Computers and surge protectors

[Richard]

Radio station fading.
Is this happening on FM or shortwave? What type of radio?
If it's happening with more than one station then suspect the radio, as stations usually don't drift in frequency. If it's happening on shortwave, or at night on standard ("AM") broadcast though, it may be due to fading, an artifact of ionospheric propagation.

[Richard]

The ionosphere is important for radio wave (AM only) propagation....
ionosphere is composed of the D, E, and F layers, the D layer absorbs AM radio waves but this D layer dissapears at night.
The E and F layers will then bounce the waves back to the earth, this explains why radio stations adjust their power output at sunset and sunrise.

Old fashioned AM (amplitude modulation) radio can carry up to 500 miles by 'bouncing' off the ionosphere at night.

[Richard]

sorry, forgot to add - if a station fades on AM (owing to ionosphere fluctuation) then if you can find the same station on FM it should be fine as FM (and VHF) use 'line of sight communication between visible transmitters.

Believe it or not radio waves can even be sent using reflection (bouncing) off ionisation columns of air from meteor trails, to cover large distances.
Otherwise satellites (expensive) would be ideal.

[moonjiver]

This is all getting rather technical for me, and complex .. as a Woody Allen film character once remarked, "Why ask me who God is? I don't even know how the can-opener works ..".

All I know is that often on FM the radio goes sharply and noisily off-tune, as if someone has physically moved or knocked the tuning knob - by a good half-inch or more, often to a completely different station! Leaving aside possible poltergeists, I have always assumed it was atmospheric conditions. Fading out does happen too of course (more often on foreign stations) but that's usually on AM where presumably the signals are weaker and have to travel longer distances?

[moonjiver]

I should add that this happens on different radios, some battery, some on mains.

Talking of spooks etc. .. I did actually know a young woman who's TV used to do just the same thing, jump to a different channel. She claimed that her little girl was psychic and was "making" the channels change over if the programme was a bit boring! All baloney of course .. except that it suddenly happened twice in front of me (no remote control around), so I was slightly un-nerved ..

[Richard]

moonjiver,
I have not experienced, and am not sure why you experience, some stations jumping sharply off when using FM.
The case for AM 'fading' is clearer, as outlined, owing to the way in which it is transmitted.

[moonjiver]

Thanks for your explanations, rather fascinating. I often listen to a particular French station on AM and also a Dutch one. Because there's not much dialogue (not comprehensible anyway!) and because the music is far better than on UK pop/light music stations.

But they are mainly easier to find in the evening and onwards. Does that mean they are not broadcasting in daytime, or can the signals "bounce" and travel further after dark?

[Richard]

my best understanding now is as follows:

Under normal conditions lower frequencies bounce and higher frequencies don't, (AM is low and FM/VHF is high).
This happens regardless of the modulation method used. The point (frequency) where this occurs depends on atmospheric conditions. Generally ''bounce' is better at night when the reflective layers of the atmosphere are stable because the sun has set and they are not "stirred up" by solar radiation.

AM broadcast radio stations generate two kinds of radio waves: Groundwave (line of sight, hugging the surface of the earth), and Skywave (by bouncing off the ionosphere at night, enabling very large distances to be covered, even beyond the horizon or obstacles such as hills/mountains).

So, either your favourite Dutch station becomes alive to you at night because the ionosphere is permitting 'skywave' to bounce longer distances than line of sight during the day.
or
The sun has set anyway and automatically removed a large source of radio frequency radiation at the lower end, which would produce noise and obscure some transmissions over longer distances.
Witness the effect of a solar flare knocking out satellites, mobiles. TV's and various electrical equipment.

Take your pick

[moonjiver]

Nobody has ever explained all this to me so clearly, you are a good teacher! I'll read again when I'm fresher ..

I was often puzzled why sometimes in the daytime I could get my foreign stations (but often not), but in the evenings they were loud and clear. Now it makes some sort of sense.

Nor has anyone explained to me the great advantages of digital radio - not enough to persuade me to spend a fair bit of money and on spec. The analogue system works perfectly well and in fact I have read several complaints that digital reception is worse in some parts of the UK. Others seem to share my warinesss - apparently the take-up of digital radio sets is still very slow.

[Richard]

Hi moonjiver,
the 'explanation' was built up going along, like "O'Rafferty's" motor car, so it may be a bit flaky.

As for digital, I tried once, in the early days, with a 'Robertsons' and didn't like it.
The presets were fiddly to program and easily lost their values.
DAB battery drainage is much higher and you need rechargeable's and most 'high-end' radios use proprietary packs which cost a lot to replace, so look out for ones that take standard/rechargeable batteries (as well as 'mains').
Amazon shows some good ratings, so things may have improved since then.

There are bandwidth issues which means that as you add stations all the others have to fit within a single fixed data-rate bandwidth and consequently get squeezed and so the bit-rate falls for each station, with loss of quality.

DAB signals are also weaker and some buildings have thick walls they can't penetrate or iron-rich cement that reflects the waves.
Just like the disruption to Free-view TV signals, digital broadcasts do seem to be adversely impacted by extreme weather / atmospheric conditions.