4 ms·

> The length of antenna you need to receive a radio signal is proportional to the signal’s wavelength, typically 1/2 or 1/4 of the wavelength Without wanting t

by myth2018 5y ago

> The length of antenna you need to receive a radio signal is proportional to the signal’s wavelength, typically 1/2 or 1/4 of the wavelength Without wanting to be too pedantic, but this particular piece of misunderstanding is what makes things like portable antennas to seem miraculous, when they aren't. The typical 1/2 wavelength size is a convention for dipole antennas. For a number of reasons -- e.g., they have a characteristic impedance of ~50 ohms at the feedpoint, which happens to be an impedance close to the ideal for maximum power transmission + minimal losses, a desirable feature for TX/RX antennas, but not so desirable for RX-only antennas (like TV antennas, which are usually designed to operate with coax cables of 75 ohms -- not so capable of transfering high powers, but they provide lower losses). Those values were found experimentally. That doesn't mean however that an antenna of a different size won't work. Different sizes (either physical or electrical) will present different characteristic impedances, and that's OK as long as it's matched to receiver's. Also, electrically short antennas like the ones in portable AM receivers are actually better than longer ones (for the intended use), as they collect much less noise due to their narrow bandwidths. Another evidence that this strict "1/2 or 1/4 wavelength rule" doesn't hold is that some antennas are way longer than that -- the resulting mismatched impedance (which probably won't be 50 ohms anymore) can be easily corrected and this disadvantage is compensated by the larger physical area covered -- it then collects more electromagnetic energy, making a great receiving antenna for weak signals.

anyfoo 5y ago
So, dumb question, what is the length of an antenna dependent on? Just desired frequency band and impedance, or is there more? And why, i.e. what physics result in that?
- mike50 5y ago
  SHM
- wrycoder 5y ago
  It’s related to the time it takes light (EM wave) to travel down the length of the antenna and back. The speed is somewhat slower when guided by the wire. The time should be one period.
  - myth2018 5y ago
    Indeed, this is one of the factors. This is represented as the velocity factor and depends on the metal used to build the antenna, and maybe other factors I'm not aware of.
- myth2018 5y ago
  One extremely important factor is that the physical length affects the radiation pattern. Roughly speaking: as the EM waves go back and forth through the antenna wire, the electrical current will reach different peak values in different parts of it. The parts where the current is higher are the ones that irradiate most. Sometimes you want to adjust and interfere in this process to create certain patterns of interest. E.g.: an antenna to talk to other ground-based stations should irradiate more to the sides, while one for satellites should irradiate more to the sky. There are many ways of building patterns of interest, and tweaking the antenna length is one of them. On RX/TX antennas, those patterns apply both to TX and RX, i.e., they will pick more signals coming from the same directions they irradiate more strongly. It's a fascinating field. There are many things I don't quite understand yet, and I see some controversy even among professionals, it seems that sometimes they can't settle on a single explanation for certain phenomena.
mlyle 5y ago
> way longer than that The main issue with "way longer than that" is that the impedance varies a whole lot with small changes in frequency when the antenna is much longer than the wavelength, and this is inconvenient. (Along with all the pattern oddities you can get).
- myth2018 5y ago
  > The main issue with "way longer than that" is that the impedance varies a whole lot with small changes in frequency It could be, but I'm not sure. Beverages (receive-only), and travelling-wave antennas in general, I'm not sure if they suffer from that. And even if they do, you may compensate this disadvantage with tunning/matching devices. That's what we do with magnetic loops, a price to pay for their reduced sizes and noise immunity.
  - mlyle 5y ago
    > And even if they do, you may compensate this disadvantage with tunning/matching devices. Yes, kind of. But if the impedance is really really high or really really low, then your receive efficiency will be really bad. > That's what we do with magnetic loops, a price to pay for their reduced sizes and noise immunity. Yah, and typical magnetic loop matched bandwidth is ~30KHz. Works fine for a narrow band of SSB or CW, or even a single AM radio station. But this is impractically small for a lot of use cases.
    - myth2018 5y ago
      > Yes, kind of. But if the impedance is really really high or really really low, then your receive efficiency will be really bad I must strongly disagree. A "really really high" or "really really low" impedance won't be a really really big problem by itself. Look at end-fed long-wires for instance. They have high impedances at the feed point (which is expected, since you are feeding them at their ends (well, almost.. because there must be a small counterpoise forming another "leg", otherwise you would have an almost infinite, unmatchable impedance at the feedpoint)). You need a 9:1 balun to bring its impedance closer to 50 ohms. And that doesn't turn them into bad antennas. On the contrary, if you have the luck of living in a high apartment and having some place down there to string it, you are going to have a GREAT antenna for long-distance communications, especially if the soil provides good reflection. > Yah, and typical magnetic loop matched bandwidth is ~30KHz. Works fine for a narrow band of SSB or CW, or even a single AM radio station. But this is impractically small for a lot of use cases. The bandwidth can actually be narrower than 30KHz, but don't forget that magnetic loops have variable capacitors for EXACTLY that reason: you must tune them to the frequency you are operating on.
ChuckMcM 5y ago
Yeah, sort of. One of the more interesting things about radio is that everything is an "antenna" :-). When I got my amateur radio license I found in talking with other HAMs that there is a lot of "lore" and not nearly enough "science" in those discussions. The ARRL antenna book does a pretty good job of pulling out the basics from electrodynamics to the point where you can use simplified "rules" to build antennas to achieve various results. One of my favorite type of antenna is a "magnetic loop" because they are smaller and more easily portable.
- AussieWog93 5y ago
  >When I got my amateur radio license I found in talking with other HAMs that there is a lot of "lore" and not nearly enough "science" in those discussions. I was taught the same in my Electrical Engineering degree! Antenna design is apparently very much a black art, even at the professional level.
  - myth2018 5y ago
    > I was taught the same in my Electrical Engineering degree! That's true. I'm not implying at all that in Engineering they do stuff the same way ham radios do, but sometimes we see Engineering building successfully upon certain phenomena, even though the comprehension of the underlying mechanisms is still evolving. A good example of that are the explanations for how an airplane wing works. It's been 20 years or so that the Coanda Effect is accepted as the explanation (afaik), but that didn't prevent engineers from building airplanes before that.
- jacquesm 5y ago
  You find this out the hard way when you design circuit boards for HF applications.
- zikduruqe 5y ago
  Many, many years ago when I was young, poor and a newly minted extra class operator (back when 20 WPM was the requirement), I had used electric fences, gutters, bed springs, file cabinets, etc... as HF antennas. Some did not work for crap, but others were surprisingly good. Anything and everything can be an antenna.

anyfoo 5y ago

So, dumb question, what is the length of an antenna dependent on? Just desired frequency band and impedance, or is there more? And why, i.e. what physics result in that?

mike50 5y ago
SHM
wrycoder 5y ago
It’s related to the time it takes light (EM wave) to travel down the length of the antenna and back. The speed is somewhat slower when guided by the wire. The time should be one period.
- myth2018 5y ago
  Indeed, this is one of the factors. This is represented as the velocity factor and depends on the metal used to build the antenna, and maybe other factors I'm not aware of.
myth2018 5y ago
One extremely important factor is that the physical length affects the radiation pattern. Roughly speaking: as the EM waves go back and forth through the antenna wire, the electrical current will reach different peak values in different parts of it. The parts where the current is higher are the ones that irradiate most. Sometimes you want to adjust and interfere in this process to create certain patterns of interest. E.g.: an antenna to talk to other ground-based stations should irradiate more to the sides, while one for satellites should irradiate more to the sky. There are many ways of building patterns of interest, and tweaking the antenna length is one of them. On RX/TX antennas, those patterns apply both to TX and RX, i.e., they will pick more signals coming from the same directions they irradiate more strongly. It's a fascinating field. There are many things I don't quite understand yet, and I see some controversy even among professionals, it seems that sometimes they can't settle on a single explanation for certain phenomena.

mlyle 5y ago

> way longer than that The main issue with "way longer than that" is that the impedance varies a whole lot with small changes in frequency when the antenna is much longer than the wavelength, and this is inconvenient. (Along with all the pattern oddities you can get).

myth2018 5y ago
> The main issue with "way longer than that" is that the impedance varies a whole lot with small changes in frequency It could be, but I'm not sure. Beverages (receive-only), and travelling-wave antennas in general, I'm not sure if they suffer from that. And even if they do, you may compensate this disadvantage with tunning/matching devices. That's what we do with magnetic loops, a price to pay for their reduced sizes and noise immunity.
- mlyle 5y ago
  > And even if they do, you may compensate this disadvantage with tunning/matching devices. Yes, kind of. But if the impedance is really really high or really really low, then your receive efficiency will be really bad. > That's what we do with magnetic loops, a price to pay for their reduced sizes and noise immunity. Yah, and typical magnetic loop matched bandwidth is ~30KHz. Works fine for a narrow band of SSB or CW, or even a single AM radio station. But this is impractically small for a lot of use cases.
  - myth2018 5y ago
    > Yes, kind of. But if the impedance is really really high or really really low, then your receive efficiency will be really bad I must strongly disagree. A "really really high" or "really really low" impedance won't be a really really big problem by itself. Look at end-fed long-wires for instance. They have high impedances at the feed point (which is expected, since you are feeding them at their ends (well, almost.. because there must be a small counterpoise forming another "leg", otherwise you would have an almost infinite, unmatchable impedance at the feedpoint)). You need a 9:1 balun to bring its impedance closer to 50 ohms. And that doesn't turn them into bad antennas. On the contrary, if you have the luck of living in a high apartment and having some place down there to string it, you are going to have a GREAT antenna for long-distance communications, especially if the soil provides good reflection. > Yah, and typical magnetic loop matched bandwidth is ~30KHz. Works fine for a narrow band of SSB or CW, or even a single AM radio station. But this is impractically small for a lot of use cases. The bandwidth can actually be narrower than 30KHz, but don't forget that magnetic loops have variable capacitors for EXACTLY that reason: you must tune them to the frequency you are operating on.

ChuckMcM 5y ago

Yeah, sort of. One of the more interesting things about radio is that everything is an "antenna" :-). When I got my amateur radio license I found in talking with other HAMs that there is a lot of "lore" and not nearly enough "science" in those discussions. The ARRL antenna book does a pretty good job of pulling out the basics from electrodynamics to the point where you can use simplified "rules" to build antennas to achieve various results. One of my favorite type of antenna is a "magnetic loop" because they are smaller and more easily portable.

AussieWog93 5y ago
>When I got my amateur radio license I found in talking with other HAMs that there is a lot of "lore" and not nearly enough "science" in those discussions. I was taught the same in my Electrical Engineering degree! Antenna design is apparently very much a black art, even at the professional level.
- myth2018 5y ago
  > I was taught the same in my Electrical Engineering degree! That's true. I'm not implying at all that in Engineering they do stuff the same way ham radios do, but sometimes we see Engineering building successfully upon certain phenomena, even though the comprehension of the underlying mechanisms is still evolving. A good example of that are the explanations for how an airplane wing works. It's been 20 years or so that the Coanda Effect is accepted as the explanation (afaik), but that didn't prevent engineers from building airplanes before that.
jacquesm 5y ago
You find this out the hard way when you design circuit boards for HF applications.
zikduruqe 5y ago
Many, many years ago when I was young, poor and a newly minted extra class operator (back when 20 WPM was the requirement), I had used electric fences, gutters, bed springs, file cabinets, etc... as HF antennas. Some did not work for crap, but others were surprisingly good. Anything and everything can be an antenna.