Whatever happened to VHF?

[Twister_Ken]

Last time I crossed the Channel must have been about 12 years back, on a boat with a 55 foot mast and one of those big, expensive Sailor green box VHFs. We just about lost Solent/Niton VHF tx before we started getting Cherbourg (or was it Barfleur in those days).

This time, on my own rather more modest boat with a Navico VHF that cost less than 250 beer vouchers, we could hear Solent and Portland CG all the way across and were receiving Cros Cherbourg from inside the Needles. At one point in mid channel we were hearing Brixham, Portland, Solent and Dover CG, as well as Jersey Radio, Cros Corsen, Cros Cherbourg and Gris Nez radio, and vessels inbound to the Casquets TSS.

Has the power of the shore stations been pumped up, or has the performance of the latest mini VHF receivers improved a heck of a lot (and on about 20 feet less mast height as well). Or could it have been some atmospheric trick?

 

[ParaHandy]

Sunspot index? It's over 193 (yes, I'm not joking...somebody in the world's getting to see it). This is 23rd cycle and it's high. Current conditions for >50mhz radio traffic are exceptional and I would think this will cover VHF at 100+mhz. Bad news, though, for <12mhz HF and, possibly, Navtex?

I, too, heard TSS VHF traffic a long way away from transmitting station a fortnight ago.

 

[tony_brighton]

It was good a month or so ago - got one half of all those "Solent Coastguard radio check please" conversations whilst in Cherbourg. Must be annoying to the French.

 

[Guest]

My experience of VHF ranges gathered over some 20 years has been that once the barometric pressure exceeds about 1015 mb the range of shore based VHF stations increases remarkably. There have been times when off Plymouth that I have heard the old coast radio stations at Niton as well as Swansea CG, Guernsey Radio and CROSSMA when at other times it has been almost impossible to hear Falmouth CG.
Shore stations are limited to power output much the same as we are, and the sensitivity of modern receivers cannot have improved that much.
Those calls for radio checks by Solent sailors are a bit of a pain to us down here, too!

 

[longjohnsilver]

Strange you should mention it, last Saturday in Lyme Regis I picked up Peter Port radio clearer than either Portland or Brixham CG!

And my mast is on the coachroof of my motorboat and the vhf is almost 20 years old, a trusty Kelvin Hughes make.

 

[ParaHandy]

It's an effect caused by high pressure, humidity, and warm temperatures in the Troposphere (<10km above the earth surface). The signals are scattered (by ionisation - Troposcatter) with some coming back to earth otherwise they would pass straight through. Apparently most evident when pressure is dropping (from high level). Whole subject is fascinating.

 

[Piers]

Ducting....

By far the most common cause of long range VHF (and normally slightly longer range with UHF) frequencies, is a temperature inversion - I suspect this is what caused the phenomonen you experienced.

Typically, a temperature inversion suitable for ducting, is associated with a frontal passage. It often happens over a large, stable high pressure area ahead of a cold front, especially where there is an influx of warm air from the Gulf mixing with colder air from the north.

When the correct conditions are formed, the 'duct' can carry the transmission along its length - like piping sound or water.

The 'duct' tends to be parallel with the front, and given the specific weather conditions, each duct will have its own characteristics, normally favouring specific frequencies in the high VHF and UHF bands.

A 'duct' will increase the transmission range between 50 - 200 miles, yet extreme distances of 1500nm have been recorded with UHF. A duct may appear and vanish in little over an hour, or last for days.

These ducts do not necessarily stay on the surface, and can 'bend' from one location to another, thereby missing out a receiver in the middle!

The problem with ducting is the interference from normally well separated radio stations.

Does this help???

Piers du Pré
MBM Cruising Club enthusiast
www.dupre.co.uk/fsPlaydeau.htm

 

[ccscott49]

Interesting thread this one!

 

[Guest]

Re: Ducting....

I can just about remember studying ducting and VHF ranges in general on my Royal Signals Young Officers Qualifying Course way back in 1958. One example of ducting was a New York taxi driver being heard in UK for a period of some hours. He was operating on about 45 MHz if memory serves me after all these years. Interestingly, nobody from this end could call him. And I needed a taxi from Catterick at the time!
By the way, we used to say that VHF started at 30 Megacycles (long before they started to honour Herr Herz) and became UHF at 300 Megs.
If nothing else, this thread has revived some memories of hard study in cold winters in Yorkshire.

 

[Piers]

Re: Ducting....

I was at Flying College at Hamble in 1969 when I was first taught Radio, and I, too, was cycles trained - hertz came later.

Really interesting story of the NY taxi!

I confess I had to check in my old notes to get the info I published!



Piers du Pré
MBM Cruising Club enthusiast
www.dupre.co.uk/fsPlaydeau.htm

 

[Viking]

I have just brought a new (old) boat here in Norway. The existing radio was removed. So I rushed out and brought a new radio, installed it. The marinas are to small to have radios. So now wondering who the hell Im going to talk to?
Being 'in line of sight' in these fjords doesnt help much either.


"man will only discover oceans, if first he has the courage to loose sight of the shore"

 

[ParaHandy]

Re: Ducting....

There are two ducting effects both of which rely on climatic conditions in the troposphere as previously described. Ducting is one but is at very low level eg <2km and will not pass over obstacles higher than, say, 1km. It is commonest over water and during periods of high pressure. Non-Ducting-Tropo uses the highest part of the troposphere and is best with falling barometric pressure.

Both can be effective.

You can get the predicted tropospheric conditions at
http://www.globalserve.net/~hepburnw/tropo_eur.html
which when compared with 1200z forecast for tomorrow at
http://www.wetterzentrale.de/pics/brack0a.gif
rather superbly (well, I think so anyway) illustrates your point.

 

[Twister_Ken]

Re: Ducting....

Para,

Thx for that. Their archive for last Monday (the day concerned) shows a smallish 'strong' patch just about covering the Cherbourg-Solent area.

Fascinating - but I'm sad to find out that I don't have a super-dooper VHF installation!

 

[Funken]

Propogation modes in VHF

Sorry, but no, this is incorrect.

Troposcatter only occurs at much higher frequencies, and requires a very high transmitter power. It has nothing to do with ionized layers, but rather is caused by scattering of the wave by dust particles in the upper atmosphere.

Frequencies above about 50MHz are not reflected back by the ionosphere - they pass directly through. Hence our ability to communicate with spacecraft, receive satellite TV etc.

The most likely reason for hearig distant stations on VHF is due to ducting. This is caused by a change in the radio refractive index of a llayer within the atmosphere. The change of radio refractive index is normally caused by a temperature inversion. In effect, this creates a waveguide, passing the signal over great distances with relatively little dispersion. The height and physical depth of the layer, together with the frequency of the signal, determine which signals become ducted, and which do not.

For further information on VHF propogation, the Radio Society of Great Britain VHF/UHF Handbook contains good reading. The American Radio Relay League also have a similar publication. You should be able to find both in good public libraries.

 

[ParaHandy]

Re: Propogation modes in VHF

Not wishing to be argumentative...just very interested in the subject..but

Dispute that ionization is not at the bottom of this because an electrical energy source must be required to refract the signal; much as happens with light thro' water. There's plenty of stuff written about refraction mechanism in the ionosphere, none attempts an explanation of what the mechanism is in the troposphere. At least none that I have found that doesn't say "it happens...". Dust particles are one factor only; temperature inversion & humidity are other factors but you agree with that anyway. Have I got tropospheric scatter & refraction confused; the same causal conditions are ascribed to both but thought that troposcatter was the main propagation factor for vhf in the troposphere?

Did you know that until quite recently (certainly my lifetime) data on HF propagation in the troposphere was classified?

There are plenty of people exploring 50mhz and they're not sitting out in space! That has to be refraction in one or other of the layers in the ionosphere - even tropospheric scattering according to some is happening.

 

[chas]

Re: Ducting....

I can remember steaming up the Med and seeing both coasts on an I band radar - also caused by ducting

 

[Piers]

Re: Propogation modes in VHF

A good question and interesting info.

IMHO (and past training), I believe the correct answer is that VHF and UHF obtain their extended groundwave from ducting, being unaffected by the ionosphere. Transmissions simply penetrate this and dissapear into space. Transmission power makes little difference to range.

HF (3 - 30MHz) transmissions get their extended range from skipping off the ionosphere and bouncing back to earth, rather like a ball off a snooker cushion. Transmission power makes a huge difference to range, increasing the groundwave hugely.

Many was the time when navigating across oceans and deserts when we had to use Loran (HF - and early 1970s before Inertial Navigation became de rigeur), having to identify the difference between graoundwave and skywave, and whether it was the E, F or F1 layers (and so on), and whether the first skip 2nd, and so on.

Getting it wrong could place you a significant distance away from your true position!

Are we together on this? Or do we need to chat 'off-line' to clear up any perceived differences (could just be the language, or I could simply be wrong....). I know confusion can always arise from what is called tropo scatter, meaning ducting rather than ionosphere scatter meaning skip and bounce.

Coo - I sound like an old man....maybe I am!

Piers du Pré
MBM Cruising Club enthusiast
www.dupre.co.uk/fsPlaydeau.htm

 

[ParaHandy]

Re: Propogation modes in VHF

We're on the same wavelength (ugh!)!

Very enjoyable thread & good fun.

 

[Joe_Cole]

We frequently hear Peter Port when we are moored up on the Exe.

Mind you last weekend we couldn't hear the night staff at Brixham Marina and they were only 50metres away! We ended up communicating by torch light. I assume that torch light really is only of use on line of sight!

Joe Cole

 

[peterb]

Re: Propogation modes in VHF

My understanding is that the radio signal is slowed down by passing through denser media. When there is a temperature inversion the higher layer is warmer, and allows the wave front to go faster than at low level. The effect is to bend the wave front towards the Earth's surface, thus allowing it to be heard beyond the horizon. (Incidentally, the statement in Relativity Theory that light always has the same velocity, no matter what velocity the observer has, is only true in vacuum.)

My own personal record is both hearing and speaking to Lerwick (Shetland) Coastguard on 16 and 67. They were asking Pentland (Orkney) Coastguard for a radio check, apparently without success. We were able to assure them that they were loud and clear just off Ostende, a distance of some 500 miles. Can anyone beat that?