VHF Receive vs Transmit

Bill_N

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I've always noticed that I can receive a whole lot further than I can transmit. Last summer I was trying to get a handle on effective transmitting range offshore. I know antenna height has something to do with it and I'm running a 26 footer with an 8' antenna on the hardtop. Late in the summer we were at the Fishtails (east wall) and I heard some guys I know that were into some fish between Atlantis and Veatches. I could hear them clear as a bell but couldn't reach them transmitting. I'm guessing they were approx 40 miles Then a 3rd guy chimed in that was at W Atlantis and I couldn't reach him either. From what I could tell he was about 25 miles east.

Late in the day while we were stopped on the way in one of the guys hailed me and we were able to talk for awhile from somewhere around 30 miles.

Has anyone else had similar experiences? Any VHF experts here?

Thanks, Bill
 

greg

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Not an expert but there are a couple of things you can check. First, check your radio and make sure you aren't set to 1W xmit power. Most radio's have a Hi/Lo or 1W/25W setting. If you are set to 1W, you will be limited to a couple of miles. It is channel specific, so you might have it set Lo on one channel but not another.

Second, get a SWR meter and check your antenna to see what your xmit strength and quality are. It's possible you have a nick in your insulation somewhere that is causing you grief during xmit. You might also just have some corrosion in your connector.

The other thing you can check is the gain on your antenna. If you have a high gain antenna and the seas were really tossing you around, the signal may have been pointing at the water and up to the sky rather than out at the horizon. The higher the gain, the flatter the lobes will be leaving the antenna, but the fuurther the signal will travel. Modt small boats and sailboats actually do better with lower gain antennas because the signal goes out like a sphere in all directions rather than as a flat disc going sideways.

Assuming flat level water, no corrosion/antenna issues, and proper radio settings, it's all determined by the heights of the two antennas and the distance between them. But if you can hear them, they should be able to hear you.

Anyway, I might have made all of this up. But then again, maybe I didn't. ;)
 

Wharf Rat

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Couldn't have put it better myself. Great explanation Greg. :geek:

Not an expert but there are a couple of things you can check. First, check your radio and make sure you aren't set to 1W xmit power. Most radio's have a Hi/Lo or 1W/25W setting. If you are set to 1W, you will be limited to a couple of miles. It is channel specific, so you might have it set Lo on one channel but not another.

Second, get a SWR meter and check your antenna to see what your xmit strength and quality are. It's possible you have a nick in your insulation somewhere that is causing you grief during xmit. You might also just have some corrosion in your connector.

The other thing you can check is the gain on your antenna. If you have a high gain antenna and the seas were really tossing you around, the signal may have been pointing at the water and up to the sky rather than out at the horizon. The higher the gain, the flatter the lobes will be leaving the antenna, but the fuurther the signal will travel. Modt small boats and sailboats actually do better with lower gain antennas because the signal goes out like a sphere in all directions rather than as a flat disc going sideways.

Assuming flat level water, no corrosion/antenna issues, and proper radio settings, it's all determined by the heights of the two antennas and the distance between them. But if you can hear them, they should be able to hear you.

Anyway, I might have made all of this up. But then again, maybe I didn't. ;)
 

jimh

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I've always noticed that I can receive a whole lot further than I can transmit....Has anyone else had similar experiences?

Radio propagation should be reciprocal. If each station has the same transmitter power, and if each station has the same receiver sensitivity, then if A can hear B, B should be able to hear A. The antenna gains are useful in both directions, and they should not affect the results. The antenna pattern works in both directions, and should not affect the results. In the VHF Marine Band radio service all recreational boats should be running the same power, 25-watts. If you can hear 'em, you should be able to work 'em.

In situations where this does not occur there is usually a good reason:

--less than 25- watt transmitter power at the antenna, either from poor output at the transmitter, excessive transmission line loss, or poor antenna efficiency. Poor antenna efficiency and transmission line loss are more easily compensated for on receive because the receiver has practically unlimited gain. The transmitter does not. It is fixed at a certain power level. Many receivers work just fine with literally no antenna connected.

--less receiver sensitivity at one end of the circuit. This occurs from several possible influences. A SQUELCH control could be cranked up to the max, reducing receiver sensitivity. There could be a high level of local noise, reducing effective receiver sensitivity. The receiver could be operating in a mode of reduced sensitivity by an intentional setting of a sensitivity control.

--the other station does not want to talk to you as much as you want to talk to them. They may not acknowledge your call. They may not recognize that you are trying to call them.
 

Bill_N

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Radio is on Hi. The antenna is a 1 year old Galaxy which is what I had previously and the connector is fine. Some of the boats I'm hearing are at extreme ranges. If I'm at Block Island some days I can hear the guys east of the Cape tuna fishing and I can hear guys at the canyon. Last year we were at West Atlantis and a buddy of mine was south of Block a few miles. Said he could hear us but he couldn't reach me or I never heard him so it seems like the same scenario.

Thought I had heard of signal bounce or ricochet, something like that, which kind of makes sense with the Cape boats. There's land between us plus about 75 miles of water and I can hear them. Can't be line of sight from antenna to antenna over those distances.
 

jimh

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...I had heard of signal bounce or ricochet...

At 156-MHz there is very little chance of consistent F2-layer reflection. There is a chance for sporadic E-layer reflection. More likely is tropospheric ducting.

There's land between us plus about 75 miles of water...Can't be line of sight from antenna to antenna over those distances.

Radio communication at 156-MHz routinely takes place at distances greater than line of sight or the optical horizon. The radio horizon is longer due to refraction in the atmosphere of radio waves. The refraction can be enhanced by certain conditions, such as stratification of the air into layers of different densities (due to temperature differences). These often enhance refraction and extend the range of VHF signals.
 
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