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I've heard if I have a dipole of some arbitrary length, or a random wire, the SWR can be very bad. An antenna tuner can present a decent match to the transmitter, but the SWR on the feedline will be very high, and thus ladder line is required.

Is ladder line really required, as opposed to coax? What are the essential properties of a feedline in this application?

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  • $\begingroup$ +1. Maybe we'll learn something. Those coax losses on Owen Duffy's site are not near as bad I would have guessed. $\endgroup$ – Mike Waters Aug 9 '17 at 20:32
  • $\begingroup$ Look at my edited answer. Are those mismatched line losses for all those different feedlines for that antenna wrong? I also used TLDetails and it pretty much agrees with Owen's calculators. $\endgroup$ – Mike Waters Aug 9 '17 at 22:41
  • $\begingroup$ @MikeWaters No they're not wrong, but you're comparing 50 ohm coax to a 5000 ohm load and an SWR of 100, to 450 ohm ladder line for an SWR of 11. Use an impedance transforming balun and you'll find the performance gap closes. $\endgroup$ – Phil Frost - W8II Aug 10 '17 at 13:18
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This can indeed be a fine setup. With a suitably low-loss feedline, and a good tuner, just about anything can be made to function as an effective antenna.

There are two factors that determine the losses in this situation:

  1. the matched loss of the feedline, a function of length and the attenuation specified in the datasheet, and
  2. the operating SWR on the feedline, which depends on the ratio of the feedline and load impedances.

When the SWR is greater than 1:1, some fraction of the power is reflected from the antenna, back towards the transmitter. The antenna tuner then re-reflects this reflected power back at the antenna.

Eventually, all the energy is either radiated by the antenna, absorbed by the feedline, or absorbed by the tuner. Since each parcel of energy is making multiple "bounces" between the tuner and the antenna, it's subject to the feedline loss multiple times. Obviously, lower feedline loss means less loss. A lower SWR means fewer bounces back and forth, so again less loss. See What is the actual loss in a feed line with high SWR?

Ladder-line does not intrinsically have lower loss than coax. For example, this 300 ohm ladder line sold by DX Engineering has a loss of 0.668 dB per 100 feet at 30 MHz. Compare to Times Microwave LMR-400 at 0.7 dB for the same. There's better coax, and there's better ladder-line, but generally the performance of a transmission line is a function of the dielectric and conductor resistance (mostly the latter at HF), not whether it's coax or ladder-line.

enter image description here
specified attenuation for DX Engineering ladder-line per 100 feet

enter image description here
specified attenuation for LMR-400

Note the significantly higher loss when the ladder-line is damp. Coax maintains its performance when wet, covered in snow, ice, or dirt, when buried, run through conduit, attached directly to a tower, etc. This is why commercial installations overwhelmingly prefer coax.

Ladder-line does tend to have a higher characteristic impedance, somewhere between 300 and 600 ohms for common designs. This is important.

A resonant dipole is at an impedance minimum, around 72 ohms. So when the dipole isn't resonant, the impedance will be higher, with a typical maximum around 5000 ohms.

Modelling software and antenna analyzers will give an SWR, but this is not the SWR on the feedline unless the feedline has a characteristic impedance of 50 ohms. Given our dipole which may be between 72 and 5000 ohms, this gives a possible SWR from 1.44:1 to 100:1.

With 300 ohm ladder-line, the antenna analyzer will still say the SWR is 100:1 in the worst case. But actually the SWR is only:

$$ { 5000\:\Omega \over 300\:\Omega } = 16.7:1 $$

This is why ladder-line tends to have a lower loss in this antenna system. Not because it is an inherently lower-loss feedline, but because the SWR is reduced.

However, if you wish to use coax, it's an easy problem to solve. A 4:1 balun on 75 ohm coax divides the load impedance by 4. So again calculating the SWR at the worst case 5000 ohm load:

$$ { 5000\:\Omega\:/\:4 \over 75\:\Omega } = 16.7:1 $$

That's the same SWR as the 300 ohm ladder-line. If the coax and ladder-line have equal matched loss, their total losses will be the same also.

A balun with a higher transformation ratio will reduce the SWR further, and losses with it. If the antenna impedance could be anywhere between 72 and 5000 ohms depending on operating frequency, then the best "compromise" feedline impedance is:

$$ \sqrt{5000\:\Omega \cdot 72\:\Omega} = 600\:\Omega $$

This gives a worst-case SWR at the extremes of high and low antenna impedance of:

$$ {600\:\Omega \over 72\:\Omega} = {5000\:\Omega \over 600\:\Omega} = 8.3:1 $$

Not such a horrible SWR. This could be realized with:

  • 600 ohm ladder-line, or
  • 300 ohm ladder-line and a 2:1 balun, or
  • 75 ohm coax and an 8:1 balun, or
  • 50 ohm coax and a 12:1 balun.

So, in summary:

  • Ladder line is not necessarily lower loss, but it does have a higher characteristic impedance.
  • Non-resonant antennas tend to have higher feedpoint impedances.
  • With an impedance transforming balun, coax can have the same SWR as ladder-line.
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  • $\begingroup$ also not that ladder/twin-lead can easily be made to have a large range of impedances by varying the spacing between the leads, or the diameter of the leads. On the other hand, ladder is usually unsuitable for higher frequencies (if in doubt, if it's > 300 MHz, don't), since the wavelength needs to be large compared to the distance between the leads. You can see that in network cabling: twisted pair, as far as I interpret it, is a 125 MHz (and maybe the 3·f harmonic of the ideal square wave) transmission line, and that works well enough for Gigabit Ethernet to with limited equalizing; $\endgroup$ – Marcus Müller Aug 9 '17 at 22:23
  • $\begingroup$ for 10 Gigabit Ethernet, it took a while until 802.3an (i.e. 10GBase-T over twisted pair) actually was implemented commercially, and 10GBASE is still mostly done using optical or Twinax direct connect cables. And that with a bandwidth of 400 MHz + sidebands! $\endgroup$ – Marcus Müller Aug 9 '17 at 22:25
  • $\begingroup$ @MarcusMüller "the wavelength needs to be large compared to the distance between the leads" and specifically it's hard to make ladder line that has a consistent spacing at a smaller scale, yes? $\endgroup$ – Kevin Reid AG6YO Aug 9 '17 at 23:28
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    $\begingroup$ Related: Why must twin-lead conductor spacing be small to avoid radiation? $\endgroup$ – Phil Frost - W8II Aug 9 '17 at 23:48
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    $\begingroup$ Just to add something to this that really helped me to understand it: The impedance at the antenna is not present at the input of the feedline. Say your worst-case impedance of your coat hanger antenna is 5000 ohms, at the input of the feedline you will have an impedance different from the characteristical impedance of the coax that depens on the load and length of the line, say 500 ohms (theres a formula for this). The tuner now matches to this impedance. There is however still a mismatch between the line and the antenna. With high-loss coax this will "eat" your output power. $\endgroup$ – GeeF Sep 25 '18 at 17:13

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