Ridiculous range on VHF?

[poonpossum]

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 :)

[Arm out the window]

Sure it's not just a retrans?

[poonpossum]

Yep. 126.7 and mostly coming from aircraft on the base leg of the circuit.

[an3_bolt]

Atmospheric ducting associated with inversion?

[Capt Fathom]

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?

[VH-XXX]

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?

[Hailstop3]

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 (http://en.wikipedia.org/wiki/Sporadic_E_propagation)

[megle2]

Used to happen quite often with common frequencies miles and miles apart usually early morning or late afternoon, just needed the right atmospheric conditions

[poonpossum]

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.

[le Pingouin]

The best I've had as a controller was picking up Townsville approach via Mt Macedon.

[thunderbird five]

Swamp gas from a weather balloon was trapped in a thermal pocket and reflected the light from Venus.:ok:

[Ollie Onion]

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.

[YPJT]

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:

[LeadSled]

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!!

[FJ44]

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.

[Tarq57]

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.

[tail wheel]

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.

[AerocatS2A]

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 (http://en.wikipedia.org/wiki/Radio_propagation#Tropospheric_ducting)

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.

And more again here. (http://en.wikipedia.org/wiki/Tropospheric_ducting#Tropospheric_ducting)

[HEATHROW DIRECTOR]

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.

[Capt Chambo]

What an3_bolt said..

Research "Duct Propagation"

[defizr]

I used to be a radio officer in the merchant navy and we often picked up Cape Town/ZSC on ch16/156.8MHz 600 miles up the west coast of Africa. It's ducting as an3 says.

Edited to read "west" rather than "east". Good job I wasn't responsible for navigation :eek:

[FokkerInYour12]

Why we can't we have more discrete frequencies?

Having a scratchy transmission from 100NM away isn't enhancing my situational awareness at all.

Surely there's enough frequencies at .25Mhz intervals to do this?

[Hasherucf]

tropospheric ducting. It's covered in the radio basics casa exams. Exam code WA

[cowl flaps]

Most times the tropospheric ducting occurs during the peak of the 11 year sun spot cycle.

[WAC]

"Skip" is the common term for these bounced transmissions. The weird thing about them is that as they are due to a "bounce" off the ionosphere, stations closer to the originating point won't be able to pick them up as they are under the bounce.
Conditions more conducive to skip happen at night and I spent many a night parked atop Mount Sugarloaf talking to some places at huge distances, even sometimes o/s on a good old single side band CB radio.

Apparently the conditions can be predicted (related to solar activity) and the amateur radio enthusiasts use it to advantage to set personal records for long distance coms.

[poteroo]

In the 60's it wasn't unusual to hear Cairns TWR on 118.1 while operating out of Port Moresby. Bit of a wake-up to hear someone cleared for 15/33 instead of the 14/32 we were used to. Seemed to be more likely if you were down a bit lower, eg, 1500 ft and coastal E or W of Moresby.
happy days,

[Hobo]

What AN3 and tarq said. Google 'VHF ducting'

[Desert Flower]

If you're near water, the water will act as a ground plane. Not sure if it would work with VHF though, but it sure as hell used to work back in the days when I used to work skip on 27 megs!

DF.

[topdrop]

Look up sporadic E.
You can also get VHF reflections caused by a boundary between 2 markedly different air densities.
In FS, I can remember getting Moresby Tower on Townsville VHF and a fishspotter landed Echucha on the Port Lincoln VHF.

[parabellum]

When I lived in Abu Dhabi, at sea level, during the hot months we could listen to Dhahran (ARAMCO) broadcasting all day on VHF, Classic, Popular, Country and Western as well as the news, read by Larry Barnes, taken from the wires of UPI! That was well over 300 miles. Ducting, as mentioned already.

[Dexta]

I remember one of the Ham radio guys nearby used to do a "moon bounce", pump enough energy (around 1000w) with a good directional antenna at the moon and you could get Japan and sometimes the states. Can't remember the wavelength, definitely 10m or shorter though so VHF.

[AerocatS2A]

Bouncing off the ionosphere is more of an HF thing. It is also HF that is affected by sun spots. VHF ducting is from low level inversion layers.

[halas]

In the old days of TV sets only having 13 channels (Channel 5A..WTF?) used to get Adelaide TV stations from 300Km down the coast on a calm summer morning.

Also cancelling SAR with Darwin FIS after landing at Troughton Is. on VHF a few times too (550Km)

halas

[triadic]

In the good old days of FS, it was not uncommon for Derby to talk to Broome and Hedland on 122.1 - (almost the whole of WA was on 122.1 - make that most of FS Aust wide) comms were good quality but did not last for long - usually early morning or thereabouts.
Also one of the towers in Indonesia would often come thru on 118.3 at Darwin Tower.

[startingout]

Been a while but I do remember that Adelaide approach used to pick up Hamilton Victoria's CTAF at times. 124.8 rings a bell.

[601]

Cancelled SAR with CV on VHF while in the Windorah circuit back in the 70s. So it is not new.

[ZAZ]

Its a result of tropospheric ducting, the signal normally would go through into outer space but is bent over or ducted down
examples are 124.2 innbound western sector into YPAD heard on 124.2 at YHML

The distance VHF can travel is huge hundreds and thousands of KM
Like from CALIFORNIA to HAWAII
MT GAMBIER to ESPERENCE

The link below will show VHF and UHF records made by ham operators not (VK2DIK) and as you can see a few hundred KM is no problem.

http://www.wia.org.au/members/records/data/documents/VHF%20Records%20080801.pdf

[flyingins]

Westbound to Perth at FL360 this evening, just north and west of Esperance, ML CTR issues a change to 118.2. Upon first contact ADL Approach instructs me to check my frequency. 880NM away!

Once the mystery of why I was negotiating with two separate controllers was solved, I could still hear ADL Approach transmitting up to 1000NM away.

Read the books but never heard of it happening before. Kinda cool.

[greybeard]

Koolan Island to Karratha, unable to get Derby early morning.

Carnarvon to Meekatharra from the circuit in Carnarvon.

HF, Halls Creek to Isa as the only station in Aust. who could hear me.

All in the 70s when the world was a better place.

:ok:

[Roj approved]

So would this be the same reason you can get YBCG ATIS overhead YWLM at about FL300? (It's only for a very short time, 3mins max).

Can't get YBBN or YSSY coming south, but it's a reasonable signal for that distance. 290nm

[Brian Abraham]

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.While training with the Army at Pucka we were told of them receiving PRC-25 transmits from Vietnam during the hostilities. The PRC-25 operated in the high HF/low VHF frequency band using frequency modulation (FM), it had a maximum range, using full power and its standard antenna, of approximately 5mi/8km, depending on terrain conditions. Nominal transmit power was 1.5 to 2 watts but it commonly produced four.

Personally heard aircraft making taxi calls at Sale while seated in an office at Essendon.

Military aircraft use the ducting phenomena to avoid radar detection at times. Similar to subs avoiding sonar detection. A Navy ship nav reported radar painting New Zealand during exercises off the coast of Jervis Bay.

Radar Basics (http://www.radartutorial.eu/index.en.html) in particular page Radar Basics - Anomalous Propagation of Electromagnetic Waves (http://www.radartutorial.eu/07.waves/wa17.en.html)

[ampclamp]

As an amateur radio operator I have experienced ranges of several hundreds of kilometres on the 2 metre band.
Most likely tropospheric ducting or less likely some sporadic E layer skip.

[Ozdork]

Back in last century (early 80's) I was a Shrike Coastwatch driver - I recall having a perfectly clear conversation with another CW Shrike - I was at 1000' in the circuit at Gove, while he was in the circuit over the other side of the Gulf at Weipa.

[YBMK Tower]

Heard Flydoc 30nm south east of YCKN whilst 10 west of YBMK at 1000agl on 126.7 this morning. Around 430nm between us.

[avconnection]

PAD app on Mel centre from ABA at 4000'. Just over 1000nm at the time. The irony of course was trying to cancel SAR on HF with no joy.

[Ex FSO GRIFFO]

Ah well, just to add one more....

Working away in KG FSU a couple of 'late afternoons' when aircraft 'EXN' or something called at 30 miles inbound.....

Now, I had nothing on any 'EX?"s at all, but I did know that in those days, the aircraft whose regos commenced with 'EX?" were usually 'Aero Commanders'....and while I was checking with Flight Data, MILDURA FSU answered, and gave the 'usual responses'....

So, I simply called Mildura on the 'common' VHF, 122.1, & and we agreed to ignore any callsigns that we knew were not 'ours'.

This situation lasted for about an hour or so, and then simply 'faded away'...

Happened a few times, but right now I cannot remember the time of the year.
'Amusing' and interesting at the time....
Cheers :ok:

Similarly, on more then a dozen occasions it would appear that KG FSU was the only unit able to hear aircraft at Moruya / Merimbula (?), S. NSW, apparently, on 8mhz HF calling SY for cancellation of SAR....5 x 5......at around 'evening time' KG.

Who do you call now...? 'Mum' on ya I-phone....

[MotCap]

As a young boy I spent many days riding in the car with my dad. He worked for the City of Chicago, Department of Electricity. His auto was equipped with a Motorola FM radio for dispatch to outages of intersection (stop & go) lights. It was fairly common to overhear broadcasts from another station in California, roughly 1,800 NM distant.

[1DC]

I was in the Merchant Navy in the sixties,sailing on oil tankers. When we entered the Persian Gulf bound for Ras Tannura (Saudi Arabia) for a period of about 30 minutes we could regularly contact Ras T. on VHF,a distance of about 300 miles. The contact was loud and clear but only for around the thirty minute period, a steaming distance of about 8 nautical miles. We then lost contact until the following day when we were about 25 miles away. The Sparks said it was the bounce of the radio waves.