DSC

[dehler25]

I am aware that transmitting DSC on an Icom 802 uses the same antenna for DSC transmit and SSB. To receive DSC on the 802 requires a separate receive antenna positioned away from the other. In the interest of power conservation is there any difference monitoring with VHF DSC. What is the range penalty? If needed one could always transmit using the 802. Is this a reasonable trade off?

 

[Bill Roosa]

Well for starters the 802 is a HIGH FREQUENCY radio and does not do VHF in the marine frequencys (160ish MHz). DSC on HF (2178.5 kHz) is a different network then DSC on VHF. The two are not linked at the USCG stations to my knowledge so making a call on the 802 will not "wake up" anybody's DSC VHF radio. Also, a quick look at the 802 specs would indicate that it uses the "well beloved" oven heater crystal oscilator, so it has a pretty high standby current draw.
The range with HF is quite a bit longer than your VHF. Without getting too technical you have VHF for ship to ship when in visual sight range and HF for longer distances, like other side of the world if need be.
transmitting on the 802 will wake up lots of folks that are not in your general area and who will not be able to respond quickly. it will also not wake up anybody who does not have a HF DSC. So when on the high seas I'd be monitoring with HF as most folks out there have HF but for costal crusing id be using the VHF and turning of the oven heater.
so to answer your question; it is not a trade off in the sense that you can use either one to accomplish the same thing.

 

[dehler25]

The oven heater was the reason I was concerned about power consumption. I had incorrectly assumed that the 802 would be monitoring the DSC spectrum and not just the HF. Upgrading the VHF for DSC would be more valuable at this time. Thanks

 

[Brian D]

I believe 2178.5 KHz has been discontinued but may still monitored in limited fashion. Below are the HF Distress DSC frequencies that the CG monitors.

HF DIGITAL SELECTIVE CALLING
Portsmouth/NMN, Boston/NMF, Miami/NMA, New Orleans/NMG, Pt. Reyes/NMC, Honolulu HI/NMO, Kodiak AK/NOJ
4207.5 DSC test calls on 4207.5 KHz will be automatically acknowledged from Portsmouth/NMN and Pt Reyes/NMC. Responses to test calls on other frequencies and at the other locations may be delayed and cannot be assured.
6312
8414.5
12577
16804.5
Note: For digital selective calling, frequencies listed are assigned. Carrier frequency is located 1700Hz below the assigned frequency.

Unlike VHF which assigns Channel 70 as the distress DSC channel, HF can use any frequency. However, there are assign HF frequencies that are guarded (monitored) by the USCG (4125, 6215, 8291, 12290) and other worldwide SAR agencies. All these are by international treaty and all will respond to the distress call.

Any boater can use DSC to call another boat using either VHF or HF. DSC is just not for distress. It is like a telephone number for you to call your buddy who is local (VHF) or on the other side of the world (HF).

To activate distress DSC on VHF just press the button and it will automatically select channel 70. On HF, I believe you have to select the desire distress frequency (6, 8, 12, or 16 MHz) and then press the button. I am not 100% sure of the process on HF.

To answer the original question, a separate antenna is not required for DSC use on HF. The HF antenna is a broadband antenna with the use of a tuner. I would recommend that you monitor the selected HF frequency that is appropriate for the location you are sailing and set the radio for that location. You will have to understand how HF propagation works to ascertain the correct frequency of the area you want cover. Makes no sense to monitor a frequency that will allow you to receive DSC from the Pacific if you are in the Atlantic.