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Ridiculous range on VHF?
Hi all, quick question, as I cannot seem to find the answer for myself..
During flying school one was taught that VHF signals operate on a line of sight basis, and cannot travel through the earth or bounce on any ionosphere, and that that is what HF is for. Only once in about 2 months or so, one can hear VHF transmissions at around 1000 ft, from aircraft broadcasting in their circuit, at an airport 480 nm south of my location, which is also at sea level. The days are usually quite still and a little hazy, and I'm wondering what effect allows a VHF signal to travel much further that what it's conventional line of sight range would be, or if anyone has any idea as to whats going on :) |
Sure it's not just a retrans?
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Yep. 126.7 and mostly coming from aircraft on the base leg of the circuit.
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Atmospheric ducting associated with inversion?
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I have seen (heard) this as well, but not for a long time!
In QLD, I have heard Dubbo Flight Service and Port Morseby. I cannot recall the details though. In the 70's, a lot of places had the same frequencies. eg., most Towers were 118.1, and the equivalent Control freq was 118.7, so it wasn't unusual to hear the adjacent TWR on rare occasions! Got no idea of the physics behind it? |
I've never heard anything like that sort of distance at that altitude.
On the face of it, I'd be inclined to think that you either heard someone using the wrong airport name (which happens a lot), or you misheard the airport name. From where-to-where did you hear these transmissions? |
I've also come across this with a marine VHF (ie in a boat on the water) in Brisbane, and receiving a transmission from Sydney. Long time ago and haven't heard it since, but certainly possible.
Edit: Found an explanation Sporadic E propagation - Wikipedia, the free encyclopedia |
Used to happen quite often with common frequencies miles and miles apart usually early morning or late afternoon, just needed the right atmospheric conditions
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Bunbury to Shark Bay. Nah, it happens a lot, and its very clearly Bunbury. You can hear Jurien perfectly at low level on the same days as well.
I thought too that someone could have been confused, so I checked the callsign on the register and the runway they were calling, and it all lined up. |
The best I've had as a controller was picking up Townsville approach via Mt Macedon.
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Swamp gas from a weather balloon was trapped in a thermal pocket and reflected the light from Venus.:ok:
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Seem to remember from my ATPL days that VHF range can be calculated by:
Range = 1.25 √height (ft) of tx + √height (ft) of rx Resulting answer is in n.m. So if you were 1000 ft and the other aircraft was also in the circuit at say 500 ft then the theoretical range is approx 60 nm. So in your example it would be unusual for a vhf transmission to go that far, must be an atmospheric anomaly or it must be getting relayed somewhere. |
Not VHF AM but military VHF FM around 50 MHz once picked up a signal from Puckapunyal in Victoria from Lancelin in WA. Low powered manpack radios.
The same explanation was given to us as what AN3_bolt stated. :ok: |
Folks,
AN3 Bolt is on the money. Long before all the repeaters and re-transmits, we regularly used to hear Port Moresby tower on the ground in Cairns. In the days before satellite comms, there was a trans Atlantic US military VHF link --- pump out enough power and some sort of a signal will get through. Tootle pip!! |
Have had it two or three times.
Once was on descent into Bathurst Island north of Darwin, I could hear traffic in the Horn Island CTAF as clear as day. Another was at 1500' on downwind at Lord Howe Island and hearing traffic back on the mainland, There was one other but I can't remember where it was other than that the location in the CTAF call was along way away and I was very low, (for VHF reception) Might have been Albury Tower and Armidale CTAF.... Has always been in the morning, probably the first hour or two after sunrise. |
What AN3 Bolt said.
In Wellington, the tower frequency used to be 118.7. It happened occasionally that aircraft in the Auckland area would break through, on the right day. (IIRC big anticyclone, very stable air, over the North Island.) Wellies was changed to 118.8 as a result. |
I recall many years ago 6 meter (50 MHZ) transmissions between Hawaii and Brisbane, with Yagi (directional) antennas and reasonable power.
A relatively rare atmospheric anomaly. |
Concur with an3_bolt.
A strong inversion layer can cause EM energy (including radio/radar/etc) to "bounce" between the layer and the ground multiple times causing unusually long ranges. You will find that it is unreliable as there are gaps between the bounces. You may hear the broadcast at 480 NM and 400 NM but not at 440 NM for example. Can be frustrating for those who spend a bit of time at low level over the ocean. You hear VHF loud and clear and think you might not have to struggle with HF for the next position report but the VHF coverage is gone as quick as it came. More here Sudden changes in the atmosphere's vertical moisture content and temperature profiles can on random occasions make microwave and UHF & VHF signals propagate hundreds of kilometers up to about 2,000 kilometers (1,300 mi)—and for ducting mode even farther—beyond the normal radio-horizon. The inversion layer is mostly observed over high pressure regions, but there are several tropospheric weather conditions which create these randomly occurring propagation modes. Inversion layer's altitude for non-ducting is typically found between 100 meters (300 ft) to about 1 kilometer (3,000 ft) and for ducting about 500 meters to 3 kilometers (1,600 to 10,000 ft), and the duration of the events are typically from several hours up to several days. Higher frequencies experience the most dramatic increase of signal strengths, while on low-VHF and HF the effect is negligible. |
I used to work in North Africa and our parent ATCC was in Malta. We used HF to talk to them but if HF was bad and under certain atmospheric conditions we could talk to them on VHF ground to ground. Radio Amateurs achieve great distances on VHF ground to ground by being fully conversant with prevailing weather conditions.
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What an3_bolt said..
Research "Duct Propagation" |
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