Mapping the Gaps: A Field Guide to VHF/UHF Coverage in the Australian Landscape

Australia is one of the most geographically extreme countries on earth for radio propagation. Within a single state you can have coastal headlands that throw VHF signals hundreds of kilometres over water, dense inland scrub that eats 70cm like it's nothing, and tropical humidity up north that bends signals in ways that'll make your head spin. For VHF/UHF operators, that variety is both a gift and a constant puzzle.

If you've ever driven out of town and watched your local 2m repeater fade into static faster than expected — or conversely, picked up a 70cm beacon from somewhere inexplicable — you've already started asking the right questions. Let's dig into how to answer them properly.

Why Bother with Coverage Mapping?

Most of us start by assuming coverage is roughly circular: powerful transmitter in the middle, signal gets weaker as you move out, job done. In flat, featureless terrain that's almost true. But almost nowhere in Australia is actually flat and featureless — even the Nullarbor has subtle undulations that influence propagation at lower VHF frequencies.

For emergency communications planners especially, knowing your actual coverage footprint is critical. A gap in coverage you didn't know about becomes a gap in your EMCOMM capability when it matters most. For home station operators, understanding your local terrain helps you position antennas smarter, choose better feed line lengths, and avoid wasting power pushing signal into a hillside.

Getting Started with SPLAT!

SPLAT! is a free, open-source RF signal propagation, loss, and terrain analysis tool that's been around for years and remains one of the best options available to amateur operators on a budget. It uses elevation data — typically SRTM (Shuttle Radar Topography Mission) data, freely downloadable — to calculate line-of-sight and Longley-Rice propagation models across real terrain.

Setting it up takes a bit of patience. You'll need to grab the relevant SRTM tiles for your region (the USGS EarthExplorer site is your friend here), convert them to the format SPLAT! expects, and define your transmitter site parameters. The learning curve isn't trivial, but there are good community guides online and the VK ham community has produced some excellent walkthroughs for Australian-specific data sets.

Once you're running, you can generate coverage plots that overlay your predicted signal footprint onto real topographic maps. The difference between what you assumed and what SPLAT! shows you is often genuinely surprising — especially in hilly coastal regions like the NSW Illawarra, the Dandenong Ranges east of Melbourne, or the ranges behind the Sunshine Coast.

For those who'd rather not wrestle with command-line tools, Radio Mobile Online (a web-based alternative) offers similar Longley-Rice modelling through a browser interface. It's less powerful but far more accessible for a first exploration.

Coastal Environments: Ducting and the Sea Path Advantage

If you're operating near the Australian coast — and a huge proportion of the VK population is — tropospheric ducting is your occasional superpower and your constant wild card. During summer and autumn in particular, temperature inversions over the ocean create ducting conditions that can push 2m and 70cm signals thousands of kilometres up and down the eastern seaboard.

The catch is that ducting is unpredictable and not something you can engineer for in a fixed coverage plan. What you can do is position your antenna to maximise the sea-path advantage during normal conditions. A coastal hilltop site with a clear water horizon to the east will consistently outperform an inland site at the same height. The ocean is essentially a perfectly flat, low-loss propagation surface.

Case in point: operators in the Sydney Northern Beaches area regularly work stations in the Illawarra — well over 100km — on 2m FM simplex during summer evenings. That's not magic, it's a combination of elevation, clear coastal path, and opportunistic troposcatter. Understanding your local coastal geometry helps you predict when these windows will open.

Inland and Arid Regions: The Terrain Shadow Problem

Head west of the Great Dividing Range and the picture changes dramatically. The terrain is generally lower and less dramatic, which sounds like it should make propagation easier — and in some ways it does, since there are fewer hard shadows. But the sheer distances involved, combined with sparse repeater infrastructure, mean that coverage gaps are vast and planning becomes even more important.

In inland Queensland, NSW, and WA, local knowledge is irreplaceable. Talk to operators who've been working the region for years. They'll know which creek valleys cut off 70cm completely, which flat-topped hills give surprising coverage to the east, and which times of year the ionosphere-adjacent effects on upper VHF start bending things around.

For site surveys in these areas, a portable 2m/70cm station and a GPS-logging receiver make for an excellent coverage test kit. Drive the roads, log signal strength against position, and compare what you find to what your prediction tool suggested. The discrepancies teach you more about your local RF environment than any textbook will.

Tropical Australia: Humidity, Vegetation, and the Wet Season Factor

Operating in the Top End or Far North Queensland introduces a whole new set of variables. High humidity increases atmospheric absorption at the upper end of the UHF spectrum. Dense tropical vegetation — particularly during and after the wet season — adds significant diffraction loss that doesn't show up in bare terrain models.

Operators around Darwin and Cairns often find that their effective VHF/UHF range contracts noticeably during the wet season compared to the dry. Part of this is atmospheric, part is the sheer biomass of wet-season vegetation. If you're modelling coverage for a tropical site, add a conservative extra margin of loss to account for these effects — your prediction tools won't include them automatically.

The flip side is that elevated sites in the tropical north can have extraordinary sea-path performance given the proximity to the Coral Sea and Timor Sea. Some remarkable 2m contacts have been logged between VK4 and VK8 stations from good coastal hilltop locations.

Practical Tips for Your Own Survey

• Start with SRTM data at 1 arc-second resolution (about 30m grid spacing) for the most accurate terrain modelling in your area.
• Do a physical drive-test before committing to antenna infrastructure. Software predictions are a starting point, not a guarantee.
• Log everything: GPS coordinates, signal strength readings, antenna height, and weather conditions. That data becomes invaluable when you revisit the site later.
• Talk to your local WICEN group or AREC team — they've often done exactly this kind of coverage assessment for emergency planning and may already have maps you can build on.
• Check the ACMA Radcom database for existing repeater and beacon registrations in your area. Understanding what's already there helps you identify genuine gaps versus areas that are already served.

The Bigger Picture

Coverage mapping isn't just a technical exercise — it's a contribution to the community. When you identify a genuine dead zone and document it properly, you're giving your local amateur radio club, your EMCOMM group, and future infrastructure planners something genuinely useful. In a country as large and as under-served by commercial communications infrastructure as Australia, that matters.

The tools are free, the data is freely available, and the knowledge you build is yours to keep. Get out there and map your patch.