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I currently operate a 20M half-wave dipole with a balun. It's a little lower than ideal, but it gets me on the air.

I've read about folded dipoles. But I've found contradicting information about them. So here are a few questions:

  • Does a folded dipole really have "wider bandwidth"? Some sources say "yes" because that's what the textbooks say, others say "no", based on their real wold experience

  • Is a folded dipole less succeptible to QRM? Would it work better in a city, compared to a regular dipole?

  • What's its impedance? The theory says it's closer to 300 ohms, but this number can vary wildly and even be much lower. If it's "much lower" than 300 ohms, will feeding it with 300 ohms line have significant loss from mismatch?

  • What is required to feed it?

  • Is it a "miracle antenna" that will work in any band when tuned with a suitable balanced tuner?

  • Is a folded dipole just a waste of time and money, giving diminishing returns over a regular dipole?

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Recently Kevin Loughin, KB9RLW made a few videos about folded dipoles and how they perform comparing to regular dipoles. I believe these videos will answer all your questions.

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  • $\begingroup$ very useful videos! $\endgroup$ – hjf Oct 28 '19 at 14:29
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Does a folded dipole really have "wider bandwidth"? Some sources say "yes" because that's what the textbooks say, others say "no", based on their real wold experience

A little more, yes, mostly just because they are thicker than a simple dipole. The same increase in bandwidth could be achieved by making an ordinary dipole out of sections of ladder line with both conductors shorted.

Is a folded dipole less succeptible to QRM?

No. I'd expect behavior exactly like a similar dipole.

What's its impedance?

When constructed of two equally sized conductors, 4 times that of a dipole. So if an ordinary dipole has an impedance of 75 ohms, a folded dipole will be 300 ohms.

As with any antenna, the actual impedance can be influenced by proximity to ground, straightness or bending of the conductors, losses, and feedline isolation.

What is required to feed it?

As with any dipole, if you intend to feed it with an unbalanced feedline (like coax) then you'll want a balun. Additionally, some kind of impedance matching may be desired if the feedline's characteristic impedance does not match the folded dipole, otherwise the feedline will operate with a higher SWR, increasing losses.

Both concerns can be addressed by constructing the dipole from tubing, and running the feedline inside the dipole, and matching with a 1/4 wavelength of 121 ohm coax. VE2AZX has a nice writeup. Many VHF commercial stations use this construction method.

Is it a "miracle antenna" that will work in any band when tuned with a suitable balanced tuner?

No.

Is a folded dipole just a waste of time and money, giving diminishing returns over a regular dipole?

Demonstrably no, since there are commercial applications of folded dipoles, and commercial applications care very much about not wasting time or money.

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Is it a "miracle antenna" that will work in any band when tuned with a suitable balanced tuner?

No more than a regular dipole. A dipole of an appropriate length, fed by ladder line to reduce the standing wave losses (often called a "doublet") can be pretty close to a miraculous all-band antenna with a little care, a little luck, and a big tuner. A "folded doublet" with an insulator at the center where the two ends meet behaves pretty much the same, only with 4x the impedance. This may be good, bad, or indifferent (depending on lots of factors) but as tuners are generally a bit better at matching high impedances than low ones, perhaps it's a slight win.

There's also the terminated folded dipole, which is commonly advertised as an all-band antenna with an SWR < 2 across several octaves of frequency — and it is, but only because of the terminating resistor that's placed at the center where the two ends meet. This resistor takes whatever voltage is left at the ends of the elements and dissipates it, turning it into heat. So you get a consistent "all-band" impedance, but the cost is that your antenna is literally part dummy load, with the efficiency varying very much with frequency.

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  • $\begingroup$ I was just asking my dad (LU9GBR) about this! That I think the "termination resistor" is just a dummy load to make the tuner happy, but it's just dissipating away any power you think you're transmitting. If a dipole is infinity SWR at a certain frequency, the resistor will just eat up all the power. You won't be getting any of that RF on the air, and yet you'll see a SWR of 1. Is this correct? $\endgroup$ – hjf Oct 29 '19 at 1:07
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    $\begingroup$ @hjf not quite -- the dipole is always radiating something if there's current flowing, the efficiency is never 0%. But it can be very low, especially as the frequency gets low and the dipole becomes an insignificant fraction of a wavelength. Then it's almost as good as if the termination resistor was directly at the feedpoint :) $\endgroup$ – hobbs - KC2G Oct 29 '19 at 3:06
  • $\begingroup$ ladder-line losses aren't significantly lower than high-quality coax (like LMR-400) operating at a similar SWR. $\endgroup$ – Phil Frost - W8II Oct 29 '19 at 19:06

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