Radio Antennas Q & A

Antenna gain basics

What is gain?

• Gain shows how concentrated the signal is in a direction.

• Measured in dBi (vs an isotropic source).

• dBd uses a dipole as reference.

• Convert: dBd = dBi − 2.15.

Are 2.5 m antennas always “high gain”?

• No. Length helps, but gain depends on design and radiating elements.

What gain should you pick?

• Omni sites: 3–6 dBi is a safe, usable range.

• Higher gain narrows the vertical beam and needs steadier mounting.

Range and line-of-sight

Why do I reach radios far away if antennas are line-of-sight?

• A hilltop repeater re-transmits your signal across wider LOS.

How do I estimate radio horizon?

• Distance (km) ≈ 3.57 × √height(m) per antenna.

• Total path ≈ 3.57(√h1 + √h2).

• Height beats most other tweaks for range.

UHF vs VHF

Which band should you use?

• UHF: better around buildings and for smaller antennas.

• VHF: better over water and open country for longer paths.

Can a UHF antenna work on VHF (or marine VHF)?

• No. Match the antenna to the radio band for safe, efficient transmit.

SWR and matching

What is SWR/VSWR?

• It compares forward and reflected power on the feedline.

• Lower is better; target ≤1.5:1.

• Up to ~2:1 is usually acceptable for many systems.

What hurts SWR most?

• Wrong antenna band.

• Poor ground plane or mount.

• Water ingress or damaged coax.

• Bad connectors or crimps.

Ground plane and mounting

Do I need a ground plane?

• 1/4-wave mobile whips need a metal ground plane (roof, bonnet, or radials).

• 1/2-wave designs can work with little or no ground plane and suit fibreglass roofs.

How close can antennas be?

• Aim for at least one antenna length between radiators.

• Separate different services (UHF/VHF/LTE/GPS) where possible to reduce coupling.

Should antennas be vertical?

• Yes. Each degree off-vertical reduces useful omni coverage.

Marine antennas

Which gain should you choose at sea?

• Sailboats (mast sways): prefer ~3 dB to keep coverage when rolling.

• Powerboats: 3 or 6 dB depending on ride and mounting height.

• Height on the vessel remains the biggest factor.

Best length?

• Small boats: ~1.8 m for practicality.

• Larger vessels or fixed masts: 2.5 m+ for more range, if the mount is stable.

Coax and feedline loss

Why does cable choice matter?

• Loss rises with frequency and length.

• LMR-400 class cables beat RG-58 for longer runs.

• Typical LMR-400 attenuation per 100 m: ~5 dB at 150 MHz, ~8.9 dB at 450 MHz.

Tips

• Keep runs short.

• Use quality connectors.

• Avoid tight bends and crushed sections.

• Weatherproof every external joint.

Materials and durability

Fibreglass vs stainless whips

• Similar electrical performance when tuned.

• Fibreglass radomes protect collinears and put the element higher.

• Stainless whips are light, tough, and low wind-load.

Weather exposure

• UV-resistant cable and sealed connectors extend life.

• Outdoor-rated coax and hardware improve service life.

Directional vs omni

When to use a yagi or panel?

• Point-to-point links.

• Fringe coverage to or from a known site.

• Interference rejection from unwanted directions.

When to use an omni?

• Mobile use.

• Sites needing 360° local coverage.

Power handling and safety

Does every antenna handle the same power?

• No. Check the rated wattage and duty cycle.

• Ensure connectors and cable match the radio’s output.

Troubleshooting quick checks

Receiving but not transmitting?

• Low battery or bad power path.

• Wrong channel plan or CTCSS/DCS.

• High SWR from a damaged cable or wrong antenna.

Performance dropped over time?

• Inspect for water in the coax.

• Check kinks, crush points, and loose mounts.

• Reterminate corroded connectors.

Practical selections (examples)

Vehicle UHF CB or commercial UHF

• Short city use: medium-gain whip on roof mount.

• Highway/rural: higher-gain collinear if your mount is rigid.

Marine VHF

• Small trailer boat: 1.8 m, ~3 dB.

• Larger powerboat: 2.5 m, 6 dB if the mount is steady.

Site or depot

• Roof mast with 3–6 dBi omni for local handhelds.

• Add a yagi toward a repeater for backhaul.

Calculating element length (quick guide)

Quarter-wave length (m) ≈ 71.5 / f(MHz).

• Example: VHF 160 MHz → ~0.45 m.

• Use the antenna maker’s cut chart for final trim.

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