIY 1/4λ whip antenna for 70 cm: math, soldering, SWR tuning

The "quarter-wave whip with radials" antenna (ground plane, GP) is the most honest DIY build for UHF: cheap, repeatable, and with the right length the SWR comes out better than many of the cheap Chinese "rubber duckies" that ship with radios. It is ideal for a DMR hotspot on a desk or balcony, or for a simple base station. Let's build it on a connector and tune it by SWR.

How it works

A GP is a vertical radiating element a 1/4 wavelength long plus 4 counterpoise wires (radials) that mimic a "ground." When the radials are bent down to about 45°, the antenna's input impedance rises toward a convenient ~50 ohms — a direct match for 50-ohm coax with no matching devices.

Calculating the length

The radiator length is calculated with a correction for the "shortening" effect in the conductor:

Radiator length (mm) ≈ 71250 / F(MHz)

where F is your operating frequency. Make the radials the same length or a few percent longer than the radiator, then trim to SWR. Ready-made reference figures:

What you'll need

• a panel-mount SO-239 connector (or an N connector) — the base of the antenna;
• copper wire 1.5–2.5 mm (a strand from power cable or a bare welding electrode will do) — for the whip and the 4 radials;
• a beefier soldering iron (40+ W), solder, flux;
• for tuning — a NanoVNA antenna analyzer or an SWR meter for the band you need.

Assembly

1. Cut 1 whip (radiating element) and 4 radials per the table (+ margin).
2. Solder the whip to the center pin of the SO-239 — it points vertically upward.
3. Solder the 4 radials to the four mounting holes/body of the connector (this is the "ground"), symmetrically, crosswise.
4. Bend the radials down to about 45°.
5. Screw the antenna onto a 50-ohm cable (for UHF use good coax, shorter is better: at 70 cm losses in cheap cable are high).

Tuning by SWR

1. Connect the NanoVNA/SWR meter and set the operating frequency.
2. See where the SWR "dip" (minimum) is. If it's below the operating frequency, the radiator is too long — shorten it by 2–3 mm. If it's above — a too-short radiator is the rare case, but then lengthen it a bit with the radials or a replacement.
3. Trim 2–3 mm at a time, remeasuring each time. The goal is an SWR of 1.1–1.3 at your frequency.
4. Fine-tuning — by the radial droop angle: steeper down = higher impedance.