4 ms·

> 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 "r

by 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
  - mlyle 5y ago
    > Personally, I don't use random wires. For a long time I had a long random wire up and had a lot of success on it. But it would not tune on small segments of the bands because the impedance was very high or very low. And it was relatively deaf near those points, too. I shortened it slightly until those dead zones were in places I didn't care about. As the wire gets longer, those scary multiples of half wavelength become more common (though narrower). E.g. if I have a 150m electrical length antenna, around 17.987MHz, 18.986MHz, 19.986MHz, 20.985MHz, 21.984MHz, are really bad. This is if the wire is straight and far above the ground. If it also has random bends, you get even more places where funny things happen. And then, of course, really crazy things happen when you start trying to use this as a receive antenna up on UHF-- 399.7, 400.7, 401.7, 402.7, etc, not to mention the unpredictable, strong, and varying directionality. > 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? https://vk6ysf.com/longwire_antenna.htm https://vk6ysf.com/longwire_antenna.htm " A multi-band end fed wire antenna presents a potential problem in that a ¼ wave length antenna cut for say the 80 metre band will present a feed impedance of about 70 ohms however when the same antenna is used on the 40 metre band the feed point impendence will be several thousand ohms and therefore difficult to achieve a match even with a quality tuner. The trick is to making this type antenna an easier match by avoiding the lengths that are 1/2 wavelength and harmonically related to 1/2 wavelengths or simply multiples of 1/2 wavelengths that will present the most extreme rang of impedances." 4NEC2 graphs are in the article. Admittedly, 15k ohms is an extreme case: at some point, earth losses, etc, prevent you from getting all the way to infinity, and ohmic losses of the wire and feedline stop you from getting all the way to 0 ohms. Of course, neither of these mechanisms makes you better at radiating power or receiving. > I can't remember of any mode demanding more than 30KHz (again, I'm not implying they don't exist). Lots of exotic, interesting modes, but also --- AM radio. 15KHz is in the human audible range (of younger people) and some AM stations carry audio this high. Occupied bandwidth of an AM radio station is 2 times highest audio frequency conveyed. Also, you probably are a few KHz off in tuning of your magloop, so you may have e.g. 20KHz on one sideband and 10KHz on the other within the bandwidth of the magloop. In turn you get the weird phasing artifacts and attenuation of higher frequencies that comes from receiving only 1.5 sidebands.

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
- mlyle 5y ago
  > Personally, I don't use random wires. For a long time I had a long random wire up and had a lot of success on it. But it would not tune on small segments of the bands because the impedance was very high or very low. And it was relatively deaf near those points, too. I shortened it slightly until those dead zones were in places I didn't care about. As the wire gets longer, those scary multiples of half wavelength become more common (though narrower). E.g. if I have a 150m electrical length antenna, around 17.987MHz, 18.986MHz, 19.986MHz, 20.985MHz, 21.984MHz, are really bad. This is if the wire is straight and far above the ground. If it also has random bends, you get even more places where funny things happen. And then, of course, really crazy things happen when you start trying to use this as a receive antenna up on UHF-- 399.7, 400.7, 401.7, 402.7, etc, not to mention the unpredictable, strong, and varying directionality. > 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? https://vk6ysf.com/longwire_antenna.htm https://vk6ysf.com/longwire_antenna.htm " A multi-band end fed wire antenna presents a potential problem in that a ¼ wave length antenna cut for say the 80 metre band will present a feed impedance of about 70 ohms however when the same antenna is used on the 40 metre band the feed point impendence will be several thousand ohms and therefore difficult to achieve a match even with a quality tuner. The trick is to making this type antenna an easier match by avoiding the lengths that are 1/2 wavelength and harmonically related to 1/2 wavelengths or simply multiples of 1/2 wavelengths that will present the most extreme rang of impedances." 4NEC2 graphs are in the article. Admittedly, 15k ohms is an extreme case: at some point, earth losses, etc, prevent you from getting all the way to infinity, and ohmic losses of the wire and feedline stop you from getting all the way to 0 ohms. Of course, neither of these mechanisms makes you better at radiating power or receiving. > I can't remember of any mode demanding more than 30KHz (again, I'm not implying they don't exist). Lots of exotic, interesting modes, but also --- AM radio. 15KHz is in the human audible range (of younger people) and some AM stations carry audio this high. Occupied bandwidth of an AM radio station is 2 times highest audio frequency conveyed. Also, you probably are a few KHz off in tuning of your magloop, so you may have e.g. 20KHz on one sideband and 10KHz on the other within the bandwidth of the magloop. In turn you get the weird phasing artifacts and attenuation of higher frequencies that comes from receiving only 1.5 sidebands.