Unraveling the mystery of VHF transmissions

Ahoy.

I would like to start this week by answering a question I

constantly hear about how far a VHF radio can transmit and receive.

Here is an excerpt from the “VHF Marine Radio Handbook” that explains

the transmitting distances and a little-known skipping phenomenon.

“VHF radios will have different transmitting powers, which is a

major factor determining distances you can communicate with another

radio station. The standard stationary radio (the one mounted in a

boat) has a typical output of 25 watts, and a hand-held radio has

between 4 [and] 6 watts of transmitting power. Thus, the 25-watt

radio (the radio mounted in your boat) will transmit the furthest and

should be used in an emergency, ... [rather] than the 4-watt

hand-held radio.

“The distance a radio can transmit depends on a few factors, such

as the wattage output, antenna height and weather conditions. Also,

it is incorrect to assume one can receive or transmit well while in

heavy seas, as the antenna is basically tossed around from its

vertical position.

“A rule of thumb for the transmitting distance of your 25-watt

radio [is to] use the formula -- the square root of the antenna plus

the square root of the receiving antenna times 1.22. Thus, the

communication distance of a radio with the antenna on a boat’s

flybridge at 15 feet and another radio with the receiving antenna

located on the boat’s gunwale at 8 feet ... [is] approximately 8.18

nautical miles.

“Remember, this will depend upon the weather conditions.... As

most mariners know, when the ‘skipping effect’ occurs, you can hear

communications hundreds of miles away.

“This phenomena is known as ‘tropospheric ducting’ and ... is very

common in areas such as Mexico’s and Southern California’s waters,

and in the southeastern states from Virginia to Florida.

“Tropospheric ducting occurs when marine radio transmissions

become trapped in warm air inversions or ducts that are lying above

the cooler ocean air.

This is so common that it is not usual to communicate with a

friend who may be hundreds of miles away cruising off the Mexican

coast, [while] you are boating off Long Beach, California.”

I hope that this helps. It is probably more than you wanted to

know about your radio, which makes a strong argument for carrying an

emergency position indicating radio beacon, or EPIRB, aboard your

boat if you sail off shore.

The tip of the week is to start exploring Lake Arrowhead. We now

have a family cabin on the mountain. We have been spending the last

few weekends there, and it looks like I will be heading up the

mountain regularly after my radio show.

Those of you who have been trying to find me know I have been out

of contact. That will change when the phone line with a DSL

connection is installed -- but then again, maybe I will keep the

lines unplugged.

After decades as a professional captain upon the Pacific Ocean, I

find lake boating a different yet enjoyable experience. I have

visited countless lakes across the nation, from the Great Lakes to

the North Lake at Woodbridge in Irvine.

Every time, I immediately notice the lack of salt in the air. The

second most dramatic difference is the swells. Lake swells are quite

different from ocean swells, which can be generated thousands of

miles away. Keep in mind that if there were no wind, there would be

no swells.

Additionally, many lakes do not have a constant fluctuation height

from tides. Only water being released from a lake’s dam (if it’s

man-made) or the longer effects of a drought or rainy season will be

noticeable.

Maybe in the near future, I will broadcast a live remote radio

show from the shores of Lake Arrowhead.

Our next local live remote is at the Lido Yacht Expo in Lido

Marina Village on Sept. 24.

Tune in to the No. 1 boating talk radio show in the nation, “Capt.

Mike Whitehead’s Boathouse Radio Show.” It airs every Saturday from

noon to 1 p.m. on KCBQ-AM (1170).

You can join me, Chandler Bell and Eric Hovland by calling the

listener line at (888) 344-1170.

Safe voyages.

* MIKE WHITEHEAD is the Pilot’s boating and harbor columnist. Send

him your harbor and marine-related thoughts and story suggestions by

e-mail to o7mike@boathousetv.comf7 or visit

o7www.boathousetv.comf7.