wavelength

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Re: wavelength

Post by Admin on Sun Jan 08, 2017 12:05 pm

Hello Densil,
> ... like "radio 290 north sea" and "luxemborg 208"
Now that's going back a few years :-)

You are quite right, the speed of light is a little slower than 300m/uS (per micro-second). Wavelength is not normally used to accurately express a signal in the frequency domain. Wavelength is a rather "loose" figure, anyway. With your radio Luxemburg example you would have to use decimals to express the exact position on the dial, but in those there was a single 180-degrees dial and a knob that couild get you in the right area, but never exact.

Today we only use cycles-per-second (Hertz) as an exact measurement, and wavelength is normally used for a rough guide, for example the 70cm hamradio band or 10cm RADAR band. In hamradio we would rarely use wavelength if it were not for the usefulness in antenna design, RF cable measurements and such. I suppose we could also use "period-time" as an alternative, but it has little function or meaning in the radio world.

If you turn the clock back 120 years, in the days of Loomis, Armstrong and Maccaroni, then wavelength was used and was more convenient at the time.

60 years ago people were used to looking at the wavelength on the wireless-set dial.

Today people look at the 4-digit precision frequency displays on domestic equipment.

In the future we will be using channel numbers, I am sure. Tele operators are already refering to ARFCN numbers for mobile telephones, often with little or no idea as to the frequency involved. ARFCN = Absolute Radio Frequency Channel Number.

So to answer your question, wavelength is a rough approximation, usually refereng to a band or range of frequencies (eg. 2m band = 144.000MHz to 146.000MHz). Frequencies are used to express the exact values.

One final point, perhaps to spark off a bit more thought, how do you know that the speed of light is a constant? How do you know that it does not slow down when leaving a distant galaxy, due to the huge power of gravity behind it? We can prove that light has mass, and that light is affected by gravity.

If this is true then perhaps the universe is not expanding? If it were expanding, and light has an absolute speed, then if you stood on the surface of a planet just a few billions of light years distant, and moving away from ut at only 10 million metres a second, then what would it look like?

If the speed of light is not a constant, then think of all the scientific knowledge we have that is suddenly not valid, including frequency and wavelength.

Anyway, back to you. Have I answered your question?
Have I provoked any new thoughts?

Very best regards from Harry Lythall

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wavelength

Post by Densil on Sat Jan 07, 2017 10:16 am

Harry I have one more question. I am trying to follow many of your projectes, especially power circuits and antennas.
I read somewhere that the speed of light is not 300 m/s but a little slower than that, 299,791m/s. So if you have 100mhz then is the wavelength really 3m or 2,99791m?

I mean that we have a lot of international "standards" that are not actually standard but approximations. Is the speed of radio one of them? LW and MW stations always quoted wavelength like "radio 290 north sea" and "luxemborg 208". These assumed speed of radio to be 300m/s but if you use 299,791 then there are a few khz difference which is 1/2 a channel.

I am sorry but questions like this have puzzled me for a while because electronics and radio calculation has to be accurate.
/D

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