VHF/DSC Send vs Receive Range

[mattkab]

Hi all.

Over the past couple weeks I installed DSC capable radios on both my main boat (4788 Bayliner) as well as the RIB dinghy (12' Rendova). I plugged in the MMSI of each boat into the radio of the other, and at the dock we confirmed we could make, answer, and end DSC calls between the vessels.

Yesterday I took the dinghy out fishing. My wife initiated a DSC call from the 4788. The radio on the RIB received it, I answered it, and a connection was made. At that point, she could hear me (RIB -> 4788), but I could not hear her (4788 --> RIB).

Then, after making contact via cell service, we tried the other way, and I initiated a call from the RIB. The 4788 heard to call, my wife answered it, and a connection was made. Again, she could hear me, but I could not hear her.

Both transmission tests were done with the boats ~6 miles apart, with clear line of sight between them.

I know there's a ton of factors in play here, with antenna heights, connections, etc etc... but I would think that if the radio could receive the DSC signal it should also be able to receive the voice transmission as well. If anything I would think the RIB outbound transmissions would be the ones to fail, as it's a smaller antennae, closer to the water, but it's the other way around.

Any ideas?

 

[diver dave]

6 miles on even terrain to a HH is pretty far for 2 way voice. I just did an experiment with a 25W boat to a HH up on a hill, and 3 miles is about it.
The other "thing" is that data can go further than voice. This is because the bandwidth for data is much lower than voice, so the system noise figure improves.

 

[Tolly Roger]

Are you sure the radio on the 4788 is on "high" and not on "low" power?

 

[mattkab]

Are you sure the radio on the 4788 is on "high" and now "low" power?

I am not sure -- in fact I'm not sure what either boat's transmission power was. I will check and re-test this.

 

[mattkab]

The other "thing" is that data can go further than voice. This is because the bandwidth for data is much lower than voice, so the system noise figure improves.

Thank you. This actually makes sense too... I do find it odd that voice works in one direction, but the digital signals having a higher range makes total sense.

 

[diver dave]

I wasn't sure what the RIB radio was. If both are 25W, and SET to 25W, and both squelches are set to threshold, it should be a balanced system. That is, the receive signal should fail at both ends at about the same range. Independent of antenna, since deficiencies of just one antenna affects both sides of the equation equally.
Data should go quite a bit further, as explained before.

 

[OldDan1943]

Start when both boats are tied together.

You do have a different MMSI for each boat?

Remember the VHF radio is 'line of sight' radio.
Also, the antenna on the RIB is much shorter than the antenna on the Bayliner.

Switch to an unused channel and see if you can communicate between the RIB and the boat.

 

[mattkab]

Start when both boats are tied together.

You do have a different MMSI for each boat?

Remember the VHF radio is 'line of sight' radio.
Also, the antenna on the RIB is much shorter than the antenna on the Bayliner.

Switch to an unused channel and see if you can communicate between the RIB and the boat.

Yes, each boat has it's own MMSI.

I can communicate between the two boats easily when they are close... with the RIB in and around the marina everything works as expected.

It was when I took the boat out that I saw unexpected results. I found it odd that the large boat was able to receive the small boat's voice transmissions, but the small boat could not hear the big boat's transmissions.

 

[OldDan1943]

Yes, each boat has it's own MMSI.

I can communicate between the two boats easily when they are close... with the RIB in and around the marina everything works as expected.

It was when I took the boat out that I saw unexpected results. I found it odd that the large boat was able to receive the small boat's voice transmissions, but the small boat could not hear the big boat's transmissions.

This occurs only when calling via MMSI?

You can get the boat or the RIB attention via MMSI? What happens if after the connection via MMSI then change to an unused channel?

 

[hmason]

Look at the size of the antenna on the big boat 8 feet or more. The HH maybe 8 inches? That could explain the receive capabilities.

 

[mattkab]

This occurs only when calling via MMSI?

You can get the boat or the RIB attention via MMSI? What happens if after the connection via MMSI then change to an unused channel?

I can "call" both the boat and the RIB via MMSI/DSC from either vessel. This works in all cases.

Once a connection is made, and the radios change to channel 72, at ~6 miles, RIB transmissions are heard loud and clear. Primary vessel transmissions are not heard.

 

[mattkab]

Look at the size of the antenna on the big boat 8 feet or more. The HH maybe 8 inches? That could explain the receive capabilities.

RIB antenna is 48", but point taken.

I thought the transmission would be limited by the same antenna/environment limitations as receive, but maybe that was a bad assumption.

I was just surprised to end up with a 1-way connection.

 

[OldDan1943]

I can "call" both the boat and the RIB via MMSI/DSC from either vessel. This works in all cases.

Once a connection is made, and the radios change to channel 72, at ~6 miles, RIB transmissions are heard loud and clear. Primary vessel transmissions are not heard.

This ONLY occurs when the initial contact is via MMSI/DSC?
I think I am stumped.

 

[mattkab]

This ONLY occurs when the initial contact is via MMSI/DSC?
I think I am stumped.

We didn't try non-DSC communication, nor another channel. I can try that test as well.

But my understanding was that any communication after "connection" was just a normal VHF broadcast -- that is there is nothing private or digital anymore, and DSC is just a hailing/acceptance method.

 

[Benthic2]

It might be useful to get a third party involved. Take the rib out and call your big boat at the marina....then call another boat at the marina.....compare the results. That way you can tell which of your radios is the weaker link.

 

[mattkab]

It might be useful to get a third party involved. Take the rib out and call your big boat at the marina....then call another boat at the marina.....compare the results. That way you can tell which of your radios is the weaker link.

Good idea. Another test to try.

I will report back to this thread what I discover. Unfortunately I won't be able to make it back to the boat until Wednesday at the earliest.

 

[hmason]

I may be wrong however I think a DSC call is relayed thru a different broadcast system which is more powerful than VHF. That system monitors channel 70 and relays the DSC calls. Then you switch to a working channel which is normal power. “I think.”

 

[OldDan1943]

I may be wrong however I think a DSC call is relayed thru a different broadcast system which is more powerful than VHF. That system monitors channel 70 and relays the DSC calls. Then you switch to a working channel which is normal power. “I think.”

hmason, you have made the fog begin to clear.
I do believe you are correct.
If I wasn't so lazy, I would grab my Chapman's book a read about the DSC calling.

 

[diver dave]

I may be wrong however I think a DSC call is relayed thru a different broadcast system which is more powerful than VHF. That system monitors channel 70 and relays the DSC calls. Then you switch to a working channel which is normal power. “I think.”

Nope; no infrastructure required; it's all peer to peer. There can be a GPS (satellite) interface, but even that is optional.

 

[MYTraveler]

IME, the range for a DSC connection (ie, the two radios talking to each other, digitally), is significantly longer than the range for voice communication. And as noted above, once the connection is made, it is a conventional analog communication.