Testing Your VHF Antenna and Coax: Radio Checks, SWR, Connectors and Mounts

The Coast Guard asks boaters to do voice radio checks with other boaters on channel 9, not channel 16, and notes that the Coast Guard doesn't monitor channel 9. (Sea Tow's automated radio check on channels 24 and 28 ended in 2020.) Ask a boat a known distance away how they hear you. Then try from farther out, and do it on high power (25 watts). A friend on a boat at a set distance is the most useful test you can do without tools; record the results.

If your radio has DSC and an MMSI, send a DSC test call to the Coast Guard: the Coast Guard's Rescue 21 coastal stations answer test calls automatically on channel 70, and your radio shows the acknowledging station's number. This proves the radio, MMSI and antenna can reach a Coast Guard station, though not how well your voice gets out. The VHF radio setup guide explains how to store the Coast Guard number and send the call.

With the radio off, inspect the antenna from deck level (binoculars help on a mast):

• Is it upright and firmly attached? A cracked or loose mount lets the antenna flop and eventually snap the cable.
• Is the fiberglass radome chalky, cracked or splintering? Boating Magazine notes the better antennas have a glossy coating that protects the fiberglass from sun.
• Is the ferrule (the metal base fitting) pitted? Chrome-plated brass pits over time; stainless lasts longer.
• Does the ratchet mount lock properly, and are its screws tight and bedded?

Replace a cracked mount and any antenna with a split radome, since water inside it ruins the antenna.

Turn the radio off and unscrew the coax plug from the back. Most marine radios use a PL-259 plug into an SO-239 socket (some small radios use BNC). Look for green or white corrosion, salt crust, water or a loose center pin, and check the body for cracks. Wipe and dry it, then refit it firmly by hand. West Marine stresses that snug is right: overtightening can crush the connector or damage its threads.

Trace the coax as far as you can. Look for crushed spots under tight cable ties or clamps, sharp kinks, chafe where it passes through bulkheads, hatches or hinges, and cracked jacket. West Marine notes that once the jacket is cracked through to the shield, water gets in and the cable should be replaced. On sailboats, look at the mast step or deck connector, a favorite spot for corrosion.

Each joint costs a little signal, and each one is a place for water to get in. Look at barrel connectors, deck connectors, the AIS splitter if you have one, and any lightning arrestor. Corrosion inside a deck connector is a classic cause of a radio that worked fine last season. Make sure any AIS splitter is one made for AIS, as the AIS guide explains.

The topside connector at the antenna base is the most exposed point. A white or green crust around it means water has been getting in. Clean it with contact cleaner, dry it and reconnect firmly. Seal it with self-amalgamating tape stretched as you wrap, then cover with UV-resistant tape. West Marine suggests a thin layer of dielectric grease on the outer connector surfaces only, never on the center pin. Leave a drip loop in the cable below the connector so water runs off.

With the coax disconnected at both the radio and the antenna, check:

• center pin to center pin, end to end: should read near zero ohms
• shield to shield, end to end: near zero ohms
• center pin to shield at one end: should read open (no continuity)

West Marine points out that many fiberglass marine antennas are DC-grounded inside, so with the antenna connected a center-to-shield reading near zero can be normal. A short with the antenna disconnected means a cable or connector fault. Keep in mind that a meter only finds hard failures; wet coax can pass these checks and still waste most of your power.

SWR (standing wave ratio) shows how much power comes back down the cable instead of leaving the antenna. Use a meter that covers marine VHF and connect it between the radio and the coax with a short jumper. Listen first to make sure the channel is clear, then transmit for just a second or two on a working channel such as 68 or 72, at low power first, then high power.

How to read it (check your antenna maker's figure too):

• Practical Sailor treats an SWR of 1.5 or lower, with the antenna connected, as a sign the antenna and coax are OK.
• West Marine calls 2:1 or lower good, and says above 3:1 is a significant fault to fix before relying on the radio for safety calls.

A high reading means a bad connector, damaged coax or a failed antenna. Practical Sailor warns of a trap: a low SWR measured at the radio can hide lossy coax, because wet cable soaks up the reflected power too.

To find wet or tired coax, Practical Sailor's field test disconnects the antenna at the top and measures SWR from the bottom with the far end open. With the antenna off, the reading should be high (Practical Sailor's figure is above 4.0); a lower reading means the cable itself is losing too much power. Reconnect the antenna and measure again; it should then be 1.5 or lower. This test needs someone at the masthead, so it usually happens when the mast is down or a rigger is aloft.

• Connector: cut back to clean, bright copper and fit a new marine-grade PL-259. BoatUS notes that solder-type PL-259s make a longer-lasting connection than crimp-on ones. Check the new connector with the multimeter before sealing it.
• Coax: replace the whole run in one piece if you can. West Marine suggests RG-8X or LMR-400-equivalent marine coax for runs up to about 20 feet and lower-loss cable for longer runs; your antenna or radio maker may specify a type. Avoid splices.
• Antenna: if the cable and connectors test good but SWR is still high or range is poor, the antenna has probably failed internally. Replace it with one suited to the boat: Boating Magazine notes lower-gain antennas suit small boats that rock a lot and masthead installs, while higher-gain eight-footers suit larger powerboats.
• Mount: use a mount rated for the antenna's length, through-bolted with a backing plate where needed and bedded against leaks.

After any repair, repeat the SWR test and the radio check with a friend at a known distance. Write the SWR readings, coax type and length, and date in BoatMend, so next year you can compare.