Something in the weather conditions today must be just right for great VHF propagation. I'm sitting aboard Shakti on my mooring at Sandringham with OpenCPN running, watching AIS plots of ships as far away as south of Cape Otway. That's 99 nm! The previous best was about 23 mile in calm conditions, and usually less with any rain, fog or wave action.

I've verified them on Marine Traffic - correct MMSI, speed course etc.

Cheers, Graeme

I have read of VHF voice transmissions being received over freakishly long distances on rare occasions. In calm weather I get about 15 miles from the pushpit antenna (about 4m above SL) and about 23 miles form the masthead (12m).

Any telecomm's nerds on SB who can fill us in on the gory tech details?

I had to leave for home and gave up at 100Nm, will post a screen photo later to dispel doubts as to my sanity.

The upper air sounding for Melbourne Airport at 00Z on the 1st shows a 1.3 degC inversion between 119 and 663 m (see THTA column, acknowledgement to the University of Wyoming).
The vertical (and temporal) resolution is not great in the publicly available data so it's not possible to be conclusive but it looks like the inversion/isothermal layer extended to about 1500 m.

The change came through Melbourne Airport about 1400 on Saturday. Cooler air from a change (or a sea breeze) can often undercut warm air giving a strong but short-lived inversion. What time did you notice the extended AIS range?

K.

The change went through South Channel Island at 1200 and Frankston Beach at 1300. Based on the timing of your obs, I'd say the change initially came through as a shallow layer of cool air (higher density) that undercut the warm northerlies (lower density) and advanced from the SW, basically following the coast up from Cape Otway. This would create a shallow layer only a few hundred metres deep capped by a strong temperature inversion. The refractive index of the 2 layers would be different, leading to some refraction of RF at that boundary, hence the ducting.

We saw similar things on the east coast of the South Island of New Zealand during an experiment looking at southerly changes in the '80s. Sometimes the shallow layer would completely detach from the main front and blast up the coast as an isolated pocket with the wind backing to north-westerly again before the main front came through. If the wind direction and topography are at 90 deg, more or less, you can get an area of low pressure in the lee of the topography and this tends to suck the low level change through faster. There's a BoM model product at 1.5 km resolution (ACCESS-C), it would be interesting to see if that resolves this kind of local flow.

Congratulations on a good bit of observational science, wongaga!

K.

Off-topic I know but;
Has anyone noticed the ferocious wind data anomolies for readings from South Channel Island aka "the Fort" recently.
Yesterday I was becalmed just before the change within 1 NM of the fort, while readings before and after showed 14 - 19kts.

hmm, might need to consider the 'Jetstar effect' then from the very local airport next time we go for a cruise.
Interesting info, you blokes may put Jane Bunn out of a job.. :)

The proof of any hypothesis is to make a prediction and then carry out an experiment to see if the prediction is correct. So, if the VHF ducting was caused by a low level temperature inversion associated with an approaching cold front, it should be possible to test the hypothesis by looking for anomalous propagation next time a change is approaching. The actual conditions might be quite specific though, so it's possible not every approaching front will produce the exact conditions needed.

Just need someone to park up in Apollo Bay with a VHF for several weeks and wait for a few fronts to go through and see if they can be heard in Sandringham ... :=)

K.