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enter image description here

If many radio waves can generate currents in the antenna ground circuit simultaneously, I am wondering do they all get carried over to the tuner circuit?, but only the current whose frequency matches the tuned frequency dominates? or is it the case that even though many currents can be generated in the antenna ground circuit, only the meant one gets carried over to the other side?

source of the pic:

https://industrial-electronics.com/measurement-testing-com/rf_design_5.html

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3 Answers 3

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Whatever frequencies the antenna picks up get fed into the radio. Typically in the receiver front end there will be a tuned circuit that filters out most of the signals. To get selectivity narrow enough to only pass a single signal will usually take multiple stages of filtering and amplification.

There are a couple of different ways this is done. In early radio receivers the selectivity was done at the RF frequency, this is called a TRF (Tuned Radio Frequency) receiver. Suppose you wanted to receive a station broadcasting on 610 kHz, there would be a series of tuned circuits that could be adjusted to that frequency with amplifiers between each filter. There are some problems with this method, notably tuned circuits change bandwidth as their frequency changes so a tank circuit that's narrow at one end of a radio band will be wider at the other. Also all of the stages must track so that they all stay tuned to the same frequency as they are adjusted.

A fix for this is known as the superheterodyne receiver. In this type there can be an RF amplifier and filter right after the antenna but sometimes not. The RF signals are fed to a mixer along with a signal called the local oscillator frequency, the output of the mixing will contain the RF signals, local oscillator frequency, and the sum and difference of those two. This is fed into what is known as an IF (Intermediate Frequency) amplifier that is tuned to a single frequency. This allows amplification with a set bandwidth and all of the IF stages are tuned to a fixed frequency so there's no need to make them track. Only the initial RF stage filter and the oscillator frequency are variable so only those need to track.

There are other methods of receiving a single signal out of all the signals an antenna picks up. Lately the trend is towards software defined radio (SDR) where the signals are separated using digital methods, but even these usually have an RF filter after the antenna to narrow the range of signals they have to deal with.

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  • \$\begingroup\$ So to sum up, all possible frequencies are picked up by the antenna, but eventually, not as naively as my figure depicts, ONLY one frequency make to the end, right? \$\endgroup\$
    – Jack
    Feb 26 at 2:04
  • \$\begingroup\$ But let's say what i call a radio is only composed of that LC circuit and the antenna ground circuit, then how things will behave, for exactly this naive model? \$\endgroup\$
    – Jack
    Feb 26 at 2:07
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    \$\begingroup\$ @Jack With an antenna coupled into a tuned circuit as shown in your picture the signal at V will contain to some degree all the frequencies that the antenna picks up but filtered so that the frequency the LC circuit is tuned to dominates. If the inductor and capacitor were ideal only their resonant frequency would pass but in real components there will be a passband due to resistance and parasitic inductance and capacitance. That passband depends on the Q (Quality factor) of the tuned circuit. \$\endgroup\$
    – GodJihyo
    Feb 26 at 2:24
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Red, green, and blue light reach your eye simultaneously. If you put a red filter in front, you'll only see the red light. The tuner is a filter that picks out the "color" of radio wave you wish to receive.

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  • \$\begingroup\$ So you're saying that yes many currents are produced simultaneously in the antenna ground circuit, BUT only one make it to the other side? \$\endgroup\$
    – Jack
    Feb 26 at 1:45
  • \$\begingroup\$ @Jack Ideally, all of the energy from the antenna at the filter frequency is transferred to the output. At other frequencies, less is transferred. How much depends on how different the signal frequency is from the filter frequency. \$\endgroup\$
    – John Doty
    Feb 26 at 1:53
  • \$\begingroup\$ So I can say that ALL make it to the other side BUT the meant one dominates, right? \$\endgroup\$
    – Jack
    Feb 26 at 1:56
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    \$\begingroup\$ @Jack Maybe one dominates. Depends on frequency separation, signal strength, filter frequency, ... You should build yourself a "crystal radio" and try it out. \$\endgroup\$
    – John Doty
    Feb 26 at 2:00
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There are many antenna types, some broadband some narroband. A half-wave diopole receives best the frequency whose wavelength is \$\lambda=2\dfrac{c}{f}\$. There is a narrow bandwidth around this value that can be received but decreasing the further away.

So only some range of frequencies will excite the primary. The transformer must be designed to operate at the frequencies of interest in order for the secondary circuitry to recive the band of signals.

So ALL the frequencies in the air are not available to the transformer. Only the ones admitted by the antenna.

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