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
Select to expand quotewongaga said..
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
Wow.
I'm seriously impressed Wongaga, that nuts!
I was happy when I got 25-30nm!
Mind you, I was never a fan of my AIS range, it used an active diplexer with a single antenna and I always wanted to run a second antenna just to see how much improvement there was to be gained.
I noticed every time there were big seas running my range would drop to less than 12nm, as did the VHF performance.
Part of it was a stubby mast for the size boat, but that was still some 19m from waterline so it should have been better than that. The active diplexer was simply a powered amplifier and a switch that would allow transmission from either the VHF or AIS, you could see the receive and transmit powers on all three legs and the numbers always looked fine. But the real results never quite matched the on paper performance.
99miles.....sheesh.
Cheers!
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.
Tropospheric ducting of RF signals, including VHF, can give extreme ranges but is usually very locallised, see en.wikipedia.org/wiki/Tropospheric_propagation. Perhaps related to the hot conditions on Saturday (warm air advection) due to Northerly winds. Would be useful to look at the radiosonde data for the day to see if there was a temperature inversion (potential temperature increasing with height) at some level. Ducting can happen between the surface and level of a temperature inversion. Signals reflecting off both can propagate beyond line of sight.
Cheers,
Kinora
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.
I first noticed it around 1.30 pm and clocked the 100Nm at about 4.30. It was certainly directional as you said, with nothing from the SE, just SW down to Cape Otway.
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.
Thanks for your explanation Kinora. I was on board Shakti at Sandringham, as indicated by the red circle. The target is the green triangle in the dashed box, SSW of Cape Otway 100Nm away.
The display is an Asus eeePC, which receives AIS via a TwinYakker wifi from a dAISy dual channel AIS receiver, fed from the masthead antenna 12m above sea level.
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.
Best I can offer on this is that I was talking to a nearby boat [on VHF] at Seal Rocks in NSW, north of Port Stephens
when another boat broke in asking where we were.
After telling him "on the east side of Seal Rocks" he said I'm there and I can't see you.
That's when the penny dropped and I asked him, "are you in Victoria" He said yes.
My reply was as we were at Seal Rocks in NSW it would be a bit hard to see us.
These long range broadcast used to be a 'why is it so" in HAM circles until some bright spark proved that the signals were
actually travelling down the disturbances created by jet aircraft between Melbourne and Sydney.
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.. :)
Select to expand quotenswsailor said..
Best I can offer on this is that I was talking to a nearby boat [on VHF] at Seal Rocks in NSW, north of Port Stephens
when another boat broke in asking where we were.
After telling him "on the east side of Seal Rocks" he said I'm there and I can't see you.
That's when the penny dropped and I asked him, "are you in Victoria" He said yes.
My reply was as we were at Seal Rocks in NSW it would be a bit hard to see us.
These long range broadcast used to be a 'why is it so" in HAM circles until some bright spark proved that the signals were
actually travelling down the disturbances created by jet aircraft between Melbourne and Sydney.
Were the HAM's talking about HF, which is their customary haunt? HF signals behaves very differently to VHF.
Given the vast amount of jet aircraft travel (at least pre-Covid) I'you'd think this would have been a very widely observed and confirmed phenomenon.
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.
In 2009 on my first Lord Howe attempt we turned back about 150nm out. At about the 100nm mark from Port Stephens we were receiving weather warnings on VHF 16 from Marine Rescue in Port Stephens. It was a few hours before a front (pretty savage one too) came through so I put it down to atmospherics.
In my Navy days we used HF a lot and it wasn't uncommon to get ducting or ionosphere bouncing and incredible ranges. I recall one when we were off the NSW south coast trying to contact a local station on HF and we got a reply from Clark Air Force base in the Philippines. There were others even longer range but the memory has faded on the details. Ramona might have a better memory for details.
