Here, we first talk about Ladder Line (a.k.a. Twinlead) and provide some information about coax at the end of this page.
Ladder line can provide a very low-loss transmission line between the antenna (usually some variation of a dipole or multi-band doublet) and the tuner.
It easily tolerates the frequency-dependent, widely varying SWR found at the junction of the feedline and the antenna. Ladder Line presents very little signal loss compared to coax. Coax will not tolerate very high SWR’s but Ladder Line will.
Where it terminates at the tuner, install a 1:1 or 4:1 Current Balun (a.k.a. Guanella type) of sufficient size to handle the power used. The 1:1 is preferred but if you already have a 4:1 installed and the system works properly, just leave it. So use what you have or can wind. See the Antennas>Baluns and Ununs section of this website.
Here’s an interesting Tidbit:
Concerning a transmission-line, US-Army manual TM11-675 (Aug 1951) states that “…the greater portion (about 90%) of the transmitted energy is in the electromagnetic waves that the line conductors guide through the space between them. In general, less than 10% of the transmitted energy is actually in the conductors of which a well-designed line is comprised.” This is very interesting (and pardon their bad grammar).
Even if a product is advertised as “450 ohms,” it is probably not 450 ohms. The approximate impedance ratings of the most common Ladder Lines are:
1 inch wide #14 ladder line = 370 ohms
1 inch wide #18 ladder line = 450 ohms
6 inch wide #12 ladder line = 600 ohms
Here is The Wireman’s table of twinlead parameters
KV5R has posted six pages of information about ladder line – read all six!
W1NPP has an excellent article about the uses of ladder line.
If you need to splice ladder line or any other line that might be under tension, use the pictorial “Western Union Splice” found at: https://www.instructables.com/How-to-Do-a-Western-Union-Splice/
Note that any splice will introduce an impedance “bump” into the feedline but this will probably not matter as the feedline impedance usually varies greatly depending on the frequency in use and/or the length of the line. This assumes a well-made splice.
Add enough to the antenna lengths in the table to allow for installing the center and end insulators; probably eight inches for each wrap (4 wraps in this case; wrap length depends on the insulator size), plus enough to attach the feedline in the antenna center (this length hangs down a bit from the center insulator.
Remember that when calculating the active part of the antenna length, the section of wire used for wrapping, after passing the wire through the hole in the antenna insulator, is not counted as part of the antenna length.
For example, an antenna of about 137 feet would be a center-fed dipole on 80 meters, but it can also be used as a multi-band “doublet” on 40 through 10 meters if fed with ladder line connected to a 1:1 or 4:1 Guanella current balun at the tuner’s antenna input. It might go to 6 meters as well; I haven’t tried that.
Since ladder line is so low-loss, this is, in my opinion, the best way to feed this type of antenna and the multi-band operation is a bonus. It can be a horizontal antenna or configured as an inverted Vee with the apex as high as possible, and the angle between the two legs about 100 – 120 degrees.
Don’t worry about what the SWR is at the point where the twin lead feedline is connected to the dipole/doublet; it will vary greatly depending on the frequency in use. Feedline loss will be minimal; much less than a typical coax setup. If you use a feedline of the recommended length (see below), it should tune nicely on HF bands.
In the event that your choice of feedline length is not liked by the tuner and a high SWR is present at the tuner on some band, just rig up a simple and quick way to add a 1/8 wavelength (at the offending frequency) of twinlead to the feedline, right there in the shack. You might have to experiment with this; try a shorter length…use whatever works on that band.
In my situation, using a 137 foot doublet fed with twinlead, I have to add quick-plug-in addition for 30 meters because I made a poor choice of feedline length. I had not drunk enough coffee when I designed it.
Note that, if you tie the two sides of the ladder line together at the tuner without the balun, you might be able to use it as an old-fashioned “Tee” antenna on 160 meters. Mine works this way but there might be a bit of RF in the shack since the feedline is now a single wire like what was used 100 years ago.
ODD MULTIPLES of 1/8 wavelength should be used for the Ladder Line feed line length; see below. This way, we won’t be presenting a maximum voltage and high impedance, at the tuner.
For example, a wavelength is about 66.4 feet at 14.1 mhz. 1/8 of this would be 8.3 feet. The third (odd) multiple of this would be 24.9 feet, the fifth multiple would be 41.5 feet and so on.
Attach the ladder line to a 1:1 current (Guanella) balun feeding the tuner. Read about 4:1 and 1:1 baluns on this website, in the menu’s Antenna>Baluns section.
DJ0IP makes it simple: The CENTER chart’s black line (bottom of chart) shows feedline lengths that will work with his 111 foot doublet, using “450 ohm” twinlead. Note that he is using a 111 foot doublet; these lengths are not to be used for the 137 foot doublet described above.
How do we know if the currents in both sides of the ladder line are equal? If they are not, the ladder line is radiating and all is not well. VK5BR has designed a simple instrument you can easily build to check each side’s current. It can work on coax, too, to detect radiation from the shield. Here is a follow-on article with enhancements, about the test meter.
Now to Coax:
Need to use coaxial cable to feed the antenna rather than ladderline? This article explains much about coax and presents a table of coax types, their losses per 100 feet and other information. It is presented by the “Off Grid Ham” website. There is much information about coax on the Web but this article provides what you need to know.
Here is an extensive chart showing wattage capabilities at varying frequencies, for many types of Coax.