HF SSB Offshore


AM modulation was first. AM contains the carrier (as much as 2/3rds of the power) and the sidebands with the information. The bandwidth of the AM signal is twice the modulating frequency (typically 300 to 3,000 Hz for voice). That gives a typical AM signal a 6kHz bandwidth or more. Received signals are always competing with noise (Signal to Noise Ratio) and with AM we have 6KHz of broadband (White) noise plus any extraneous noise to contend with.

Double Sideband DSB modulation removes the carrier so we can now concentrate all the power in the sidebands for a 2/3 boost in the usable signal. The sidebands contain all the intelligence but are identical so we don’t actually need both of them.

Single Sideband SSB modulation removes the carrier and one sideband to concentrate the signal even more. With SSB we get a six times signal strength boost since all the power is now concentrated in one sideband. We also reduce the bandwidth to 3kHz so we have only half the noise to contend with (double the S/N ratio) if we employ proper transmit and receive filters. For SSB reception and detection, the carrier is reinserted in the receiver so the mixing result can produce the original voice signal for the speaker.

Audio Frequency Shift Keying AFSK – For a single audio tone, we can now generate a single RF frequency with all the power, usually reported as Peake Envelope Power, concentrated there. A 1,000Hz tone added to a 3,800 KHz suppressed carrier frequency provides a single 3,900 RF signal with all the power concentrated there. In order to transmit useful data, we need at lease two frequencies usually referred to as mark and space in older teletype terminology. When transmitting, five pulses plus a start pulse and a double width stop pulse are used in the TTY Bardot code (Note ASCII adds a parity pulse).

AFSK and bandwidth – For reliable reception, the two AFSK frequencies must be sufficiently separated so we can reliably detect them. AFSK shifts that are used vary from 170Hz to 425Hz and 850Hz but 170 is recommended to reduce bandwidth. There may be some in-band fading, where both mark and space frequencies fade, but the reduced bandwidth is generally better for signal to noise considerations. With 170Hz shift and proper filters we could theoretically reduce the noise to five percent and some of this is achievable.

Power needed – When using digital techniques, as seen above, we can understand why less transmit power is needed to overcome the background noise. Voice SSB operators run somewhere between 100 and 1,000 watts but SSB voice is possible, especially in controlled, multiuser networks with as low as 10 watts PEP transmitted power. It does require good antennas and transmission lines but those should always be used for efficiency. QRP (decreased power) SSB voice transmissions typically use 10 watts or less so that should be completely sufficient for narrow band AFSK data transmissions.

Frequencies to use / Propagation Predictions – HF signals between 3 and 30 MHz travel about 20 to 30 miles by ground wave and from 100 to several thousand miles by sky wave. There is most often a “skip zone” (no signals) between the maximum ground wave and minimum sky wave distances. Most HF frequencies are reflected or refracted back to earth by the F1 and F2 ionization layers during daylight and the combined F layer at night. Ionization within these layers and hence reflective power varies by path, time of day, day to day and year. This ionization can be estimated by the smoothed sunspot number and it varies in 11-year cycles plus the hourly and daily variations. Lower HF frequencies especially, are absorbed by the lower (in height) D atmospheric layer. HF signals above the Maximum Usable Frequency (MUF) are not reflected back enough to be useful. Signals below the Lowest Useful Frequency (LUF) are covered by noise and thus unusable.

HF propagation predictions are an estimate of the best frequency, Optimum Working Frequency, OWF (OWF = MUF * 0.85) to use based on the anticipated smoother sunspot number, particular path location and distance plus other factors.  For more propagation information see  https://hamwaves.com/propagation/en/index.html. Predictions can be refined by atmospheric soundings or actual observations. WINLINK uses observations to correct their estimates.


Top layer = 5, Internet based, worldwide Common Message Servers CMS

These CMSs collect, exchange and store WINLINK email type messages. Only one of these

servers is necessary for the system to function but having more provides more flexibility and

adds robustness to the system.

Middle layer = Many 24/7, automated, HF receiver/transmitter, Radio Message Servers (RMS)

There are RMSs located all over the world, insuring that at least one and normally many will be

in range of anyone at sea or ashore ham station wanting to use the system via HF (or VHF)

AFSK. These servers/stations automatically receive messages from users (at sea or if needed on

land) via HF (or VHF) and upload them to one CMS and then it is replicated so that all five have

it for redundancy. If a user requests that the system check for WINLINK messages for it, the RMS retrieves any messages over the Internet and transmits it via HF to the ship station user or

via VHF or HF for land-based users especially if a disaster prevents them from having cell or

Internet access.

User/client layer = HF at sea or HF/VHF Ham radio stations wanting to use the system

User/client Ham radio stations at sea use appropriate HF SSB (with digital capabilities)

Transceivers to contact an RMS station. WINLINK software allows:

  • addressing and building the message
  • selecting an RMS station from a list based on the client’s location, time of day and anticipated propagation
  • automatically controlling the radio (if possible) for RMS station selection and frequency setup (radio needs to be capable of auto antenna tuning)
  • checking for email waiting for the client and returning any it finds
  • sending any locally cashed messages to the RMS for forwarding to the CMSs

The client runs WINLINK software, usually on a shipboard laptop computer, to retrieve incoming messages, build and cash outgoing messages and transmit outgoing messages. There is no automatic incoming message notification facility so the client must periodically check for messages for its station.

Anytime with Internet access, ashore or in coastal waters, this whole process can be accomplished via the Internet and does not require any radio access. Messages can be addressed to regular email addresses or special WINLINK email addresses for your Ham radio station.

The WINLINK Express software, www.winlink.org, handles generating and decoding the necessity audio frequency tones and allows setting output audio sound levels. You need to set your transceiver for Voice Operated Keying (VOX) mode so the software can transmit when necessary. The software functions properly with modern sound cards. If your computer can not function with the software only there are interface boxes available like the SignalLink USB at http://www.tigertronics.com/ .

For sailors/boaters WINLINK is sponsored by the Amateur Radio Safety Foundation, Inc and is used by:

  • Boaterwatch.org
  • Seven Seas Cruising Association

“Quick Start Links for Mariners

Quick Start:

Get your General Class amateur radio license: http://www.arrl.org/getting-licensed

Operating with a ship’s, marine or general radiotelephone license is illegal on the Winlink network as it operates on the amateur radio bands, NOT the marine bands.

Download Winlink Express: http://www.winlink.org/RMSExpress

Read the program help files, configure it.

See: http://www.winlink.org/user  Create your account. Use ‘Telnet’ sessions to practice and experiment until you can set up a solid radio installation and go on-air.

Installation Basics:

Good installation tips: https://sailmail.com/category/problems-installation-faq/

An overview of what makes a good radio installation. A bit dated and inaccurate regarding Winlink and Sailmail, but technically very good and easy to read: http://www.tongacharter.com/ssb.htm

Good advice about marine lightning protection: http://adagiojournal.com/?p=349

Download a PDF of the excellent guide from Florida Sea Grant: Lightning and Boats: http://ufdc.ufl.edu/IR00007610/00001


Equipment Sources:

Pactor Modems, cables, accessories (if needed): Farallon Electronics (http://www.farallon.us/webstore ) 415-331-1924 (USA) SCS (http://www.scs-ptc.com ) (Worldwide)

Source for grounding copper, connection hardware: Georgia Copper (http://www.gacopper.com ) 770-536-1108

Voltage booster: https://mfjenterprises.com/search?q=voltage+booster

Download this document below. It’s filled with links to tips, reliable references, and equipment sources. It’s especially useful for the new marine mobile ham, but anyone new to Winlink will find it helpful.

Tip: If you use Microsoft Word to open the .odt (open document text format) file, it will say the file is corrupt. It isn’t. Just continue and ask Word to recover the file contents. It’s all there. Thank you, Microsoft!