On Friday the 10th, the Global Disaster Awareness Coordination System (GDACS) announced a Red Alert for Tropical Cyclone SINLAKU, travelling from South-east to North-west, and threatening the islands of Guam and the North Mariana Islands. With maximum wind speeds of 194 km/h, and as a category 3 hurricane, it was threatening 180000 people in its path. The maximum effect is expected to be felt on Monday the 13th and Tuesday the 14th of April, as SINLAKU first makes landfall over Guam.
In last week’s bulletin, I promised to share the other half of the report on the SASOL Marathon of 28th March, received from Riaan Greeff ZS4PR, Divisional Director for HAMNET in the Free State. He wrote further:
”HAMNET acquired “Single Frequency Repeater” (SFR) Digital Mobile Radio (DMR) technology in the form of 3 repeaters and 20 hand radios for the race. The principle of SFR is to make use of the 6.25kHz channels with 2 alternating timeslots and transmit AND receive simultaneously on the two RF channels at either timeslot 1(TS1) or timeslot 2 (TS2).
“DMR is a digital radio mode utilizing Time Division Multiple Access (TDMA) modulation. This feature allows a repeater network to be deployed systematically into areas of need fast and effectively. Only one frequency is used, so no cavity filters are needed. The hand/portable radios are set to follow TS1 and TS2 in the two channels and seamlessly switch between simplex and repeater mode as the signal’s strength indicator is evaluated in real time.
“The Technical team set up three repeaters in a triangle around the route. This worked well. However, when all 15 radio users were active it became troublesome to decode the DMR signals. On investigation, it was found that the two 6,25kHz TS1 and TS2 signals were creating heterodyning for the receiving radios. The better way to set up three or more SFR units would be to deploy the repeaters in a straight line and not in a triangle/circle/looping arrangement.
“Why is this important for HAMNET? The Barberton aeroplane accident in 2025 highlighted the need to set up repeaters quickly and easily at local high sites. A SFR is extremely portable, and an ordinary ham can set it up in 10 minutes. The handheld radios are issued to specialists on the ground to do their work. HAMNET provides the infrastructure to deploy the hand held radio capabilities with the SFR repeaters. SFR allows effective radio signal selection management and relays signals autonomously. “
Google tells me that “a Single Frequency Repeater is a digital radio device that extends communication range using only one frequency, rather than the two required by traditional repeaters. It works by using Time Division Multiple Access (TDMA) to receive a signal on one time slot and simultaneously retransmit it on the other time slot within the same 12.5kHz channel, eliminating the need for a duplexer and allowing for cost-effective coverage extension.
“The repeater receives a snatch of data in time slot 1 and immediately forwards it in time slot 2 (that is: 60-90 milliseconds later), enabling direct mode operations to extend range without needing to change frequencies, as is used in analogue communications. The timing is so fast that the listener cannot discern the very short breaks in the received signal. Effectively, the repeater is alternating between receiving and transmitting the signal every 60-90 milliseconds but on the same frequency.”
Furthermore, if the received signal from the sending station is strong enough, as monitored in the Received Signal Strength Indicator algorithm of the repeater, it will not be repeated, but allowed to pass directly. So the transmission proceeds in the least complicated manner.
Thank you, Riaan, for telling us of your experiments with this technology.
If you have ever worried about the efficiency of your car’s brakes, spare a thought for the Orion capsule astronauts, as they came in to splashdown at about 2am our time on Saturday morning. While still approaching our atmosphere, they were travelling at 11km per second, about 40000km/hour, 40 times faster than a typical passenger jet.
After smashing through the Earth’s atmosphere, travelling at more than 30 times the speed of sound, the spacecraft had to slow to a safe speed for landing.
As it crashed into the top of the atmosphere, the craft created a shockwave that saw the surrounding air reach temperatures of 10,000C or more. To keep humans inside safe, Orion’s heat shield is made of Avcoat, a mixture of silica, epoxy and resins that NASA also used for the original Apollo missions to the Moon.
After the initial shockwave, the heat shield experienced temperatures of around 2,700C as the crew module plummeted through the atmosphere on re-entry. The coating of 200 Avcoat tiles is just a few inches thick and is designed to burn away, transferring heat away from the spacecraft.
The astronauts then fell 122000 metres through Earth’s atmosphere and were completely out of communication with mission control as superheated plasma around the capsule disrupted radio signals.
During this descent, the crew experienced 3.9G’s, or nearly four times normal gravity. This means their bodies felt four times heavier than normal. Astronauts had to train to withstand these high G-forces on re-entry. As Orion slowed it started to cool and radio communications resumed.
At around 11000 metres, around the cruising altitude of a passenger plane, Orion opened its first of two sets of parachutes designed to slow it for the final landing. The main chutes opened at around 1800 metres, and at 900 metres they fully opened for a gentle landing.
After splashdown, a system of five airbags on top of the capsule inflated, turning Orion right side up if the capsule landed upside down.
Thank you to mirror.co.uk for that summary of events.
Finally, a reminder to all HAMNET members who are unable to attend the SARL Annual General meeting from the 1st to 3rd of May, to give your Proxy to vote to a HAMNET member near you who is attending. Download the Proxy Form at https://forms.gle/GeWRwJnT1aSzyZPw9, fill it in, and return it as instructed on the form. Thank you.
This is Dave Reece ZS1DFR, wondering if I should install a deployable parachute on the back of my little car to guarantee my ability to stop, and reporting for HAMNET in South Africa.