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Marine Electronics Journal Blog
 

A primer on VHF radio
12/11/2017


 

Bow shotJohn Barry is a marine electronics dealer located in Pleasant Prairie, Wisconsin. His company, Technical Marine Support Inc., is on the western shore of Lake Michigan. He also instructs NMEA’s technical training courses and writes a regular column for Marine Electronics Journal. He says there are four topics that stick out in those training sessions—radars, autopilots, data and radios. Below is an introduction to marine VHF radio. Next week he’ll ramp up the discussion with advice on installations. We'll take on the other topics in weeks to come.

 

 

By John Barry

 

The discovery of electricity spurred many inventions, and one of the most consequential was the radio. We stand on the shoulders of giants. When Mr. Hertz discovered that electromagnetic waves can propagate through space, Mr. Edison got the first patent—and Mr. Marconi purchased it from him soon thereafter. The commercialization of the radio that followed was a revolution in communications. Grassroots development accelerated and continues to this day by marine radiomen and ham operators, whose demise has been greatly exaggerated.

Today, we use highly accurate, extremely sensitive, rock-solid reliable and affordable VHF radios to keep boats in communication with other vessels and shore. These devices still need a power source and a working antenna to function and there is little chance that this will ever change. 

Radio waves

To understand radio waves, their production, transmission and reception you need to start by ringing a bell. When you strike a bell, it vibrates. Big, thick bells sound a low tone and little, thin bells have a higher tone. They resonate at different frequencies based on the material used, the weight and the shape. In radios we use an AC voltage changing very quickly to resonate our antenna. An antenna is an element designed to resonate at a given frequency or range of frequencies.  Marine VHF antennas are designed to resonate at marine VHF frequencies of around 150-162 MHz.

When we increase the frequency of our AC voltage above about 100,000 Hertz (100 kHz) we call it RF—or radio frequencies. Radio frequencies act differently than lower frequency AC or DC voltage. The characteristics of RF energy are not always obvious or intuitively understood. When we get over 100 kHz, the energy leaves the wire and travels through space. We must contain the RF in a coaxial cable, known as a transmission line, to get it from the radio to the antenna. Once it gets to the antenna, it leaves the wire in what we call a radio wave. A radio wave will travel down the coax to the antenna and propagate through space if the output impedance of the radio is matched to the impedance of the coax and antenna, together called the antenna system.

Impedance, like resistance, is expressed in ohms, but impedance is frequency dependent. DC resistance and impedance should not be confused. Typically if you measure an antenna with a DC ohmmeter, you will measure an open circuit, (low ohms if base loaded antenna) even though marine VHF antennas are 50 ohms impedance. NMEA’s Marine Electronics Installer class teaches coax, antennas and VHF radio sections as part of the curriculum. To ensure a good impedance match, we measure the forward and reflected power of the antenna system (coax + antenna). A mismatch will show up as reflected, undesired radio wave action (radio signals not getting out) in an RF power test or an SWR (Standing Wave Ratio) test.

Power supply

The marine VHF outputs a maximum of 25 watts according to Federal Communication Commission rules. This translates to about 6-7 amps at 12 VDC. We normally supply this power from a battery, sometimes from a power supply. Adequate power supply is critical to proper performance. The NMEA requires a voltage test during transmit to ensure that the radio is getting enough juice. Properly sized wiring and batteries, with tight, clean connections are imperative. 'Gotta' have those excited electrons for everything we do.

Radios have evolved from crystal sets using tubes to synthesized solid state marvels. The price has consistently dropped even as features and capabilities have increased. These days we can get a VHF that uses DSC for messaging, receives AIS targets, and costs less than $500. Sensitivity to receive weak signals as well as accuracy of frequency, power and modulation have combined to deliver products unheard of by Marconi and Hertz. Many lives have been saved by the marine VHF, and it continues to be a first-tier marine electronics safety device.

Next week: VHF installation

 

 

 


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Comments | Leave a Comment
Page 1 of 2 ( 10 comments)

 
trawlerdeejay:(10/13/2017 3:46:51 PM) "Excellent article. I had no idea what the differences were between o183 and 2000, Thank you so much."
 
 
Darryl:(3/27/2017 10:17:15 PM) "Putting the MSRP with each unit reviewed would have been helpful. If each unit was actually tested, the reports on each unit would have been helpful too.


Thanks Darryl---we generally don't mention prices due to confusion over so many variations---MSRP (mfg. suggested retail price), MAP (min. advertised price), MRP (min. resale price) and then there are internet prices on some websites that go their own way. But your point is well taken--buyers need to know if something is in their price range. We'll work on it.
There is independent testing of some of these products on sites like panbo.com but the information we receive from manufacturers rarely cites the results of any shootouts they may conduct against the competition's products. "
 
 
Laurie Seibert:(2/16/2017 2:00:20 AM) "Thanks EV Collier for sharing this informative blog. It is important to know the causes of EMI filters. We use these parts in our daily life in the electronic products so we should know that what are the causes are cures of EMI Filters.

Great job and keep updating!

Regards
Laurie Seibert
http://www.lcr-inc.com/"
 
 
Yes:(2/10/2017 7:22:40 AM) "EMI/RFI filter causes and cure. There are very few people who share such information with everyone. I was looking to read such informative blog!

Great job!

Regards
Lisa Wilson
http://filterconcepts.com/
"
 
 
hugo:(1/30/2016 2:00:32 AM) "Why is no integrated ais transceiver available? Only recivers.

Hugo---

Each AIS system consists of one VHF transmitter, two VHF TDMA receivers, one VHF DSC receiver, and standard marine electronic communications links (IEC 61162/NMEA 0183) to shipboard display and sensor systems (AIS Schematic). Position and timing information is normally derived from an integral or external global navigation satellite system (e.g. GPS) receiver, including a medium frequency differential GNSS receiver for precise position in coastal and inland waters. Other information broadcast by the AIS, if available, is electronically obtained from shipboard equipment through standard marine data connections. Heading information and course and speed over ground would normally be provided by all AIS-equipped ships. Other information, such as rate of turn, angle of heel, pitch and roll, and destination and ETA could also be provided. Check out: http://www.navcen.uscg.gov/?pageName=AISworks"
 
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