Good Receive, No Range, and a Display That Dims
Your 32-foot cruiser has a fixed-mount 25 W VHF on the hardtop, fed by about 15 feet of RG-58 coax with a PL-259 you soldered on yourself last spring, and powered by 14-gauge wire run about 18 feet from a bus bar behind the dash. Lately you can hear the Coast Guard and nearby boats fine, but hails to a marina 8 miles away go unanswered. On 25 W the display dims and the radio occasionally reboots; on 1 W it transmits fine to boats a mile out. At the radio's power leads, a multimeter reads 12.4 V at rest and 10.9 V while transmitting. The antenna is a 3-foot whip on a stainless ratchet mount, and the mount shows white corrosion at its base. The battery is two seasons old and reads 12.5 V at rest.
1. Separate these symptoms into distinct faults and say what each one points to.
Read the worked answer
Fault 1 - DC distribution (the dimming and rebooting). A 25 W VHF draws roughly 5 to 6 amps on transmit. A 1.5 V drop at that current implies about 0.25 to 0.3 ohms of resistance somewhere in the path - that is far too much. Suspects, in likelihood order: corroded or loose terminals and fuse holder, an 18-foot run of 14 AWG carrying transmit current, and a two-season battery that is marginal at 12.5 V rested. 10.9 V at the radio is right at the point where a modern set browns out and resets. This alone explains the 25 W symptom.
Fault 2 - the RF path (the missing range). Hearing well but not being heard over 8 miles is a transmit-side problem: high SWR, lossy or water-ingested coax, a bad shield connection at the PL-259, or a corroded mount that has left the antenna without a proper ground/counterpoise and detuned it. RG-58 at 15 feet costs only about half a decibel, so the cable type is not the culprit - the terminations and the mount are.
The trap: these are two independent faults that mimic each other. If you fix only the wiring, the radio stops rebooting and still talks to nobody. If you fix only the antenna, the radio still browns out at 25 W.
2. What tests do you run, in what order, and what should each read?
Read the worked answer
DC side first (cheap, fast, and it protects the radio during later tests):
- Measure voltage at the battery posts and simultaneously at the radio's power terminals, keyed on 25 W into a dummy load if you have one. A difference of more than about 0.5 V means the drop is in your wiring, not the battery.
- With the radio off, measure resistance from the radio's negative lead back to the battery negative; anything beyond a fraction of an ohm is a bad connection.
- Inspect and clean every terminal and the fuse holder; feel for warmth after a transmit. Replace 14 AWG with 10 AWG on a dedicated fused circuit direct to the battery switch, and shorten the run where possible.
RF side, radio off and disconnected:
- Ohmmeter, center pin to shield at the radio end of the coax: should read open (very high). A low reading means a short - stray braid, water, or a bad solder joint.
- Shield continuity from the connector shell to the antenna mount base. Look inside the PL-259 for a good braid solder, no whiskers, and no green in the dielectric.
- Clean the corrosion at the ratchet mount and confirm the mount is electrically bonded as the antenna's ground reference.
RF side, radio on, into a wattmeter/SWR meter:
- First into a known-good dummy load - this tests the radio, not the antenna.
- Then into the antenna: forward and reflected power. SWR should be below 2:1 and ideally near 1.5:1. High reflected power confirms the antenna/coax/mount fault.
Order matters: fix DC, then verify the radio makes rated power into a dummy load, then fix RF, then do a radio check on low power on a working channel - not on 16.
3. Your slip-mate says: 'Just key it for a few seconds with the antenna off - that'll tell you if the radio is putting out.' Explain why not, and what you do tonight when you need to reach the marina.
Read the worked answer
Why not: transmitting into an open or badly mismatched load reflects power back into the final amplifier stage. Short tests sometimes survive; repeated ones often do not, and the failure shows up later as reduced output that you cannot diagnose at sea. It can also radiate spurious signals and interfere with other stations - which is a regulatory problem on top of a hardware one. Keying with no antenna is a diagnosis method for someone with a dummy load and a wattmeter, not a dockside shortcut.
Tonight, before repairs:
- Treat the fixed VHF as degraded, not as your primary. Its transmit path is suspect and its power path is failing under load.
- Use a charged handheld for the short-range calls you actually need - a handheld at 5 W reaches the marina at 2 or 3 miles.
- Do not use the failing set for anything safety-critical. If you had to raise the Coast Guard tonight, the reliable paths are the handheld on 16, the EPIRB if it became a distress, and a cell phone as a backup to both.
- Keep transmissions on the fixed set brief and on 1 W if you must use it, and note in your own records that the installation was known-degraded - that is exactly the kind of thing that belongs in a station log entry.
Then repair in this order: dedicated 10 AWG fused power run with clean terminals; verify output into a dummy load; replace the coax and connectors (RG-8X or LMR-240 with professionally fitted connectors, waterproofed at the mount); clean and bond the mount; verify SWR below 2:1; radio check on low power.