posted 07-13-2012 04:24 PM ET (US)        

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My one-year-old Cobra MR80 VHF radio has developed a problem receiving. I found this out when I was out boating last time around Anacapa Island (which is about 12-miles away from port). I transmitted a radio check to the Harbor Patrol office but could not get a reply. I’ve done successful radio checks from this spot before. After trying again, I heard a very faint response that I determined to be from my hand-held back-up that was in the ditch bag under the rear bench.

So I know the radio transmits, but may either not be receiving signals or not annunciating them. My first inclination would be to say that the radio’s speaker is not functional but it does work at shorter ranges.

Should I be looking at my antenna connections or getting a replacement radio under the warranty?

Thank you for your input.

posted 07-13-2012 05:49 PM ET (US)            

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You can easily test your VHF Marine Band radio receiver using NOAA Weather Radio broadcasts. See

http://continuouswave.com/whaler/reference/antennaRange.html

If your antenna, transmission line, and receiver are working properly, you should be able to receive NOAA Weather Radio broadcasts from a local station and also from a more distant station. If you can receive NOAA Weather Radio at a range of 50-miles or more, it is a good sign your antenna, transmission line, and receiver are working well.

There is no simple way to test the transmitter, other than calling another station (that is known to be in good working order) that is at least 5-miles away.

posted 07-14-2012 05:24 AM ET (US)            

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I am in a quandary as well. This thread brought up [another VHF radio installation problem] for me, so I hope [readers] will allow another question: Why are splices in coax bad? Especially if the splice is done well and heat shrink protected, water proof, as well as secured.

I have to move my brand new installed antenna because of the bimini installation.

posted 07-14-2012 08:56 AM ET (US)            

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Coax splices are bad because mechanically they are hard to do right, and, more importantly it is pretty impossible to make a splice that does not alter the RF electric characteristics of the transmission line at VHF frequencies.

Although still not ideal, you are better off using a connector designed for that cable which will try to better maintain the geometry of the transmission line (including inner conductor, inner insulator, and shield).

And I second checking the VHF antenna connector on the back of the set. Those tooth-lock and large screwdown collars sure look substantial, but I have had several start to unscrew (or completely unscrew) during normal boat usage. Don't know why - seems like bayonet type connections would be less prone to that. But those ocnnectors go way back and are used in all sorts of much higher powered and more complex radio equipment.

posted 07-14-2012 09:29 AM ET (US)            

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A coaxial cable is a transmission line that conducts radio frequency energy from the transmitter to the antenna. It has a characteristic surge impedance which is part of the radio frequency circuit. If there is a physical discontinuity in the coaxial cable, like a splice, there will also be a radio frequency discontinuity in the transmission line, and this will cause a disturbance in the flow of the radio frequency energy down the transmission line.

Most VHF Marine Band antennas for recreational boats come with about 20-feet of coaxial transmission line attached to them. A transmission line of this length is long enough to be practical for most installations, and short enough that there is little advantage to be gained by shortening it. If you have to re-locate the antenna mount, just re-route the transmission line from the new location to the radio. If you need to pass the transmission line through a bulkhead and don't want a hole large enough to pass the connector, cut off the connector at the radio-end of the transmission line. feed the line through all the obstacles, and re-install the connector when the line has been routed to the radio.