Image: Mossel Bay Area (Click on image for larger view.)
The ZS1I DMR Repeater in Heiderand, Mossel Bay has been running from time to time since June 2023. It is permanently on the air from the 1 May 2026 after several hardware and software modifications were done for optimum functioning. Several radio amateurs have provided reports and positive comments with regard to the repeater. It is quite strange that I never plotted the coverage area using Radio Mobile since June 2023. I have now plotted the expected coverage area of the repeater.
Before I publish the images it is important to first publish the repeater- , equipment- , feedline- and antenna information.
ZS1I Digital Mobile Radio (DMR) Repeater
DMR Repeater Talkgroup 65522: This repeater is NOT located on a remote mountain site but is situated in the Shack of ZS1I in Heiderand, Mossel Bay. This allows ZS1I to monitor and control the repeater while it is on the air.
Mossel Bay DMR Repeater Information:
Mode: DMR
Band: 70cm
TX Frequency: 438.262500 Mhz
RX Frequency: 430.662500 Mhz
Radio Mode: Duplex
Talk Group (TG): 65522
Colour Code: 1
Time Slot: 1 or 2
RF Power Output: 15 Watt
Logarithmic power level: 41.76 dBm
Antenna EIRP: 46.96 dBm
Antenna: Diamond X50
Antenna Gain: 7.2 dBi
Antenna Height: 12 Meters
Coax Cable: RG213 Mil-Spec (West Germany)
This repeater is linked to the ZS1I AllStar Hub Network (Node 49355) (Analog Repeaters / Simplex Link Radio / Echolink / SVXLink / AllStar / South Cape Reflector) via the ZS1I DMR Bridge and Repeater.
With your system operating at 440 MHz (70cm UHF band) with an EIRP of 46.96 dBm from an antenna height of 12 metres at sea level in Mossel Bay, your real-world coverage will be highly asymmetrical.
Because UHF signals rely almost entirely on line-of-sight propagation and are easily blocked by solid earth, your coverage splits into two completely different zones: vast open coverage over the ocean, and a sharp cutoff to the north caused by the Outeniqua Mountains.
Here is how your 46.96 dBm EIRP system will perform under these specific local conditions:
Line-of-Sight Horizon Limit
The theoretical radio horizon for an antenna 12 metres above sea level is calculated using the standard RF horizon formula:
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RADIO HORIZON CALCULATION
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Formula:
d = √(17 × h)
Where:
d = Distance to the radio horizon (in kilometres)
h = Antenna height above the ground/sea level (in metres)
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Your Setup Calculation (12-Metre Antenna Height):
d = √(17 × 12)
d = √(204)
d ≈ 14.28 km
Result:
The theoretical radio horizon for your repeater antenna is 14.28 kilometres.
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To a Handheld Radio (Ground Level): If communicating with a person holding a radio at ground level (approx. 1.5 metres high), their radio horizon is about 5 km. Adding your horizons together means you will have clean, high-clarity Line-of-Sight coverage up to 19–20 km away over the flat ocean surface or open coastal flats towards Hartenbos and Klein Brak River.
2. Terrain Obstacles: The Outeniqua Mountains
To the north of Mossel Bay, the Outeniqua Mountains rise sharply to heights between 800 and over 1,500 metres (such as the Robinson Pass area).
The Shadow Effect: At 440 MHz, radio waves behave much like light beams. When your signal hits the massive sandstone slopes of the Outeniquas, the mountains will cast a massive "radio shadow" directly behind them.
The Cutoff: Your signal will cleanly illuminate the southern, seaward-facing slopes of the mountains. However, coverage will completely drop off on the northern side of the ridge. You will not be able to reach deeper inland areas like the Little Karoo (Oudtshoorn region) unless you bounce a signal off a mountain-top repeater.
3. Signal Penetration in Town (Urban Factor)
Because your antenna is mounted at 12 metres, it is likely sitting just above or level with standard two-story residential rooftops in Mossel Bay.
Structural Losses: 440 MHz UHF is excellent at bouncing between buildings and penetrating walls.
Local Range: You can expect highly reliable, punchy coverage throughout the immediate town, even over the hilly terrain of the Cape St. Blaize peninsula. The 46.96 dBm (approx. 50 W) of effective directional power is more than enough to overcome urban attenuation within a 10 to 15 km radius through town structures.
✅ Summary of Estimated Range
Over Ocean / Flat Coastline: 20–35 km (Excellent clarity to marine traffic or coastal stations with elevated antennas).
Urban Mossel Bay: 10–15 km (Robust signal piercing through local neighborhood obstacles).
To the North (Mountains): Up to the ridge line (Signal stops abruptly at the mountain peaks; no coverage in valleys behind them).
Images: Courtesy Radio Mobile (Click on images for larger view.)
Above image: Mossel Bay wide coverage area
Above image: Mossel Bay close-up image 2
Above image: Repeater coverage Albertinia Town. Bad coverage!!
Above image: Repeater coverage George Area. Good coverage!!
Above image: Repeater coverage Mossel Bay and Hartenbos Areas. Good coverage!!
Above image: Repeater coverage West of Mossel Bay / Gouritz River Areas. Spotted coverage!!
Above image: Repeater Coverage - Still Bay, Heidelberg, Riversdale, Albertinia and Herbertsdale. Bad coverage!!
