VHF Transmit Problems

The most common piece of electronics found on coastal and offshore boats is the VHF
radio. Like most other electronics they’ve progressively gotten smaller and less
expensive. Despite the small size and price, it’s still one of the best investments a
mariner can make in the area of safety. Not only can you communicate with most any
boat around you, it’s used by the US Coast Guard to monitor for distress calls. Google
“Rescue 21” to get more information about the nationwide system established and
maintained by the USCG.

One common complaint from boaters is that they can receive other transmissions but
have a difficult time getting other boats to hear them. Reports of “weak” and/or noisy
transmissions are something we hear a lot. More often than not, these types of
problems are not defects in the radio itself but caused by something external to your
radio. Here are a few of the more common causes for VHF radio transmitter issues.

“Weak but clear” transmissions that extend out to your normal distances are signs of a possible microphone issue. You might have a defective mic but it could simply be that you’re not speaking directly into the element or there is an obstruction. Today’s noise rejecting microphones usually have a small hole in which to speak. Your mic may have a large “grill” that looks like all your other microphones in the past but look closely at the lower portion of the face and you’ll probably see a small hole about 1/8” in diameter. The mic element, or the part that converts sound to electrical signals, is behind that little hole. Make sure you’re speaking directly into it and that dust, dirt, or moisture isn’t blocking the hole.

A buzz or hum during transmissions often combined with a loss in transmit range may indicate a voltage problem. One of the most basic troubleshooting rules in electronics is to check the power source. My estimate is that approximately 1 in 4 problems reported with marine electronics has an associated input power issue.

Every boater should have a multimeter on their vessel. A small inexpensive digital unit
is just fine and can be bought for $20 or less. Establish a benchmark by measuring the
voltage to your radio when everything is working fine. Most all VHF radios work on
12VDC. Your radio will either operate from a 120VAC to 12VDC power supply or a
battery. Batteries will either have an AC charger or it may be charged directly from an
engine alternator. A normal system will typically measure 12.4 to 13.8 volts DC.

Measure the voltage to your radio with the charging source off or disconnected. If the
voltage drops below 12 volts without a charging input, your battery is probably bad.
Also measure the voltage with the radio on but not transmitting then key your mic and
see how much the voltage drops. If the voltage falls below 12 volts, you may have a
bad connection somewhere on the power cable. Remember, even the best electronics
is only as good as it’s power source.