4 ms·

> 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

• 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.
    • mlyle 5y ago
      > I must strongly disagree. A "really really high" or "really really low" impedance won't be a really really big problem by itself. 450 ohms isn't really high. That's easily manageable with a balun, as you state. It even matches some feedlines. But random wires end up with impedances that vary considerably from crazy high to crazy low across relatively small changes in frequency, and this in turn sucks. You can't fix it with a fixed balun or reactance. And if your impedance is 15k ohms, you're going to need to reach a rather ridiculous voltage to get any power out. > 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. Yes, which is fine as long as your signal comfortably fits in the bandwidth, as mentioned. Note that e.g. a reasonably wide commercial AM station might -not- and have rolloff of the high audio frequency response.
      • myth2018 5y ago
        > But random wires end up with impedances that vary considerably from crazy high to crazy low across relatively small changes in frequency, and this in turn sucks. Could you send a link with a practical case? I'm not implying I don't believe you, it's just that I've never seen such complain. Personally, I don't use random wires. But I never see someone complaining like you about how wildly the characteristic impedance varies. Granted, it varies, but well within the range of most couplers. Besides, this is another source of myths: you often encounter folks excited about how a given coupler "tune everything" and "turn any piece of metal into an antenna", but they are never concerned about things like radiation patterns, radiation resistances, losses from the antenna, the cables, the coupler etc. They just see a SWR close to 1:1, celebrate and go to the internet complain about how empty are the bands. > And if your impedance is 15k ohms, you're going to need to reach a rather ridiculous voltage to get any power out. This is another case I haven't faced so far, neither myself or in the literature. 15k ohms? If the impedance really reached that high, maybe it's for the lack of a counterpoise, even if a small one? In a real-world end-fed, if you don't go high power (low power DX is perfectly feasible on days with good propagation and narrow bandwidth modes), you can keep the voltages at the feed point within the limits of commercial connectors, so that shouldn't be a problem. > Yes, which is fine as long as your signal comfortably fits in the bandwidth I can't remember of any mode demanding more than 30KHz (again, I'm not implying they don't exist). Sure, you have Amateur TV, but this mode is reserved for clubs and, if they are really meaning business that hard, they will certainly not use a magloop. Ah, I've also seen some guys transmitting signals to generate pictures at waterfalls of web SDRs. They were using a HackRF and probably a very high bandwidth signal (I'm not sure), but again, if you are getting into experimental modes, you are probably not going to use a small, portable antenna anyway