ICOM IC-M803 SSB-Radio

[Hermia II]

I'm installing this using the backstay as the Transmitting and receiving antenna as usual. However in the Installation Manual it says I need to install yet another HF-antenna (with a normal connector the type you use for a VHF antenna) in order to receive distress calls (DSC I imagine).
It does not say anything about what type of antenna I need and the only thin I can think of is a 6 - 7 meter whip and I hate these things on my sailboat. At the same time receiving distress calls from hundreds of miles away does not seem relevant. Would it do with a shorter whip, say 2 meters?

[sv_pelagia]

Waiting for others to reply, more technically knowledgeable than I, here is a quick reply:


The "need" is for a DSC antenna, not another HF antenna. There are many easy ways to accomplish this (with excellent-to-less-than-best results...).

There are MANY posts about this... for example, this thread:
https://www.cruisersforum.com/forums...d.php?t=175681

FYI, the radio receives/broadcasts HF just fine without the DSC antenna; without the DSC antenna, howver, you might miss a urgent message sent by another radio with DSC.

[belizesailor]

Yes, DSC uses VHF frequencies, not HF frequencies.

You dont need a big whip antenna for this application. Most any small basic VHF antenna will work. Like any of those below:

https://www.practical-sailor.com/mar...f-antenna-test

Personally, Im a fan of Metz. Ive owned them for decades...and that has been only 2 antennas...they are very durable.


However, how high you mount it will affect DSC range. A friend has his mounted on the stern rail. Range is limited to few miles, but thats good enough for him.


Do you already have a VHF aboard? While a separate antenna is technically better, you can successfully set these up w a splitter. Some will no doubt howl at that idea, but it does work.

[Brian.D]

What? DSC uses VHF not HF? What? Yes, there are DSC frequencies on VHF, but there are also DSC across just about all the bands. LF, MW, HF, VHF, UHF, and so on.

Several maritime frequencies are used for digital selective calling (DSC), and they are also monitored for DSC distress signals:[6][7]

• 2.1875 MHz
• 4.2075 MHz
• 6.312 MHz
• 8.4145 MHz
• 12.577 MHz
• 16.8045 MHz
• 156.525 MHz, Marine VHF radio Channel 70

The DSC connector on the back of the radio is for receive only. Therefore, you can connect anything you want since it will not be used for transmitting. If you want to connect your VHF antenna you can, but you will degrade your receive capability because it is too short in length for those frequencies list above. You are best served by running a separate length of wire up your shroud and connecting it to the designated DSC connector. Length is semi important but not too short. Maybe as long as the shroud itself.

[Jammer]

Originally Posted by belizesailor

Yes, DSC uses VHF frequencies, not HF frequencies.

Brian.D is correct, DSC is used on HF as well as VHF.

[Jammer]

Originally Posted by Hermia II

Would it do with a shorter whip, say 2 meters?

There are many choices. The Metz 201, which is 54" (about 1.5 meters) is a common one.


I don't have HF on my boat but I have been told that, under good propagation conditions, the DSC alarm can go off often enough to become problematic. If you can receive the entire Pacific Ocean then someone is always having an emergency. A shorter receive antenna can serve as a compromise so that nearby vessels can still be heard.

[Brian.D]

I could be wrong on this, but...

I don't think you will receive every emergency DSC transmitted. I believe this is only received by the emergency agencies. Once the specifics have been established, then the emergency agencies broadcast an alert, which you will receive.

Now, do vessels in the Atlantic receive alerts for vessels in the Pacific? This I don't know either. Maybe the MMSI is sent occasionally giving agencies a heads up of who is where (re: ASI). So maybe the agencies have a list of what vessels are in close proximity of a vessel in distress.

[Jammer]

https://gmdss-radio.blogspot.com/201...lls-tx-dx.html


Less sophistication than we might hope for

[Hermia II]

Thank you so much for all your kind and knowledgeable input, what would we cruisers do without this forum providing so much help. The solution I think will be best for me is to insulate a lower shroud and connect to that. I'm really excited to get this up an running!

[Dockhead]

Originally Posted by Brian.D

What? DSC uses VHF not HF? What? Yes, there are DSC frequencies on VHF, but there are also DSC across just about all the bands. LF, MW, HF, VHF, UHF, and so on.

Several maritime frequencies are used for digital selective calling (DSC), and they are also monitored for DSC distress signals:[6][7]

• 2.1875 MHz
• 4.2075 MHz
• 6.312 MHz
• 8.4145 MHz
• 12.577 MHz
• 16.8045 MHz
• 156.525 MHz, Marine VHF radio Channel 70

The DSC connector on the back of the radio is for receive only. Therefore, you can connect anything you want since it will not be used for transmitting. If you want to connect your VHF antenna you can, but you will degrade your receive capability because it is too short in length for those frequencies list above. You are best served by running a separate length of wire up your shroud and connecting it to the designated DSC connector. Length is semi important but not too short. Maybe as long as the shroud itself.

The above is the correct answer.


But most people use the Metz 201 antenna for this, which is made specifically for receiving HF signals for weatherfax and DSC:


https://www.hamradio.com/detail.cfm?pid=H0-007076


This is simpler than insulating a shroud (although that will also work).

[Jim Cate]

You might try just connecting it to the UN-insulated shroud. This will make a quite satisfactory general purpose (including DSC) rx antenna. Or to a life line (if stainless wire) or a toe rail, if metal. You really don't need anything fancy for practical DSC usage.

Jim