Copyright © 2002-2019 by Harold Melton, KV5R. All Rights Reserved. Rev. 02/19/19
Spacing and Impedance
Don’t worry about it. A non-resonant antenna will present a feed point impedance of perhaps 35-5000ω, at various frequencies, so who cares about the exact feed line impedance? Matching the antenna to the feed line simply has nothing to do with efficiency (unless you’re using coax).
Anything from 1 to 6 inches is acceptable spacing. 1-inch #14 line is 370 ohms. 1-inch #18 line is 450. 6-inch #12 is about 600. It just isn’t at all critical - and don’t let any geezer or guru tell you different! The spacing should not be over 1 percent of the highest-frequency wavelength, and that’s the only real consideration with ladder-line spacing. On the other hand, it needs to stand-off from metal at least 2-3 times the line's width, so wider lines need longer stand-offs. I think 4-inch open line, with 12-inch stand-offs, is a good place to start.
As mentioned above, parallel feeders can pick up RF from the antenna and transport it into the shack as common-mode current (this means the two wires acting as one, in-phase). The way to avoid this is to avoid resonant lengths of ladder-line, and, if possible, bring the line away from the feedpoint perpendicular to the antenna (90°) for a far as possible before turning parallel to one side of the dipole. Setting your bend-point will also help you take up slack when using a non-resonant length, without having to cut off the excess.
In other words, measure your total run, then increase that to the next available “good” number, then route the line to take up the slack. A 300-foot (or 100 meter) open-reel tape measure is handy, and Harbor Freight has them for about $20.
Lengths to avoid (in feet): 32, 65, 96, 130, and 260 - and multiples of any of those. Don’t let them make you buy 100 feet when you know that’ll be too close to 96! Make them sell you 110 feet, for example.
Good lengths: Somewhere around 40, 80, 110, etc. This is from the ARRL Antenna Book.
Ladder-Line Length, Update 2019
Since writing this article in 2002, I’ve discovered there are several theories about calculating ladder-line length. It turns out that the ARRL Antenna Book is right about using non-resonsnt lengths, but that doesn’t present the whole picture. Common-mode currents may be effectively blocked with a good 1:1 current choke/balun.
However, there’s an even better reason to select non-resonant feed-line lengths, and that is: To avoid impedance extremes, and present an easy match to the tuner.
The Odd-Eighth-Wave Method
Various experts are now recommending that such systems should use odd-eighth-wave multiples of the antenna’s fundamental frequency. The easy formula is:
LL(feet) = 123 ÷ MHz × VF, x1, x3, x5, x7, etc. Since full-wave (feet) = 984 ÷ MHz, eighth-wave = 123. If you want to get it really close, multiply that by the Velocity Factor, which is about .91-.92 for window-line, and about .95 for insulated open-line.
Example: Dipole resonant at 3.75: (123 ÷ 3.75) × .91 = 29.848 feet; x3 = 89.5; x5 = 149.24; x7 = 208.9; etc. Not to imply it’s that critical — we just need to ensure that no current or voltage anti-node falls within any amateur band, to the greatest possible extent.
The reasoning is that such lengths will place impedance extremes between bands, and more moderate impedances within bands to make it easier on balun and tuner components. Or, put another way, keep the voltage and current anti-nodes out of the ham bands, and therefore, away from the tuner. For example, a certain length of line that is odd-eighth-waves of the fundamental will transform the low impedance of a current anti-node up to perhaps 500, and at double that frequency, transform the very high impedance of a voltage anti-node (like running an 80-meter dipole on 40 meters) down to around 500. So the tuner never sees an extreme mismatch, and we avoid both overheating things with high current, and arcing things with high voltage.
NOTE that I did NOT say using an odd-eighth-wave WILL give you 500 ohms on all the ham bands. I just picked 500 because it’s the approximate geometric mean of antenna impedance extremes, to use examples to explain the concept. If you have a VNA, feel free to do some experiments and let me know what you found.
Transforming both low and high impedances to medium impedances is also desirable because antenna tuners are less efficient with low antenna-side impedances, and are the most efficient when the antenna side sees impedances around 500Ω. See also, charts in G3TXQ’s Tuner Balun article.
For example, if our long and low dipole has a feedpoint impedance of around 25 ohms (a typical case for 75-meter dipoles well under 1/2-wave high) at resonance, and we duplicate that at the tuner end by using a half-wave of feed line, the tuner sees 25 ohms and its efficiency is a poor 60-70%. And if we use an odd-quarter, we invert the low impedance to extremely high, and things start arcing. But if we use an odd-eighth feed line, the tuner will see several hundred ohms, and its efficiency soars to around 95%! The extremes are still there, but they are an eighth-wave up the line, not at the tuner’s terminals.
Now when we double the frequency (160 to 80, or 80 to 40), suddenly we are running a full-wave dipole, and feeding a voltage anti-node, with a very high impedance. If we duplicate that at the tuner (with a half-wave of line), we arc the capacitor plates with high voltage, causing tuner damage (and maybe amplifier as well). Not good! Odd-eighth-wave to the rescue—it will transform both low and high impedance to medium, right where the tuner is both safe and efficient! Nifty.
I used 5/8ths-wave with my 80-meter (130') doublet, and found that it tuned well on all but 10-meters, so I had to trim the line a little. There is no magic number or formula for ladder-line length, because every antenna is different. But the odd-eighth-multiple is a good place to start.
From the MFJ-976 Balanced Line Tuner Manual:
“The following suggestions will reduce the difficulty in matching an antenna with a tuner:
- Never center feed a half-wave multi-band antenna with a high impedance feed line that is close to an odd multiple of a quarter-wave long.
- Never center feed a full-wave antenna with any feed line close to a multiple of a half-wave long.
- If this tuner will not “tune” a multi-band antenna, add or subtract 1/8 wave of feed line (for the band that won’t tune) and try again.
- Never try to load a G5RV or center fed dipole on a band below the half-wave design frequency. If you want to operate an 80-meter antenna on 160 meters, feed either or both conductors as a long-wire against the station ground.
- To avoid problems matching or feeding any dipole antenna with high impedance open wire lines,
keep the lines around these lengths. [The worst possible line lengths are shown in brackets]:
160 meter dipole: 35-60, 170-195 or 210-235 feet [Avoid 130, 260 ft]
80 meter dipole: 34-40, 90-102 or 160-172 feet [Avoid 66, 135, 190 ft]
40 meter dipole: 42-52, 73-83, 112-123 or 145-155 feet [Avoid 32, 64, 96, 128 ft]
Some slight trimming or adding of feed line may be necessary to accommodate the higher bands.”
Several other balun and balanced-line tuner manufacturers offer similar numbers. Good advice from them, but to me it’s just simpler to calculate an odd-eighth-wave multiple and put those impedance extremes between bands. What about 60 and the WARC bands? Well, you might not avoid an impedance extreme in all of them, but if you have trouble tuning a band, trim a little feed line and move the impedance extremes a bit. How much to add or subtract? An eighth-wave on the band that will not tune. Makes sense, eh?
What About “Skywire” Loops?
Myth (heard nightly): “Horizontal loops only radiate straight up!” Really? How is it then that many hams use them with great success, both regionally and, on the higher bands, for DX? The elevation pattern for a full-wave horizontal loop is almost identical to a dipole at the same wavelength high. High-angle NVIS on 160, 80, and 40; and lower angles on 20 and above. The loop also suffers in the azimuth plane from multiple nulls and lobes on the higher bands.
Keep in mind that a full-wave loop is resonant (~100j0 free-space, lower near earth) on every multiple, not just odd multiples like a half-wave dipole. That means that a 160 meter loop cut for ~1.78 MHz (1005 ÷ 1.78 = 564 feet) will be current-fed also on 80, 40, 20, and 10.
Like the big multi-band dipole, the big multi-band loop works best at about 68 feet high, where it’s a half-wave high on 40 meters (but it’s still okay at 30-45 feet), and ladder line feed. Please see NVIS Page 3 and the elevation angle plots therein.
Articles still abound, from the original Loop Skywire article decades ago, to the present, showing the “Skywire” fed with coax. And everyone I’ve ever known, or heard of, who did so was disappointed. But those that feed big loops with ladder line say they will never go back to dipoles, or coax.