h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

When you drive through the Blackwall tunnel reception on the car radio disappears, of course. Except that whenever you get to a corner in the tunnel, radio reception comes back for a moment. Why is this?

(I'm sure it happens in other tunnels, it's just that I happen to notice it when going through the Blackwall tunnel.)

Hi Douglas,
Usually, radio reception disappears when you are surrounded by reinforced concrete (Faraday's cage effect), so perhaps the cage has gaps in it when the tunnel turns a corner. A wild guess.

Radio waves can be reflected by walls of buildings and (presumably) tunnels. In a tunnel, there's probably a massive number of interfering reflected waves which usually cancel each other out. At a corner the waves are probably reinforcing themselves, so you get clearer reception.

That would be my guess, anyway.

Before entering the tunnel - if I was on my daily commute - I would pop a favourite tape into the car-radio-cassette to avoid such a suprise - and simultaneously avoid the irritation caused by listening to "Mourning Radio" (as it is called over here in N. America).

Regretably, I would be blissfully ignorant of such observations that may cause, with a streak of genius, realisation of something profound - like the solution to the generalised unified field theorem problem - But at least my attention would be uninterestingly absorbed watching the traffic while I negotiate the flow of Volvo 740s, refurbished Ford Prefects and crazed, tranquilised van drivers sharing the thoroughfare with me.

Such are the benefits of modern urban living.

But where's the fun in keeping one's mind on their driving?

As for the question, I vote with Jim. 'Twould have been my guess, too.

Radio waves do not pass through much rock. They do pass through air and some specially shaped structures can reflect and transmit waves - although often this is the realm/art of microwave engineerign. Perhaps something more profound is at work here. Anyway I can't hear you, the music's too loud

That's something I've wondered about. Modern mircrowave ovens use sound waves to 'nuke' food.. so why do they call it 'nuke' ?

(Though I suppose that's still a lot easier than, "Hey, I'm gonna go ultrasound my coffee." )

And yes, I realize it's not quite the same tech.

Actually.. microwave ovens use microwaves to 'nuke' your food. Micro meaning very small wavelength - a high frequency radio wave. Your domestic microwave oven has a device in it that generates very high energy 'radio' radiation. To stop you from being baked by the radiation the oven is covered in metal and the glass door in a perforated foil that will stop these microwaves of such a high frequency - Otherwise, putting your ear to the oven to listen to the food bake would severly shorten your lifespan, by cooking your insides. These miscrowaves do not travel well through rocks and tunnels, or oven doors - They are used to transmit signals for mobile phones, satellites, etc. through the air. Likewise if you unfortunately stand in front of one of those dishes on a radio tower you will bake in the classic "inside-out" fashion - Horrible stuff.

Lower frequency radio waves can bounce a bit more. The Longwave adio signals common in Europe for example, or the "shortwaves" that can bounce through the atmosphere around the Earth.

Whatever frequency station DNA was tuned into was probably not microwave. Even so, I still think there is something more profound at work here.

Actually the 'inside-out' method is a myth. The high-frequency waves work by exciting the molecules in food (to a depth of about 1 inch) so that they vibrate faster, thus causing friction to heat your food. These outside molecules then bounce against the ones next to them, causing a chain reaction (much the same as in conventional ovens) which heat the inner sactums. This is why liquid (such as water) heat faster than, say, a baked potato, since the molecules in water are much freer than potato molecules. In fact, the order (iirc) is water, grease (or fat), food. Corn will cook faster than a steak, because there's a higher water content (unless you buy your steak from Big Bear ).

But that still doesn't explain why the nickname "nuke" ... We're just moving molecules, not splitting atoms.

Correction: Sean tells me that the answer is more prosaic - that the Blackwall Tunnel has a radio repeater in it which re-broadcasts certain frequencies.

This makes a lot of sense, as the BT is a notorious traffic blackspot so it's useful for people to be able to listen to traffic reports while stuck in the tunnel.

That's sort of a dull ending, isn't it?

My brain is mostly water. If I stuck my head in a microwave oven it would bake evenly from a few cm into my cranium as my skull is a bit drier. This is why I stick to the old methods of fire and hot things to radiate heat - You know when it hurts, it's time to get away from it before any serious damage occurs.

If folks call the "art" of microwave "cooking" - nuking - would old fashioned cooking by radiation be "burning and pillaging by barbaric hordes"? I just "burned and pillaged" my toast. Time to spread on some congealed bovine milk fat.

heh heh.

Let me get this straight *so it's useful for people to be able to listen to traffic reports while stuck in the tunnel* well not that useful, they know they're stuck in a tunnel

But it makes a change from listening to microwave ovens......

*ahem* ... That wasn't nice.

Anyone wanna buy a copy of my latest CD?

Is it like the one before?

I'm not sure that Sean's explanation can be correct. If you could get radio in one place in the middle of the tunnel, you should be able to get it all the way along. I have a more likely theory...

When tunnels are built, venting shafts (for access, power, ventilation, etc.) are usually sunk and joined at corners for ease of construction. The radio waves from outside can get down these shafts from the surface (but not bend round and zoom off down the tunnel when they get to it). That would explain why you can hear radio about 20 yards from the corners (i.e. under the shaft exits), but not at any points in between.

Dartford tunnel has some radio piped in (they have an unshielded coaxial cable running the length of it), and if this were the case in the Blackwall tunnel, you would get perfect reception all the way through.

As far as the wave-cancelling theory goes - it's not true. That's not how EM waves behave (at least not at these distances, and not because of the corners). I did go to some of my lectures!