Radio Antennas Q & A

Antenna Gain Basics

Gain shows how concentrated a signal is in a given direction, not how much power it adds. It's measured in dBi (against a theoretical isotropic source) or dBd (against a dipole reference). Converting between them is simple: dBd = dBi minus 2.15.

A 2.5m antenna isn't automatically "high gain." Length helps, but gain genuinely depends on the design and number of radiating elements inside it.

Range and the Radio Horizon

If antennas rely on line-of-sight, why can you sometimes reach radios far away? A hilltop repeater re-transmits your signal, effectively extending the line-of-sight path well beyond what your handheld could reach alone.

You can estimate your radio horizon with a simple formula: distance in kilometres is roughly 3.57 times the square root of the antenna height in metres. For a full path between two stations, add both: 3.57 times the sum of the square roots of each height. Height beats almost every other tweak for genuine range improvement.

UHF vs VHF Antennas

UHF antennas work better around buildings and can be smaller for the same performance. VHF antennas work better over water and open country, giving longer paths where there's nothing in the way.

A UHF antenna will not work properly on a VHF radio, or on marine VHF, and vice versa. The antenna must be matched to the band your radio actually operates on for safe, efficient transmission, never assume they're interchangeable because they look similar.

SWR and Matching

SWR (or VSWR) compares forward power against reflected power on the feedline. Lower is better, aim for 1.5:1 or below, though up to roughly 2:1 is usually acceptable for many systems.

What hurts SWR most: the wrong antenna band for your radio, a poor ground plane or mount, water ingress or damaged coax, and bad connectors or crimps. Most SWR problems trace back to one of these four.

Ground Planes and Mounting

Quarter-wave mobile whips need a metal ground plane, a roof, bonnet, or a set of radials, to work correctly. Half-wave designs can work with little or no ground plane, making them a better fit for fibreglass roofs and boats.

• Spacing: aim for at least one antenna length between radiators, and separate different services (UHF, VHF, LTE, GPS) where possible to reduce coupling.
• Orientation: antennas should be mounted vertically. Every degree off-vertical reduces useful omni coverage.

Marine Antenna Selection

Gain choice at sea depends on your vessel. Sailboats, where the mast sways, generally do better with around 3 dB gain to keep coverage while rolling. Powerboats can go 3 or 6 dB depending on ride and mounting height. Height on the vessel remains the biggest factor either way.

On length: small trailer boats generally suit a practical 1.8m antenna, while larger vessels or fixed masts can benefit from 2.5m or more for extra range, provided the mount is genuinely stable.

Coax, Feedline Loss and Materials

Cable loss rises with both frequency and length. LMR-400 class cable comfortably beats RG-58 for longer runs, with typical attenuation of around 5 dB per 100m at 150 MHz and 8.9 dB per 100m at 450 MHz. Keep runs as short as practical, use quality connectors, avoid tight bends and crushed sections, and weatherproof every external joint.

On materials: fibreglass and stainless whips perform similarly when properly tuned. Fibreglass radomes protect collinear elements and put them higher, while stainless whips are light, tough, and low wind-load. UV-resistant cable and sealed connectors genuinely extend service life in NZ's weather.

Troubleshooting and Practical Selection

Directional vs omni: use a yagi or panel for point-to-point links, fringe coverage to a known site, or rejecting interference from an unwanted direction. Use an omni for mobile use or anywhere needing 360-degree local coverage.

Receiving but not transmitting? Check for a low battery or bad power path, the wrong channel plan or CTCSS/DCS setting, or high SWR from damaged cable or the wrong antenna. Performance dropped over time? Inspect for water in the coax, kinks or crush points, loose mounts, and reterminate any corroded connectors.

As a quick reference for element length: quarter-wave length in metres is roughly 71.5 divided by the frequency in MHz. At 160 MHz VHF, that's about 0.45m, always confirm the final trim against the antenna maker's cut chart. Browse our Antennas and Mounts collection for genuine, correctly rated options.