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VHF antenna choices & troubleshooting PDF Print E-mail
Written by Carlos Alvarez   
Tuesday, 01 June 2010 13:12

Many people ask why there's a $30 marine VHF antenna and another that looks the same but costs $120.  Is there a benefit to the expensive one?  And just as importantly, how do you install it properly and fix any issues that come up over the years?

 



The marine VHF radio is not only your lifeline to emergency help, but a great convenience in daily boating life.  Most people will carefully shop for a radio and spend a lot on a good one, but give the antenna very little thought.  However the antenna is the most important part of the radio installation.  It is widely recognized that a cheap radio with a great antenna will always out-perform a great radio with a cheap antenna.  So what makes a good antenna?  To determine this we'll look at construction quality and internal design, which is most commonly expressed as "db gain" or just "db."  Gain is a measure of how much the signal is boosted in one direction by not allowing signal to go in other directions.  It's not free energy, it's just redirected to where you want it.  In the case of a marine antenna, we generally want all the signal going outward in all directions.  It doesn't do any good for signal to go upward or downward, so we can redirect that energy to where it matters.

 

This article necessarily delves into some technical details of radio and antenna operation.  While I try to keep my articles as short and simple as possible, this one demands a little more detail.  In summary, I'll say that yes, there is a vast difference between the cheap and good antennas, and the better one is well worth it if you travel any distance away from help.  On small inland lakes or very close to shore I'd take the cheap one, but not on any larger lakes or away from land.

So why not just buy the highest gain antenna possible?  In a moving and pitching vessel, an antenna with more than about 7db of gain may be too directional and cause reception problems as the boat moves.  The effect would be a signal that is only usable when the boat is level.  For boat use, 6db is considered optimal.  Unfortunately, we still have the question of construction quality, and the fact that not all db specs mean the same thing.  There is dipole gain, and isotropic gain, and most manufacturers don't tell you what they are using.  The dipole measurement is a real-world specification based on a comparison to a simple straight wire, and isotropic uses a theory of equal radiation in all directions.  There is no such thing as an isotropic antenna, so there's really no reason to compare to one.  We want 6db gain over dipole.

For some round numbers, a unity (0dB) antenna is about 1.5’ long, a 3 dB is about 3’, and a 6 dB is about 6’.  That’s dB gain over dipole, of course.  That said, ever wonder why the $30 antenna (Shakespeare 5206) and the $120 antenna (Shakespeare 5225XT) are both rated at 6dB?.  They are simply rated differently, and their interior construction is very different.  The cheap antenna uses a wire element with a coax cable matching network. The element is only about 3’ long, and is 3 dB dipole, and 5.2 dB isotropic.  The good antenna uses two phased or brass elements, which are actually 6dB dipole, and 8.2 dB isotropic. On top of that you get much better internal construction, with materials that resist oxidation.  The cheap antenna is almost guaranteed to have internal corrosion within a few years, further weakening the signal.

Some people talk about SWR and antenna tuning or matching.  I don't consider this critical with any good antenna; it's going to be close enough.  You can pick up an SWR meter to check it for about $50.  You can somewhat tune SWR by cutting the cable an inch at a time, but again, it's probably pretty good to start and if it's not, you probably have a problem this can't fix.  Speaking of cutting the cable; don't!  I would recommend leaving it as it is, though some say you can go as short at four feet.  Without an SWR meter you can't really tell, and of course, once you cut you're done with that cable (splices create signal loss).  An SWR meter can be useful to verify that the antenna is good from the start, and then you can check it for degradation in the future.  If the SWR starts to climb, you should cut off and replace the connector, then test again.  This is usually the source of the trouble.  If not, it's a sign that the antenna is corroding internally and needs to be replaced.

Now how do you mount that nice new antenna?  Stainless steel.  Period.  The price difference isn't that big, and you'll see a difference in how much the antenna whips around in rough water.  The plastic mounts eventually rot and break.  I've never seen a broken stainless mount.

Last Updated on Tuesday, 01 June 2010 17:32
 
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