Coaxial Cable Selection for Antenna Systems: The Installer's Guide

The coaxial cable connecting a transmitter or receiver to its antenna is rarely the first thing on a specifier's mind. It should be. Cable loss directly reduces effective radiated power on transmit and receiver sensitivity on receive. In a system that is already working close to its coverage limits, a poorly chosen cable run is often the difference between a reliable link and a marginal one.

How Cable Loss Works

Coaxial cable loss is measured in dB per 100 metres at a given frequency. The higher the frequency, the greater the loss per metre. At VHF (150 MHz), a common cable such as RG-58 loses around 7 dB per 100 m. At UHF (450 MHz), the same cable loses around 14 dB per 100 m. At 2.4 GHz WiFi frequency, it is effectively unusable for runs beyond a few metres. Cable selection must always be made with the operating frequency in mind.

Common Cable Types

RG-58 is a thin, flexible cable suitable for short indoor runs at VHF and UHF. Its high loss per metre makes it unsuitable for anything other than very short connections. RG-213 / LMR-240 offers a good balance of loss and flexibility for VHF and lower UHF runs up to 20–25 metres. LMR-400 / Westflex 103 is the professional standard for longer runs, offering significantly lower loss than RG-213 at both VHF and UHF. For runs over 30 metres at UHF or over 50 metres at VHF, LMR-400 equivalent is the correct starting point. Heliax / Andrew LDF series is used for demanding base station applications, with loss per metre well below LMR-400 but at the cost of rigidity and cost.

Connectors and Weatherproofing

N-type connectors are the standard for professional UHF and VHF installations. PL-259 (UHF) connectors are widely used but should be avoided for outdoor installations above VHF due to their poor weathering characteristics. BNC connectors are for short indoor patch leads only. All outdoor connectors must be weatherproofed: wrap with self-amalgamating tape from below, working upward, with a minimum 50% overlap, then add a UV-resistant outer layer where UV exposure is high.

Calculating Your Cable Loss Budget

Before ordering cable, calculate the total system loss. Add the cable loss for your run length and frequency, plus estimated connector losses (0.1–0.2 dB per connector pair), and subtract from your transmitter output power or antenna gain. If the result compromises the link budget, choose a lower-loss cable. Most antenna manufacturers publish link budgets that specify maximum acceptable feeder loss.

Contact Renair for technical support on cable and connector selection: renair.co.uk/contact-us.

Frequently Asked Questions

Can I use the same cable for VHF and UHF?

Physically yes, but the loss will be higher at UHF. A cable that provides acceptable loss at VHF may not be good enough at UHF for the same run length. Always calculate loss at the highest operating frequency you intend to use.

How often should I replace coaxial cable on an outdoor installation?

Outdoor coaxial cable should be inspected annually. Jacket cracking, connector corrosion or physical damage to the cable run are all reasons to replace. A cable with an intact jacket and properly weatherproofed connectors can last 15–20 years in UK outdoor conditions.

What is the minimum bend radius for LMR-400 cable?

LMR-400 has a minimum bend radius of approximately 25 mm for installation and 50 mm for repeated flexing. Exceeding the minimum bend radius collapses the cable's internal structure and permanently degrades its electrical performance.