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Old 01-12-2011, 09:48   #1
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DSC Watch Receiver on 2187.5 Securité Over a Long Distance

I made some tests on the canary island
with the DSC antenna for the ICOM M801E.
The Metz Weatherfax/DSC receiving-only antenna is designed for 2-30MHz and I was astonished last night at 2000.
One hour after dark the DSC Watch was alerting a Securite Message from Iceland: a geographical call for a region in the northern atlantik.
My receiver had no GPS input information, so he was unable to block out the geographical call.
When switching to 2182kHz I heard the Security message poor, but readable.
My background:
I am teaching LRC courses and the examination is made on an outdated Sailor radio HC4500.
So I like to train the students after the examen on a real life radio, as the ICOM.
I found the distance Iceland to Canarias quite tremendous taking into account the propagation of the 21xx kHz.
So the Metz Antenna makes a good job and I would like to recommend this antenna.
I switched off the speaker and in the morning I found in the Rx Memory a repeated Security call from Iceland.
My question:
Can you give me some other real life examples of receiving long distance DSC calls with antennas for the DSC input of the ICOM M801/M802.

How often are there DSC calls coming in? When to often during night, they will wake everybody up.
So I very good antenna is distorbing.

Is there a practical way to set an attenuator in the CoaxCable from Antenna to Rx input:
- to use a path thrue when sending out an own DSC call, in order to get a ACK
- to attenuate when avoiding alerts.
Thanks, Wilhelm
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Old 08-12-2011, 07:27   #2
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Location: Florida
Boat: Catalina 470
Posts: 1,849
Re: DSC Watch Receiver on 2187.5 Securité Over a Long Distance

I am NOT surprised at all by your results, as this is not unusual...
The path you desribe is about 2200 - 2500 miles, all over sea water....
(Here are a few points/facts that may help you along the way...I'll be breif....I won't ramble on and on....)

1) Nightime Long-range communications in the upper MF / lower HF region is very common, especially in winter when local atmospheric noise (lightning crashes, etc.) is diminished......
Although this is NOT commonly written about in the "maritime world", it is nonetheless a fact.
It is the Signal-to-Noise ratio that determines whether the communications path works or not.....
And on these lower freqs the limiting factor is almost always received noise, not the signal.....

{{Some persoal examples:
a) I quite regularly communicate on 3.6mhz - 3.8mhz SSB from here in Florida to stations in the UK and EU, (as well as Argentina, etc.) with paths of 4500 - 5500 miles...all year long.....but it's much easier in wintertime, since the lightning crashes from local (< 2000 miles away) T-Storms makes summertime long-range comms a bit frustrating for lower-powered stations.....
(Just this October while talking with my best friend on 3664khz early one evening, from my boat, we were interupted by a station in Argentina, asking us to move off a few khz as they had a net starting soon, and my signal was so strong that it would seriously interfere with their net....we moved down about 4-5khz and all was well.....a vertical antenna over sea water makes a nice antenna!!! TRUE story!!!!)
This is normal, from both from my boat with 150 watts / Icom M-802 / insulated backstay / copper strapping and Dynaplates, well as from my home station, 50 - 60 miles inland, with 1500 watts with simple wire antennas strung thru the trees of a normal-sized city lot.....

b) When the communications path is in darkness, or even when one is in early morning sun (D-layer takes a while to form), communications over very long distances on these lower frequencies is quite common....
And, again, when in darkness, the limiting factor is almost always atmospheric noise.....
(I've regularly worked Australia, etc. 10,500 miles from my home station in Florida, on 75m SSB, as I near sunrise.....and even have usable signals up to an hour after my sunrise, then it's like the D-layer closes the door.....signals diappear.....)

c) I have worked some long-distance paths (Caribbean and EU, etc. 2000 - 4500 miles) on 1850khz / 160m SSB from the boat, with good sucess, but haven't spent too much time down there....
And, I don't have the room at home for 160m antennas, so I don't work 1.8 - 2.0mhz much from home...BUT, I have done so from remote locales.....
My friends and I set-up a portable station (in a field, far away from city noises) a few years back for the 160m CQ WW SSB contest....and again using simple wire antennas we worked stations worldwide.....and placed first in the contest from our call area (W4)......}}

2) Iononispheric "D-Layer" absorption fades quite rapidly after sunset, so even an hour after dark D-layer absorption has fallen-off by as much as 95%.....
And, remember that this time of year, by the time of your sunset in the Canaries, Iceland has been dark for your entire path has been dark longer than you first think.....

3) Maritime DSC is a low-speed / narrow-bandwidth FSK signal (SITOR-B) with FEC......
Reducing the rx bandwidth, reduces the noise bandwidth, and the theoretical advantage in Signal-to-Noise ratio compared to a SSB signal is quite high.....12db to 16db....
(this is assuming a SSB rx bandwidth of 2400 - 2700hz and a DSC rx bandwidth of 300hz to 600hz.....assuming the rx signal stays constant, everytime you "half" the bandwidth, you improve the S/N by 6db....)
So, when comparing your results to a regular voice (SSB) contact, be aware of the S/N advantage of a DSC signal.....
(This is the primary reason that so much DX can be worked on CW, at lower powers, than on SSB.....)

4) While your 4' Metz antenna doesn't capture as much signal as a longer antenna would (I used the 4' Metz rx antenna myself for a couple months, before changing to a 22' insulated shroud for my DSC rx antenna).....remember that it doesn't capture as much atmospheric noise either!!!!
And, on the lower freqs (nightime on 2mhz and 4mhz in particular), it is the atmospheric noise that limits most communications.....

a) Don't forget how well signals (MF and HF) propagate over salt water, as the ocean makes multi-hop paths very do-able.....

b) And, don't forget that vertical antennas (even short, inefficient ones) have low angles of radiation (tx and rx), allowing for good signal strengths over long ionospheric paths....AND reducing signals from more local areas (< 200 miles), thereby reducing local noise.....

6) As for my personal experiences receiving DSC calls......
a) When first using the Metz antenna as my DSC rx antenna in summer of 2004, I did not reqularly receive many DSC calls....
BUT, I was NOT using the radio much at nightime, so most of the DSC calls were from 8mhz and 12mhz (where daytime atmospheric noise is farily low).....

b) Then, later in 2004, installed an insulated shroud as my DSC rx ant (approx. 22' long)....and since then I regularly receive DSC calls.....
Most daytime calls are on 8mhz and 12mhz, from 1000 - 4000 miles away.....and most nighttime calls are on 2mhz and 4mhz, from 500 - 2000 miles away....

7) As for not wanting to awaken the crew with alerts.....
The following are just MY OPINIONS.....not facts...

a) I would NOT install an attentuator.....nor would I switch off the DSC antenna......
b) I would either learn to live with the occasional alert, or unplug the speaker.....
c) Note that the "alert" alarm should only sound with "alert" messages.....(at least that's how my M-802 works)....
With "routine" messages, such as ALL-Ships Calls, Geographic Calls, (both of which I HAVE received), etc. my M-802 just emits a gentle "beep", the same sound heard when pressing the "enter button", etc.....
The only time the LOUD Alarm has sounded is with "alert" messages, such as DISTRESS and DISTRESS RELAY messages (and I've gotten a few).....and when that happened one night while I was sitting there listening to the BBC thru the headphones it scared the "beejeebez" out-of-me!!!!
(I've never received a "PAN PAN" or "SECURITE" DSC Call, so I cannot say how the M-802 reacts to them....)

I do hope this helps....

s/v Annie Laurie
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