5
$\begingroup$

I heard that an antenna picks up signals of all frequencies, and then these signals are filtered by a circuit using an inductor and capacitor, and that this is how a radio is tuned.

However, I also heard that an antenna has a resonant frequency, and that this is determined by its length.

  • Does this mean that it will only pick up signals at or near to its resonant frequency?
  • What then is the point of a tuning circuit?
  • And what about transmitting antennas? Will they naturally transmit signals at their resonant frequency without the need of a tuner?
  • If antennas can receive signals of any frequency, and the tuning
    circuit filters out all the unwanted frequencies, why is it
    necessary to design antennas with a specific resonant frequency? Couldn't we use an antenna of any resonant frequency and just use
    the tuning circuit to get the frequency we want?
$\endgroup$
8
  • $\begingroup$ It may help your understanding, if you know that the receive circuit on most radios are very good at amplifying small signals. Even a poor choice of antenna for a certain band could allow the receiver to pick something out and provide enough gain to hear. That doesn't mean such an antenna will be able to transmit useful power on that band. $\endgroup$
    – wbg
    Feb 9, 2022 at 1:02
  • $\begingroup$ But if the antenna can receive signals of any frequency, and the tuning circuit filters out all the unwanted frequencies, why is it necessary to design antennas with a specific resonant frequency? Couldn't we use an antenna of any resonant frequency and just use the tuning circuit to get the frequency we want? That's what I want to know. $\endgroup$
    – Urthona26
    Feb 10, 2022 at 19:23
  • $\begingroup$ It cannot receive ANY frequency only a set. Just because it receives doesn't mean there's enough gain to be useful. Antennas also transmit so even if there's enough gain to receive you will not be able to transmit. Also, the receiver tuner doesn't have infinite ability to tune. Read about heterodyne detection. The incoming frequency is shifted and there's some limiting factors there. $\endgroup$
    – wbg
    Feb 10, 2022 at 20:44
  • $\begingroup$ Recall that a resonant antenna has a very narrow band pass. Also, many antenna are NOT resonant. Being resonant means you will get maximum power transfer as I understand it but it also acts like a filter. Also read about RLC filter circuits. $\endgroup$
    – wbg
    Feb 10, 2022 at 20:45
  • $\begingroup$ I think I understand. So an antenna can pick up a signal that's so weak that humans can't use it. So resonant frequency is important. But if that's the case, how can a very short antenna on my personal radio tune into longwave stations? Doesn't an antenna need to be hundreds of metres long to have this sort of resonant frequency? Is it because resonant frequency is more important for transmission than reception - so an antenna needs to be very tall to transmit low frequencies but even a short antenna can pick them up? $\endgroup$
    – Urthona26
    Feb 11, 2022 at 17:28

6 Answers 6

5
$\begingroup$

All frequencies.

But, there's a big difference between "picking up" a signal, and how well an antenna will "pick up" a signal.

Any metal object will pick up almost any RF signal at almost any frequency (I've used unbent paperclips for a wide range of Rx testing.) But the problem in that certain antenna geometries might pick up a signal too weakly, when compared to receiver noise or the local ambient RF noise floor, to detect. Other antenna's have better gain and directionality towards the frequencies and signals of interest.

But lots of people use random length wires to SWL (short wave listen) all the way from MF (and lower) to VHF, even though the length might be far from resonance at the shortwave frequencies received.

With transmit antennas, there is a similar problem regarding efficiency and providing a proper output load for the transmitter. But people have accomplished DX contacts using old incandescent lightbulbs for antenna's (possibly also radiating RF off of the random lengths of feed lines, power cords, and ground lines connected to the transmitter.) Or accomplished QSOs with poorly shielded dummy loads. But a proper half-wave dipole well above ground level will probably radiate a signal even farther with less power.

$\endgroup$
5
  • $\begingroup$ Not all antennas get all frequencies. The small loop is an extreme case -- it receives nothing outside of its pass band, not even static. $\endgroup$
    – user10489
    Feb 8, 2022 at 1:47
  • 2
    $\begingroup$ An ideal perfect small loop is the extreme case. IRL, most actual small mag loops have enough parasitics that they won't totally zero a strong AM station in the null. (I have a small portable one.) $\endgroup$
    – hotpaw2
    Feb 8, 2022 at 2:57
  • $\begingroup$ Mine does, but then, I don't have a close AM station. It would be interesting to calculate the depth of that null with the filtering (in dB). $\endgroup$
    – user10489
    Feb 8, 2022 at 12:22
  • 1
    $\begingroup$ @user10489 let me assure you, you simply don't have the equipment to measure what you think it doesn't pick up. True "nulls" are kind of rare in physics, and in the case of a loop antenna, would require an infinitely thin winding of superconductive wire :) $\endgroup$ May 31, 2023 at 9:10
  • $\begingroup$ Saying I don't have the equipment is naive and rude. "True null" is meaningless. A "real" null is where the signal is attenuated deeply. To have meaning, you put a number on that, usually in decibels. I know I can easily measure dB down to probably -60 or maybe -80. $\endgroup$
    – user10489
    May 31, 2023 at 11:21
3
$\begingroup$

It depends on the antenna. Some antennas are very broad band and do more or less pick up every frequency, but may have more sensitivity on some than others. Typically, it would pick up more on its resonant frequency and harmonics of that, but there may not be a lot of difference across the spectrum.

Some antennas act like RLC circuits themselves, and act like filters. As a rule of thumb the more complicated the antenna, the more narrow band it is, but there are many factors that affect antenna bandwidth. Most antennas are a compromise between factors, and bandwidth is one of those.

The point of an antenna tuner is in part to enhance this effect, but more typically it is used to impedance match the antenna for transmission where it is more critical, mostly to keep reflections from impedance mismatches from overheating the radio. But the radio itself has a completely different "tuning circuit" whose purpose is to narrow the bandwidth down to a single channel and frequently shift it to a new frequency (heterodyne) to make further processing easier.

The small loop antenna has a bandwidth so narrow (~100KHz maybe) that it would be nearly useless without the tuning capacitor integrated into the antenna. This is an extreme case of the antenna acting like a filter.

$\endgroup$
6
  • $\begingroup$ So, if I understand correctly, radios pick up frequencies around its resonant frequency and this can be narrowed down using the tuning circuit. How is it then that radio receivers can pick up stations of any frequency? Or are radio stations only allowed to operate within a narrow range of frequencies? $\endgroup$
    – Urthona26
    Feb 7, 2022 at 17:52
  • 1
    $\begingroup$ Non-resonant antennas can also pick up signals. And yes, radio stations are only allowed to operate within a narrow range, usually collectively called a "band". $\endgroup$
    – user10489
    Feb 8, 2022 at 1:44
  • 2
    $\begingroup$ A perfect example of a non-resonant antenna is a Beverage. $\endgroup$ Feb 11, 2022 at 2:27
  • 1
    $\begingroup$ Rereading your comment, I think you miss the point of the radio's tuning circuit. The point is that the tuning circuit allows you to change the frequency it is listening to and tune into the station. $\endgroup$
    – user10489
    Feb 11, 2022 at 2:38
  • $\begingroup$ Reflections from impedance mismatches do not overheat the radio. Impedance mismatches, however, can overheat the radio, not because of reflected power, but because (in the excessively high impedance case) the pullup FET amp current has nowhere to go, and its voltage rises above Vds. A similar effect occurs in the low-impedance case. $\endgroup$
    – Noji
    May 31, 2023 at 8:06
2
$\begingroup$

I think your question is basically : "Does an antenna receive signals at all frequencies?".

The answer is in the antenna's S11. The S11 is a measurements were you send a wave to an antenna and watch what is coming back. What is lacking surely have been radiated. So when you make that measurement for all the frequencies you get an S11 chart. The lower the S11 in dB for a given frequency the less there is wave coming back from the antenna == the more have radiated.

Now that you know that, antenna have a principle, which tell us that all characteristics are the same in transmission and in reception. ***

So basically watch the S11 of your antenna if the manufacturer gives it, or better measure it with a VNA. (Spoiler) You will see interesting things, such as a 400MHz dipole antenna having a resonance at 800MHz and at 200MHz.

The part where the antenna has a very small S11 is called the resonnant frequency, and the width of this zone is called the bandwidth of the antenna.

In short: Antenna pick up all signals at a very (very very) small level compared to the signal at their resonant frequency.

Look at this Link page 5 on the return loss diagram, you can see at 420MHz the S11 is -2dB which means outside of it's band (not very far away in this case) the antenna absorbs, and radiate a bit.


*** For purists : Yes we don't often use ring antenna for emission but for reception, that is because it's a poor antenna (in both case) but for low enough frequencies it is good enough for reception. And the receiver can make a pretty decent use of it, despite being poorly adapted. But it's cheap. And simple to mass manufacture. So..

$\endgroup$
1
$\begingroup$

Your question is similar to others that have been asked here before. Here are a few:

An antenna being resonant means that its impedance is purely resistive, and has no reactance. This happens when the antenna is a half-wavelength long electrically for dipole antennas, or a quarter-wave long for vertical antennas. Typically antennas are designed to have low SWR near the point of resonance, but not always. Whether an antenna is resonant or not really isn't important to hams most of the time; what's usually more important is the SWR of the antenna at the bands or frequencies you're interested in.

If the antenna has a low SWR, that means that it will be easy to couple to the transmitter. Most HF transmitters can handle a mismatch of 2:1 or better without the need for a transmatch (antenna tuner). Receivers typically don't much care about the antenna being resonant. Most ham receivers hear better when the SWR is low, but usually that's a small effect because receivers have plenty of gain. Because the effect is usually small, antennas typically don't do much receive filtering of their own.

The point of a tuning circuit is typically to keep the SWR low on a coaxial feed line for transmitting, because coaxial cable tends to be very lossy at high SWRs. Also, most solid-state transmitters are sensitive to SWR, and will automatically reduce power if the SWR is too high. A tuning circuit is typically not needed if the SWR is lower than 2:1. Many antennas are designed to have an SWR in that range for all the frequencies of interest. A transmatch (antenna tuner) can extend the useful frequency range for an antenna, or it can make a multi-band HF wire antenna practical.

Transmatches (antenna tuners) can be great problem-solvers, but it's important to note that just because the SWR at the transmitter is 1:1 doesn't mean that power isn't being lost in the feed line or in the transmatch, or that the antenna is working efficiently. My HF transmatch can tune up a 3' (1 m) piece of coax with nothing connected to the other end, but that piece of coax is a terrible antenna!

$\endgroup$
2
  • $\begingroup$ To be honest, that explanation was far too advanced for me. I'm a complete beginner. I think my problem is that I was thinking that an antenna can pick up all frequencies and that the tuning circuit can filter these to get the one you want, so I didn't understand why we should care about the resonant frequency of an antenna. Since asking the question, I have learned that AM and FM operate in different bands, so an AM and FM antenna have very different resonant frequencies and an FM antenna can be tuned to pick up other FM stations, but not AM stations. Is that correct? $\endgroup$
    – Urthona26
    Feb 8, 2022 at 17:32
  • $\begingroup$ It's more about antenna and radio design than resonant frequency. The antenna on a car picks up both AM and FM signals. If you have a stereo receiver that can receive AM, it probably uses a dipole antenna for FM and a loop antenna for AM, and then the FM antenna won't work for AM. Sorry, it's complicated. I've enjoyed learning about antennas, and if you start learning about them I hope that you enjoy learning also. $\endgroup$
    – rclocher3
    Feb 8, 2022 at 19:21
0
$\begingroup$

In order to "receive" a signal, the power of the desired signal at the receiver's detector needs to be above the "garbage" present by some amount. The "garbage" may be just thermal noise, but it might also be interference from other signals that are present as well as the distortion that is produced in the receiver by all of the signals coming into the detector (desired and undesired).

One sometimes specifies the required ratio of desired to undesired power in terms of a "D/U" ratio, "SINAD" (signal-to-noise-and-distortion), "SINR" (signal-to-interference-and-noise ratio), or just "SNR" (signal-to-noise ratio).

Different types of signals require different minimum D/U (SINAD, SINR, etc.) for the signal to be reliably received and processed by the receiver. High bit rate digital modulation schemes such as those used in 5G wireless require very high ratios. Morse code (with a human in an analog receiver chain doing the final demodulation in his or her head) can be reliably received with D/U ratios less than 1 (i.e. when there is more garbage than signal present). The received signal strength at the minimum D/U ratio is sometimes referred to as the minimum discernible signal (MDS).

All this having been said, the antenna, bandpass filter, amplifier, and other parts of the front-end receiver chain are really only the first stage of signal selection. Regardless of their construction, they will always pass some finite band of signals through to the rest of the receiver. It's really the backend of the receiver (demodulators, matched filters, etc.) that demodulates and decodes the signal of interest. If the front-end is too wide (i.e. "non-resonant"), though, a lot of extra noise and interference will make it through to the backend and eat into the receiver's interference and noise margin and result in errors in reception or blockage altogether of the desired signal.

$\endgroup$
0
$\begingroup$

An antenna, or any conductive material, subject to the radiation field of a radio wave, will "receive" that wave, regardless of frequency. According to Faraday's Law of Induction, the conductor that's exposed to an electromagnetic field will produce ("induce") a voltage that changes as the field. Acting like a capacitor, the changing voltage in the conductor results in a changing current through it $i(t) = C\frac{dV}{dt}$ because of the movement of charges through it $C = \frac{dQ}{dV}$. This charge "movement" is governed largely by the conductor geometry and medium (substance it's made of). During any frequency cycle, the moving charges accumulate at one end of the conductor, then the other. If the conductor is precisely the size (due again to geometry and medium, which controls the velocity factor) at which the accumulated charges reach a maximum accumulation at one end and a minimum accumulation at the other end of the conductor, the voltage displacement (potential difference) is at maximum, resulting in maximum current flow at that frequency. A signal whose frequency that does not produce this maximum displacement will cancel some of the effect, resulting in non-maximum current flow.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .