Question about Cobra Mr F75b Marine Vhf Radio Fixed Mount 25watt

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I have an icom m304marine vhf with a shakespeare

I have an icom m304marine fixed mount vhf with a shakespeare 5' antenna. seems the unit receives very few transmissions. weather channel comes in strong on portable vhf but is staticy on fixed mount. the other day, i monitored an emergency call from a boat less than a mile away and was very scratchy. marine police were conversing with that boat, they were also about a mile away and i never heard their transmissions. another boater told me he heard the entire conversation between those parties. he too was less than a mile from me but for some reason my radio didnt pick it up. i was expecting better performance from the unit. i routinely use a vhf on land for work and am familiar with their limitations. all connections were done as per instructions...any thoughts??

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Sound like you have an antenna issue. Try a different antenna with coax. If the problem is still there, than you have a problem with the radio. Icom makes really good marine radios but you could have a defective one.

Posted on Sep 13, 2009


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On the ICOM M604 will not receive the weather channels, will receive other close radio's.


This is almost ALWAYS caused by a problem (a short OR an open) in the coax cable from the VHF radio to the antenna. An easy way to check is to JUST insert the PL-259 connector into the socket of the radio, making sure that ONLY the center pin is making contact with the socket. If the weather channels come in clearly, then it's usually an easy fix. Simply check the connections at each end of the coax, looking for a poor center pin solder connection or perhaps a bit of the ground braid is shorting out the center lead. A magnifying glass is a great tool to spot problems.

If the coax proves NOT to be the problem, then the fault is likely to be in the radio itself and is probably a bad solder connection where the antenna chassis mount attaches to the PC board.

Hope this is helpful.
Good Luck!

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My VHF Marine Radio transmits very well -over 30 miles but only recieves about 6 miles. The boats that are calling me also have very high antennas so hight is not the problem. The boat is new and we are on...

I don't have much experience of radio transceivers but I do have a couple of ideas.

The big question is how do you know your radio transmits 30 miles but receives only 6 miles?
Clearly you have a friend with a transceiver and you have been playing around together until you formed those conclusions.

What if the other radio received well but didn't transmit so well? Have both radios been checked with a field strength meter to ensure they are both radiating similar power outputs?

Do you receive the carrier wave but no audio at a range greater than 6 miles? If so your friend's radio (and others) might have a modulation problem. Has the signal(s) been analysed with an oscilloscope?

It is often imagined the transmission envelope is circular radiating from the antenna at the centre but that is often wrong. The radiation envelope is often distorted by a ground plane. Imagine an antenna mounted at the centre of an aluminium boat hull, the radiation envelope would be elliptical in line with the hull.

Even with a fibreglass hull the presence of metal masses can pull the radiation envelope out of shape lengthening the range of both transmission and receiving in one direction and shortening them in another.

There is another possibility to poor reception and that is a high amount of interference that might be airborne being received by the internal radio circuitry, by several means - antenna or the poor screening of the downlead and could be aggravated by poor grounding. Interference can also be on the supply line and both types can be generated by a host of electrical or electronic equipment.
Generators and motors, voltage regulators or stabilisers, local oscillators, fluorescent lighting, inverters, computers...

The basic radio of the old days could have the Squelch turned off and an experienced operator can hear much of what is or isn't happening.

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OTA stands for "over the air"and for a std TV it is searching for VHF/UHF signals as beamed from satellite. If the antennae is properly mounted and connection don't have any sulpher deposits, then the tuner has to be replaced. As far as the antennae, the basiscs are:
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I had this problem about a year ago. If all you need is the firmware update they will update it at no charge and they told me if it needs repairs they most likely will not have the parts becaus it is a discontinued model. They also said they did checked it and found no other problems. Sent it in to Icom:
Icom America
2380 116th Ave NE
Bellevue WA 98004
They did the firmware update and sent it back to me free of charge. It works great now and my chart cards work great.

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