CH16 Transmission Question

[redfish727]

I'm trying to gain a better understanding as an in-shore fisherman of how VHF marine radios work and would appreciate if someone could answer this question.

Scenario: I transmit a radio check or distress message on Channel 16 to the Coast Guard over my handheld 5W Cobra VHF Marine Radio 1/4mi from shore, the coast guard can hear me and responds.

Question: Is my low-power message automatically relayed by the Coast Guard station over 25W or higher signal so that a boater 20 miles from shore could hear my message? Or, is it more likely that the Coast Guard heard my message because I'm close enough to shore, but my 5W transmission was too weak to be picked up by that boater 20 miles offshore (let's say he has a fixed-mount 25W radio)?

I'm not looking for "why would you do this" responses, I'm looking to better understand how tx/rx works and more specifically how far a 5W radio transmitting over CH16 near shore will realistically be heard by other boaters.

Thanks so much.

 

[krokus]

VHF is a line of sight coverage. The USCG would have heard you better than others, as they have antennas in optimal locations. In this case, optimal means some place high up, with a wide "view" of the water.

Your transmission was not retransmitted on VHF, and would only cover a few miles. Over the open water, it could cover about 10 miles.

Sent via Tapatalk

 

[ecps92]

The Coast Guard have many Towers along the Shore Line and up some of the Major Rivers, all located at excellent Reception Points

A Boat is on the water at 10-20 ft of height vs the USCG Rescue 21 Tower at typically 100+ Ft All of these towers can DF [triangulate] the signals received to point out where the transmission is coming from

Height makes a difference, as does terrain

PS - Radio checks on 16 are discouraged, use 9 for them. Or determine if SeaTow or one of the other AAA of the Seas has enabled 26/27 for automated Radio Checks

I'm trying to gain a better understanding as an in-shore fisherman of how VHF marine radios work and would appreciate if someone could answer this question.

Scenario: I transmit a radio check or distress message on Channel 16 to the Coast Guard over my handheld 5W Cobra VHF Marine Radio 1/4mi from shore, the coast guard can hear me and responds.

Question: Is my low-power message automatically relayed by the Coast Guard station over 25W or higher signal so that a boater 20 miles from shore could hear my message? Or, is it more likely that the Coast Guard heard my message because I'm close enough to shore, but my 5W transmission was too weak to be picked up by that boater 20 miles offshore (let's say he has a fixed-mount 25W radio)?

I'm not looking for "why would you do this" responses, I'm looking to better understand how tx/rx works and more specifically how far a 5W radio transmitting over CH16 near shore will realistically be heard by other boaters.

Thanks so much.

 

[PACNWDude]

Your transmission would only be repeated automatically if you were using Digital Selective Calling (DSC). Your radio would have a button with a red cover if it has this feature. Then it helps if you have an assigned Maritime Mobile Service Identity (MMSI) number loaded into the radio, and accurate information for your vessel in the MMSI database. This tells the Coast Guard how big a vessel, number of personnel to expect and safety features the vessel has.

 

[mmckenna]

Redfish,

Since your header says you are in Florida, here is the USCG District 7 "Rescue 21" system radio coverage. It'll give you and idea about how it works:
http://www.navcen.uscg.gov/images/marcomms/cgcomms/Rescue21/D7.jpg

Keep in mind that commercial vessels as well as the USCG boats, cutters and aircraft monitor channel 16, so there is a chance one of them can hear you too.

 

[redfish727]

Very, very helpful all. Thank you everyone for the thoughtful responses.