I have been wondering about this for long time.I see the AM transmitter antennas are quite tall. This is due to the fact that the wavelength of the AM carrier wave is not as short as a cellular communication's carrier wave. At 1MHz the wavelength is 300m.

I also know that the AM radio receivers I encounter do not have that long antennas. In a question in this site I remember one was writing that the receiving antenna size do not need to be half of the wave length to detect the radio signals. It could be much smaller. How much smaller I dont know.. A mystery.

So in antenna theory some texts relate the wavelength to antenna size. But when it comes to practice I see that wave length antenna size relation is followed by transmitter size but not at the receiver side.

I mean transmitter antenna size is more or less similar to carrier wavelength. But receiver antennas can be much much smaller..

If we can receive the AM radio signals by a realtively much smaller antennas, why then the AM transmitters also not short but very tall? Is it possible to radiate AM modulated signals by a lets say 1m size antenna for a radio broadcast?

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    \$\begingroup\$ This is similar to the question from the Physics SE: Radio antennas that are much shorter than the wavelength. I'd look at this question and see if it answers your question :) \$\endgroup\$ – KingDuken Oct 24 '17 at 15:08
  • \$\begingroup\$ Not really.. One says "Transmitting antennas must optimally radiate" doesn't even explain why and how.. Another one just copied and pasted the equations about path loss in wave propagation. Im really curious "why" "transmitter antenna size" must be similar to wavelength and why receiver antenna size does not.. \$\endgroup\$ – Genzo Oct 24 '17 at 15:29
  • \$\begingroup\$ The opposite would work equally well: A tiny antenna at the transmitter, and everybody having a 50 m mast on their car. But the arrangement we have is more economical and more practical. \$\endgroup\$ – The Photon Oct 24 '17 at 15:47
  • \$\begingroup\$ @ThePhoton I thought it was because if the transmitter size is less than 1/16 of the carrier wavelength or even less, most of the power will be dissipated in the transmitter circuit and there will be no radiation. So basically are you saying it is more efficient to use higher antenna to not to consume too much power? But I would like to see some model or formula which shows these. A formula or circuit model which relates the transmitter antenna size to the radiated electric field strength at a particular far field distance. \$\endgroup\$ – Genzo Oct 24 '17 at 15:59
  • \$\begingroup\$ An antenna close to say 1/4 the wavelength is more efficient. But this applies equally to transmitter and receiver. Matching networks are used, especially when the antenna length isn't well-matched to the wavelength. This applies equally to transmitter and receiver. When more power is involved, the requirements on the matching network are more stringent (higher voltages applied, more heat generated by any losses). This is more important in the transmitter. \$\endgroup\$ – The Photon Oct 24 '17 at 16:23

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