DSC Receive Antenna for SSB?

[Dockhead]

I bought the Metz special antenna for DSC receive for my M802, which I am only just now getting around to installing.

However ---

I have been reading that for receiving HF radio, that an uninsulated stay or shroud works just as well (or even better?).

I have a shroud chainplate almost next to my HF radio, so this would be simplicity itself to connect.

Does it make sense? And could people do the same thing for dedicated Navtex receivers and the like?

I've actually got a Navtex antenna on my pushpit, with cable run, but if I use shrouds for Navtex and DSC, I could replace that antenna with a regular VHF antenna, and have a spare not dependent on the mast being up -- something I have kind of wanted for a while.

[SoonerSailor]

Give the shroud a try. If you want to experiment, connect your shroud to the normal txcvr input of your m802 and tune around. You should pick up stations just fine. Also, If when you attach that shroud 'antenna' to the normal txcvr input and you hear an increase of noise (atmospheric not onboard rf interference) you can feel pretty sure your shroud is going to receive DSC about as well or better than a short little whip. That is what I did anyway, and it works fine for the DSC receive only antenna connection.


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[Roy M]

Dockhead, I noticed that requirement for the DSC antenna while reading the instructions for installing my own M-802. I have an "antenna farm" atop my sterncastle that I keep for my AIS, VHF (plus one at the masthead, planned, using a selector switch), GPS (times two) and FM radio antennas. What model Metz are you using?

[Dockhead]

OK, thanks. That's very useful. Anyone else doing this?

How did you ground the shield, by the way?

[Dockhead]

Originally Posted by Roy M

Dockhead, I noticed that requirement for the DSC antenna while reading the instructions for installing my own M-802. I have an "antenna farm" atop my sterncastle that I keep for my AIS, VHF (plus one at the masthead, planned, using a selector switch), GPS (times two) and FM radio antennas. What model Metz are you using?

This one: Metz Communication General Coverage/WeatherFax Antenna

[ka4wja]

Dockhead,
1) I actually have been using an insulated aft lower shroud, as my DSC Receive and WeFax Receive antenna for 10 years now...and it works great...

Yes, mine is insulated at the top with a rigging insulator, and fed at the chain plate with the center-conductor of RG-8x coax....with the shield connected to a simple ground wire, running about 8' to a keel bolt....(actually never noticed any difference whether the shield was connected to the keel bolt or not...)

I realize this is not exactly what you asked, as you were asking about using the un-insulated shroud/rig as antenna....but thought I'd pass this on anyway...

Originally Posted by Dockhead

OK, thanks. That's very useful. Anyone else doing this?

How did you ground the shield, by the way?

2) But also important here, is a probable peculiarity of the M-802....
-- We DO know that the separate DSC antenna (and dedicated DSC Receiver) is used to receive your DSC-Distress ACK (a DSC acknowledgement of your DSC-Distress Call), while the main antenna and main receiver is monitoring the corresponding GMDSS SSB-Voice freq...
-- We also DO know that the dedicated DSC Receiver (and the separate DSC antenna) continuously scans all six GMDSS DSC freqs, all the while the M-802 is on, looking for "Safety", "Urgency", and "Distress" calls from others...
From the M-802 manual:

Monitoring the frequencies, 2187.5, 4207.5, 6312.0,
8414.5, 12577.0 and 16084.5 kHz, for distress, urgency,
etc., no operation is necessary with the transceiver.
These frequencies are monitored at all times.

-- But apparently, when waiting for a response (ACK) from a DSC Test Call, or DSC Routine Call (both are "individual" calls), the M-802's Main Antenna and Main Receiver is being used for this, while the dedicated DSC Receiver is still scanning the six GMDSS DSC Safety and Calling freqs looking for "Safety", "Urgency", or "Distress" calls...
(note that I have not personally verified this myself, but will do so within the month....this info was posted here recently by another Cruiser's Forum member...)

-- So, while having the DSC Receive antenna working well is important in order for you to receive an ACK from your own DSC-Distress Call, and to receive "Safety", "Urgency", and "Distress" calls from others....it appears to not be used for ACK's when you are making other individual DSC calls....
(again, I have not confirmed this personally....and I just finished some major family business, and now am away from the boat for a week or so....so, it will be a few weeks 'til I do confirm all of this....)



I hope this helps...

John

[SoonerSailor]

I didnt ground the shield. Of course it grounds at the receiver end, but no need at the antenna end. There is of course a large impedance mismatch (high SWR) between the long wire antenna (shroud) and the 50 ohm receiver/coax on most bands, but that makes little difference to receive strength. If the coax run is fairly short there will be minimal SWR related signal loss at DSC frequencies.


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[Dockhead]

Originally Posted by ka4wja

Dockhead,
1) I actually have been using an insulated aft lower shroud, as my DSC Receive and WeFax Receive antenna for 10 years now...and it works great...

Yes, mine is insulated at the top with a rigging insulator, and fed at the chain plate with the center-conductor of RG-8x coax....with the shield connected to a simple ground wire, running about 8' to a keel bolt....(actually never noticed any difference whether the shield was connected to the keel bolt or not...)

I realize this is not exactly what you asked, as you were asking about using the un-insulated shroud/rig as antenna....but thought I'd pass this on anyway...




2) But also important here, is a probable peculiarity of the M-802....
-- We DO know that the separate DSC antenna (and dedicated DSC Receiver) is used to receive your DSC-Distress ACK (a DSC acknowledgement of your DSC-Distress Call), while the main antenna and main receiver is monitoring the corresponding GMDSS SSB-Voice freq...
-- We also DO know that the dedicated DSC Receiver (and the separate DSC antenna) continuously scans all six GMDSS DSC freqs, all the while the M-802 is on, looking for "Safety", "Urgency", and "Distress" calls from others...
From the M-802 manual:


-- But apparently, when waiting for a response (ACK) from a DSC Test Call, or DSC Routine Call (both are "individual" calls), the M-802's Main Antenna and Main Receiver is being used for this, while the dedicated DSC Receiver is still scanning the six GMDSS DSC Safety and Calling freqs looking for "Safety", "Urgency", or "Distress" calls...
(note that I have not personally verified this myself, but will do so within the month....this info was posted here recently by another Cruiser's Forum member...)

-- So, while having the DSC Receive antenna working well is important in order for you to receive an ACK from your own DSC-Distress Call, and to receive "Safety", "Urgency", and "Distress" calls from others....it appears to not be used for ACK's when you are making other individual DSC calls....
(again, I have not confirmed this personally....and I just finished some major family business, and now am away from the boat for a week or so....so, it will be a few weeks 'til I do confirm all of this....)



I hope this helps...

John

My shrouds are massive 16mm cable which is not really realistic to insulate -- do you think this will work ok without being insulated? And in general, do you think this is a good plan, or should I use the Metz antenna?

And should I ground the coax shield?

Grateful as always for your expert advice, which will benefit others on here, too.

[Dockhead]

Originally Posted by SoonerSailor

I didnt ground the shield. Of course it grounds at the receiver end, but no need at the antenna end. There is of course a large impedance mismatch (high SWR) between the long wire antenna (shroud) and the 50 ohm receiver/coax on most bands, but that makes little difference to receive strength. If the coax run is fairly short there will be minimal SWR related signal loss at DSC frequencies.


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Does it work ok?

The coax run will be less than 2 meters if I do it this way.

And it seems to me that I wouldn't actually need coax at all over this kind of distance. I could just use a bit of GTO or even plain copper wire.

[SoonerSailor]

Yes GTO or insulated wire would be as good or better, unless it runs very close to a source of RFI that coax shield might mitigate. In fact a banana plug will fit into the center of the SO-239 on the m802 so you can solder your wire to that and not bother with using a PL-259 like you need to on your main antenna connection. A 2 meter coax run would not have be any problem though.

It works well on my boat. The shroud is not insulated at the top, and not grounded at the chainplate.


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[ka4wja]

Dockhead,
I do think your plan will work....but, you can just try it and see...use this new antenna as your "main" antenna for receive testing of various signals / compare this to your regular "main" antenna, and you should be good to go...

Originally Posted by Dockhead

My shrouds are massive 16mm cable which is not really realistic to insulate -- do you think this will work ok without being insulated? And in general, do you think this is a good plan, or should I use the Metz antenna?

And should I ground the coax shield?

Grateful as always for your expert advice, which will benefit others on here, too.

But, I'm afraid I disagree slightly with SoonerSailor here....
I would not run a simple wire / GTO-15, etc. from the M-802 to the chain plate....
I WOULD use coax....even if all you do is connect the center conductor to the chain plate, I would still use coax....
(a few feet of RG-8x or even RG-58, with a PL-259 on it, is cheap!!!)


Now, having written the above....I need to disclose that I DID use a 35' length of black GTO-15 wire (soldered into the center of a PL-259), right from a radio, out thru a thru-deck fitting, and run up along a shroud (twisted it around the shroud tightly, and secured with some cable ties), and used this for years on another boat as both a WeFax receive antenna, and a SW receive antenna....going to an Alden MarinFAX and to a Yaesu FRG-7 (yes this was 35+ years ago)....and I even had a Palomar Engineering VLF converter to allow LW / 200khz reception of UK shipping forecasts, etc., using this antenna and the Yaesu....
So....

So, not only CAN you run your antenna wire directly to the receiver input, but I've done this on-shore often, as well as on-board years ago...
BUT...

But, again this was years ago....and/or on-shore, where I could control received RFI, etc...

So, I still think feeding the chain plate with coax is the better approach...



I hope this helps.

John

[Dockhead]

Originally Posted by ka4wja

Dockhead,
I do think your plan will work....but, you can just try it and see...use this new antenna as your "main" antenna for receive testing of various signals / compare this to your regular "main" antenna, and you should be good to go...

But, I'm afraid I disagree slightly with SoonerSailor here....
I would not run a simple wire / GTO-15, etc. from the M-802 to the chain plate....
I WOULD use coax....even if all you do is connect the center conductor to the chain plate, I would still use coax....
(a few feet of RG-8x or even RG-58, with a PL-259 on it, is cheap!!!)


Now, having written the above....I need to disclose that I DID use a 35' length of black GTO-15 wire (soldered into the center of a PL-259), right from a radio, out thru a thru-deck fitting, and run up along a shroud (twisted it around the shroud tightly, and secured with some cable ties), and used this for years on another boat as both a WeFax receive antenna, and a SW receive antenna....going to an Alden MarinFAX and to a Yaesu FRG-7 (yes this was 35+ years ago)....and I even had a Palomar Engineering VLF converter to allow LW / 200khz reception of UK shipping forecasts, etc., using this antenna and the Yaesu....
So....

So, not only CAN you run your antenna wire directly to the receiver input, but I've done this on-shore often, as well as on-board years ago...
BUT...

But, again this was years ago....and/or on-shore, where I could control received RFI, etc...

So, I still think feeding the chain plate with coax is the better approach...



I hope this helps.

John

As always, it helps a lot, of course!

There's no cost issue -- I have tons of coax lying around. I just want the best solution. It seems to me that coax might cause impedance problem which GTO or plain wire wouldn't. Or maybe it doesn't matter, since it's a run of less than 2 meters?

What concerns RFI: I have a blessedly quiet boat. I have a couple of rogue LED lighting unit which put out a little RFI which sounds like white noise, but that's it (and I'm going to hunt those down and kill them eventually). I doubt if 1.5 meters of GTO inside the cabin is going to make much difference in RFI.

[SoonerSailor]

Your Coax impedance is same as the radio's input impedance if you are using the right stuff. Even if the shroud happens to be exactly 1/2 wave at a frequency of interest, with an SWR of 40:1 (a worst case scenario), and that frequency is 16 mHz (highest of the DSC frequencies I think), your 2m coax SWR loss would only be about 1 dB if using RG8x, a trivial figure. At lower frequencies loss is even less. If you use RG-8 losses would be half of those figures, but I wouldn't bother. Bottom line is dont worry about coax loss on such a short run.


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[SoonerSailor]

On top of that, when you talk of coax losses on receive, both signal AND noise are attenuated that little tiny bit - the signal to noise ratio is unchanged, which figure is by far going to be the limiting factor at the HF frequencies used by DSC.


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[Jd1]

Originally Posted by Dockhead

OK, thanks. That's very useful. Anyone else doing this?

How did you ground the shield, by the way?

I am using a side stay for my DERA. I do not run coax but just a straight 18 ga marine wire. This works reasonably well when connected to the primary antenna input so I assume it is fine for the DERA. I have NOT been able to verify correct operation though!!!
Please note, only emergency related DSC messages go through the DERA. Although the 'test' function of the M802 DSC system is supposedly going through the DERA, I was able to verify that I can get an Ack to a test call without the DERA connected.
Testing the DERA system remains a project in progress.
Late addition:
the stay is grounded at the mast (not the chain plate)
RFI is generally not an issue unless you run your cable right past a big source of RFI. The stay that is your antenna will pick up any significant RFI even if you use coax for the lead-in of the DERA