Why is it that very low frequency signals cannot be transmitted by antennas?

[AML195]

Why is it that very low frequency signals cannot be transmitted by antennas? (Or why is it inefficient?) Thanks!

 

[DaveC426913]

Sounds like homework.

What do you think?

 

[jtbell]

Define "very low." What frequency range are you thinking of, specifically?

Note that a Google search on "very low frequency antenna" turns up a lot of hits.

 

[AML195]

It's not homework (I'm not in school). Just curious, that's all. I'm thinking that it has something to do with the fact that at very low frequencies, the wavelengths are much longer and as a result, you would need an extremely long antenna in order to obtain a resonant frequency that matches that of the signal. But that's just my guess.

 

[Dadface]

Any frequency no matter how low can be transmitted.Apart from the problem of matching the antenna length to the wavelength very low frequencies will have a limited detectible range due to the low power transmitted.

 

[nucleus]

There was a low frequency antenna at Siple Station in the Antarctica with another in Quebec with the length of 42 Km. It was shut down about 20 years ago.
http://www-star.stanford.edu/~vlf/Antarctica/Siple/index.htm

 

[LURCH]

It's not impossible, it just requires very long anteni. Here in Michigan, we have a set of 28-mile-long anteni for sending ultra-low frequency (ULF) radio signals to submarines. It's part of a project known as ELF.

 

[pallidin]

...very low frequencies will have a limited detectible range due to the low power transmitted.

That is not correct.
VLF's can exist in any power range available to other EMF emmisions.

 

[Dadface]

In the original question AML195 referred to signalling and the efficiency of doing this and I interpreted his query as being signalling in a practical sense ie carrying detectable information from transmitter to reciever.There are several factors that determine detectable range the frequency of any sort of carrier wave being a major one of these and if we have to comprimise on frequency then we have to make up for it elsewhere and as the frequency reduces further the practical problems become more severe.Nobody in this thread has answered jtbells question asking for a definition of very low so let's take it to the limit and gradually reduce the frequency to zero.What would the length of the antenna be when the frequency reaches 1Hz and how will we then supply it with power?What then happens as the frequency reduces further?The difficulties, therefore.are more practical than they are theoretical.In future times we may see antennae which make those seen in Michigan and Quebec seem miniscule by comparison

 

[Delphi51]

Power lines (50 or 60 Hz) emit radio waves and the power loss is significant enough that some long transmission lines have been converted to DC to avoid it.

 

[Dadface]

Any frequency can be transmitted as I stated in my original post.The point I am making is a practical one concerning detectable range.In theory the em field around a source carrying an ac supply spreads to infinity but at what distance can a practical receiver actually pick up a discernable signal?I think we are a bit at cross purposes here-I agree with the theoretical reasoning and comments given (although I have not heard of power lines going back to d.c supplies)but I am pointing out practical limitations.

 

[radioHam]

There are numerous amateur radio publications about VLF, and amateurs have allocations in this segment of the spectrum - 500KHz is popular worldwide. For a half wave dipole at this frequency, the length would need to be 660m long, (c/freq) times the velocity factor of the wire, and, to be efficient, this height above ground, so it is the very few who have enough land and hardware to be able to utilise this wavelength, let alone the technological knowledge to build transmitter/receiver as these aren't commercially available. So these wavelengths are the true 'experimenters' domain. However, once a decent antenna is established, worldwide propagation is possible using the F1/F2 layers of the atmosphere, or lower layers in high sunspot activity.
If you think about it, the bands we used to use a lot - eg long wave, go VERY LF - eg the BBC used to be on 1500m. The antennas used were primarily vertical and very high, together with a massive amount of metalwork under the ground to act as a ground plane. Google the Crystal Palace antenna. The BBC World Service still uses 648Khz and others

 

[Phrak]

What are the theoretical values of current and voltages on the antennae required for equal power transmission at different frequencies? We can hold the length constant and use loading coils, or let the length be proportional to wavelength.

These is a complicating factor at the lower frequencies. Take a typical distance on the Earth to be one radian, or 4000 miles. This is about 1/21 Hz. (There are some practical matters in a theoretical approach.) For any transmitter sending in the order of less than 21 Hz, each leg of the antennae begin to look more like half of a capacitor and transformer with the other half at the receiving end. You're not just talking about propagating waves any more; the power coupling is not the same.

 

[radioHam]

Heh heh - yes I did say the wire needed to be a half-wave above ground - that would negate the capacitive coupling. In practical terms the antenna would be matched to the (usually 50ohm) transmitter output with an 'Antenna Matching Unit', basically an impedance matching tank circuit. The losses would be quite high I'm sure.
I think the only people using frequencies down in the 'few Hz' are the military - submarines - who tow several miles of fine wire behind the sub.