Question about wavelength

[rata478]

If lets say the wavelength is 100km how would this possibly reach a antenna?
I mean, it doesnt really make much sense to me :s.

 

[Audioguru]

What is the lowest frequency of a radio wave? Wikipedia says it is the Extremely Low Frequency of 3kHz. Its wavelength is 100km. I think it is used for communicating with submarines beneath the surface of the water.

 

[rata478]

Yes but if the wavelength would be this long how would it reach the antenna without any problems?
Just seems really strange to me.

 

[Audioguru]

High frequencies are attenuated by salt water, very low frequencies are attenuated less.
Google tells you all about it like I found out today. Why don't you look in Google yourself?????

 

[ted]

I guess you have problems figuring out the relation between antenna size and wavelength: The usual "prototype" antenna is a 1/2 wavelength dipole, which resonates nicely on received frequency. It's most konwn brother is 1/4 wave antenna, where the ground side substitutes the other half of the 1/2 wave dipole. In general, good antennas very often have a maximum dimension, which is a significant fraction (or a multiple of) wavelength. That would usually make for example 100km wavelength half-wave dipole impractically long.

However, an antenna CAN be much smaller than the wavelength, and still work pretty well. The antenna efficiency might not be as good as for larger antennas, but performance for given size can be reasonable.

One common technique for very low frequency antennas is using so-called loop antenna, which connect mainly with the magnetic field component. Other techniques exist as well, including simply accepting a reduced efficiency, using added coils, or high dielectric constant materials. Or even using the environment, such as seawater for a submarine application, as part of the antenna system.

As Audioguru already pointed out, using Google and Wikipedia will give you a lot of useful information - as in a discussion like this it is impossible to present everything of such a large subject as antennas.

 

[goldsmith]

If lets say the wavelength is 100km how would this possibly reach a antenna?
I mean, it doesnt really make much sense to me :s.

Hi rata478
I know exactly what you mean ! who told that size of antenna must be exactly lambda/2 or lambda/4 or ... ? of course for the best radiation there are some certain rules but when the distance of aerial is being calculated very large we have two choices :
1- decrease size of antenna ( in some allowed dimensions ) and increase the power injected into the antenna and use strong receivers .
2- use antennas in parallel with X axis instead of Y axis ! and then make it spiral on ground .
For better understanding of the 2nd option i suggest you to have a look on project harp . ( search it on google )( the principle is somehow correspond )
Best Wishes
Goldsmith

 

[chuckey]

To transmit VLF with a reasonable ( ) sized aerial means that the vertical section is top loaded with a lot of radial wires , so giving it more capacitance, they looks like a VERY BIG umbrella.
Frank

 

[D.A.(Tony)Stewart]

I once asked the same question, when my very 1st design project was to design a Doppler triangulation Receiver for VLF on a floating weather station in 1975.

What I learned was that any shorter length can be used at the expense of efficiency.

One of the channels I recall receiving from the Beaufort Sea was Jim Creek, a Navy transmitter to submarines like all they have, which are cesium time synchronized.

Notice the antenna length.