Table of Contents
CCIR493 Selcall
HF Selcall Experiments
Background
I became interested in HF selcall when I was looking for a way of setting up impromptu QSOs with specific stations, for doing tests etc., without the need for listening by ear for calls. Various ideas were considered for controlling an audio mute, and alerting the operator, on HF, but nothing seemed appropriate (CTCSS, DTMF etc) until I stumbled on CCIR493 HF Selcall, specifically designed to cope with conditions on the HF bands.
The protocol was familiar, as it's very similar (but slightly simpler) than GMDSS DSC. This led to more reading and research. CCIR493 is widely used in Land Mobile HF SSB in Australia and in other countries which use HF SSB as a communications method. It seems largely unused in Europe. In Australia it is used on the networks set up to assist 4WD outback travellers, with Selcall used to initiate contact with the various base stations.
Selcall's beacon and 99 beacon functions can be used to check if a particular station is reachable before attempting a voice call.
This seemed ideal…. down the rabbit hole of HF Selcall…..
Basic details
CCIR493 HF Selcall is a predecessor to GMDSS DSC used in Marine Radio.
- Two Tone FSK with 170Hz shift
- 100baud signalling rate
- Tone frequencies 1700 = 0, 1870 = 1 (tone centre = 1785)
- compare with DSC: 1615 = 1, 1785 = 0 (tone centre = 1700, inverted shift)
- Symbols are 10-bit words, 7 bit + 3 parity bits
- Dotting pre-amble of alternating 1010….. 2 to 20 seconds depending on expected scan-group size
- Same “symbol repeat after 4” RX/DX interleaving as DSC
- Same phasing sequence as DSC
- No overall Error Check Sum, unlike DSC
- At its simplest a selcall transmission can be either:
- Directed call to a specific address
Selcall= send a revertive signal over the air AND alert the remote operator. TheFMTsymbol =120Directed-Beacon= send revertive signal over the air, but do not alert remote operator. TheFMTsymbol =123
99-beaconcall to a group of addresses = all stations send revertive signal over the air, but do not alert remote operator. The99-beacon callis a special form of a directed selcall, with theFMTsymbol of120and the final two digits of the destination address =99. Receiving decoders understand to treat the call as a special type ofbeaconcall despite theFMTbeing120(Selcall)
The directed-beacon to a specific address will confirm if that station is reachable, without disturbing the operator.
The 99-beacon call confirms whether any station in the group are reachable - the group is “any station with the first two digits matching”. The address used for the 99-beacon call is XX99, where XX is the leading digits of the desired group. For example, a group stations 3610, 3602, 3603, 3654 would all respond to a 99-beacon call addressed to 3699
Jenal SC2 Mic
HF Selcall is widely used in Australia, and there is a large market for add-on Selcall devices (to convert non-selcall radios) there. One manufacturer of Selcall equipment is JENAL.
At the time of my first exploration of Selcall I found that the SC2 microphone seemed like the perfect solution. The SC2 had been out of production, but Jenal had just released a new batch and I bought TWO of them, allowing me to experiment in isolation, should I fail to find any other amateur stations suitably equipped.
One SC2 microphone can be connected to a Xiegu G90 portable HF transceiver, capable of 20W pep, and the other SC2 can be connected to a home made 60m SSB transceiver, also capable of 20W PEP, at the main station.
Scanning
The SC2 is capable of controlling the transceiver to scan, waiting for incoming selcall dotting signals, when it will pause the scan to decode any selcall message and act upon it as appropriate. Scan will resume after a certain period. The simplest scanning method is to pulse the channel up or channel down line present on most ham-grade transceivers on the mic-connector and thus the radio will step through its memory channels looking for selcall signals.
Previous Selcall Activity
In previous periods of activity with Selcall I have managed a few contacts and QSOs. Mainly with two stations who were already equipped with Barrett or Codan radios which can use CCIR493 “out of the box”.
- GM4WMM - with a Barrett 950. We successfully operated on a small set of predetermined channels in the 80m, 60m, 40m bands, scanning, and selecting the optimum channel using
beaconcalls. - F6EMT - with a Codan. Made several contacts on 20m, some of them while mobile, initiated as random Selcall QSOs.
For 2023 I hope to find some more Selcall-equipped stations willing to try some experiments.
Selcall Activity 2023
- [✓ John Pumford-Green, 2023-04-30]
Attach SC2 to Xiegu G90 - [✓ John Pumford-Green, 2023-04-30]
Set up 60m homebrew radio & amplifier & attach SC2 - [✓ John Pumford-Green, 2023-05-05]
Install mobile whip on van for use with G90 - [✓ John Pumford-Green, 2023-04-30]
Make adaptor for IC-M710 to allow easy connection of SC2 in place of usual datamode interface, for access to more bands - Try to make contact with like minded experimenters for on-air tests
UPDATE — John Pumford-Green 30/04/23 16:57
When not otherwise engaged I'll leave my main station IC-M710 monitoring 14.343MHz or 18.163MHz with Selcall ID 3658. I may not be able to respond directly to any calls the Selcall will send revertives (at 50W) for successful selcall or beacon calls to 3658or for 99-beacon calls to 3699
For skeds please contact me by email - address at https://www.qrz.com/db/GM4SLV
I have re-enabled emails from the hf-link groups.io https://groups.io/g/hflink to see what's happening here - the only place where CCIR493 appears to be discussed/used.
Further Information
Code snippets
— John Pumford-Green 29/04/23 11:42
Last Modified : 02/04/24 19:08 BST