# Wavelengths in Transmisson Lines

Discussion in 'Physics' started by JBmtk, Feb 8, 2010.

1. ### JBmtk Thread Starter Member

Jun 29, 2009
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I am having a hard time comprehending wavelengths. I understand that it is a measure of distance between two repeated points in a waveform, but why is it that the wavelength of say,a 22kHz signal is 13,636meters yet we can still receive the whole signal!

Wouldn't the distance between 2 same points be much smaller than that?

2. ### studiot AAC Fanatic!

Nov 9, 2007
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515
The reason we can still receive signals even though the wavelength will not 'fit into' our apparatus is that a part of the wavelength will fit. This causes an electrical disturbance in our apparatus which varies at the frequency of the signal wave.

It is this disturbance we detect. However the price we pay is that the magnitude is small.

3. ### BillO Distinguished Member

Nov 24, 2008
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Studiots explanation is a good one so Ill respond anecdotally.

22 KHz is a very, very low frequency and it would be difficult to say the least to effectively both transmit and to receive a radio signal at this frequency. 1/4 wavelength antennas make good use of the signal by being the shortest length that can benefit from the full amplitude of the wave. However, at 22KHz, a 3.4 km piece of wire is not practical. At frequencies like that, loop antennas work well for both transmission and reception, the system acting much like a loosely coupled transformer

Back in the early days of radio, lower frequency carriers were used because of the problems with the electronics of the time operating at higher frequencies. Along the way, practicality and advancements in electronics bought higher and higher frequencies. With FM a 1/4 wave antenna need only be around 0.78m in length

Today, most serious radio communication happens at frequencies where the wavelength is measured in cm or less. Antennas do not have to be so big to be efficient.

One of the nice things about lower frequencies though, is their ability to disperse. They can work over great distances and not be shadowed by the curvature of the Earth. The higher the frequency, the more line-of-sight the relationship needs to be between transmitter and receiver.

4. ### davebee Well-Known Member

Oct 22, 2008
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The same issue is encountered when receiving sound. Your ears are much smaller than many audio wavelengths, yet we hear those sounds with no problem.

A receiver, or receiving antenna, does not have to be sized by the wavelength; all that is needed is for enough energy to be received to be above background noise, and that condition can often be met with antennas much smaller than the wavelength being received.

5. ### JBmtk Thread Starter Member

Jun 29, 2009
25
0
Thanks guys for your answers...it really puts things into perspective. I really don't understand why some professors tend to get bogged down in the math in explaining things when sometimes the simple, practice explanations are what it takes. Of course the math is where one would truly understand the "magic" behind everything...

6. ### studiot AAC Fanatic!

Nov 9, 2007
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Glad it helped. The feedback is appreciated.

7. ### BillO Distinguished Member

Nov 24, 2008
985
136

True, as an example, you can 'see' about 1/4 of the signal's amplitude with an antenna of about 1/25 of the wavelength.

8. ### redlight000 Member

Feb 26, 2010
66
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Hi BillO,
can you kindly eleborate on saying 22khz is "very very low" in Frequency..? not saying your wrong... but on my audio charts.. it says 20HZ as low.. the starting of hearing & over 20khz of which one cant hear(Depending on age) I thought 22khz is high just going into Ultrasound.. is this right? or wrong..?
many thanks.. Just curious here.
regards
redlight000

9. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Audio is not usually what the charts show, think electromagnetic spectrum.

10. ### Duane P Wetick Active Member

Apr 23, 2009
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With regards to audiophiles everywhere, audio is mechanical in nature and is not associated with the electromagnetic spectrum and wavelength.

Cheers, DPW [ Everything has limitations...and I hate limitations.]

11. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
22kHz

Seems pretty conclusive to me that Bill was discussing electromagnetic waves, not audio.

12. ### Clay Member

Feb 12, 2010
21
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Very low frequencies were used in the early days of radio with
'Beveridge' long-wire antennas. They could be a couple of miles
long, usually strung atop farmer's fence lines. Today ELF or
extremely low frequency is used to communicate with submerged
nuclear submarines. But the wavelengths are so long the best
rate you can achieve is 30 seconds per character sent! The U.S.
navy wanted to build a 5,000 mile antenna through Wisconsin and
Michigan, but were stopped by environmentalists .In 1991 President Reagan authorized an 84 mile long antenna for testing.
Anybody know the status today?

Best regards,

/Clay