Why Ladder Line?

When faced with the need to feed an antenna, most people automatically reach for coax. Coax is convenient and easy to install, it is shielded so you don't have to worry about RF, and everyone uses it so it must be much better than balanced open line, right?

So let's have a look at how coaxial cable (say, RG-213) compares to typical 600 ohms ladder line.

  • RG-213: Shop-bought at considerable expense; radiates RF when mismatched, typical loss: 4.8dB / 100m @ 30MHz.
  • Ladder line: Easily constructed for a few cents per meter; radiates very little RF even when severely mismatched; typical loss: 0.33dB / 100m @ 30MHz.

Any questions?

RF Radiation

First of all, it is a common misconception that because coaxial cable is shielded, it will not radiate RF. True, the shielding does prevent RF from being radiated by the cable, but only if there are no shield currents, which occurs whenever current flows in an unbalanced manner along a transmission line. With coaxial cable this means that current exists on the outer surface of the braid and since there is no equal and opposite current in close proximity to counterbalance its effects, the coax will radiate. That such a current exists is purely and simply a matter of balance; it has nothing whatsoever to do with the degree of mismatch that may exist between the impedance of the load and the characteristic impedance of the coax. Standing waves, so caused, thus make no difference to whether the coax radiates or not. The only factor that causes outer-braid current to occur is the degree of unbalance that exists between the line and the load. It is for this reason that a balun is used to correct for any such unbalance. (With thanks to Prof. Brian Austin for correcting our previous statement and Dr G.W. Immelman for his assistance).

And of course no antenna is 100% ideal in the real world. The balun (if there is one at all!) may be imperfect. The frequency at which the antenna is being used is unlikely to be the exact single frequency at which it resonates. There may be an unbalance in the dipole's feeding arrangement. The shield of the coaxial cable may pick up a part of the RF that is being radiated by the antenna. These and many other real-world factors practically ensure that in practice a coaxial cable will generally radiate RF, both in and out of the shack. Which is of course why so many radio hams resort to ferrite and chokes in an attempt to suppress shield currents and the TVI they cause!

A balanced feed line, on the other hand, relies on two parallel conductors positioned very close to each other, i.e the distance between the conductors is minute in comparison to the wavelength of the signal. This means that the radiation from both conductors almost cancel each other out. Almost, but not quite, because 100% cancellation would require the conductors to be so close to each other that they literally occupy the same space, which is of course impossible. In practice, though, the distance between the conductors is still easily small enough so that radiation is neglegible. Note that this is not dependent in any way on match or mismatch: as long as the system remains balanced, radiation will be extremely low (usually much lower than that of coax) even when the feed line is severely mismatched.

This is perhaps the most important difference between a balanced feed line and coaxial cable: the amount of radiation from the cable is constant, and not dependent upon length, frequency or resonance. With coaxial cable, a bad SWR will immediately result in RF in the shack, as well as TVI. With a balanced feed line, on the other hand, even the worst mismatch will not increase the amount of RF it radiates. Due to its intrinsic low loss characteristics a balanced feed line will also allow for a very high SWR without significant power losses. This means that this type of feed line can be used to feed the proverbial random length dipole on any frequency (with a transmatch between the feed line and the radio) without any serious consequence. Case in point: before coaxial cables came along, few radio amateurs had ever used an SWR meter, and the antenna systems they used in those days often presented "bad" SWR's of 8:1 or 10:1 or worse. But because balanced feed lines were used, this was never a problem. All that mattered was to tune up the plate and load capacitors on the transmitter to match impedances, so that RF power would be transferred correctly to the antenna system, and off you went. Only when coax became popular, the SWR meter became the most often used instrument in the shack, and radio hams started to spend a lot of time hunting for that elusive 1:1 SWR.

Cable Types

Balanced feed line cable comes in various forms, shapes and sizes. One of the oldest commercially available types is the well-known 300 ohms ribbon cable (also known as tape or twin lead) which was used for FM radio and VHF television antennas before 75 ohms coaxial cable became the standard for that application. Ribbon cable comes in two varieties: one for indoor use and one for outdoor use. The latter is black, and can be either a thin ribbon with a dumbellshaped cross section, or a thick ribbon with a cylindrical or lenticular cross section. Solid and foamed dielectrics may be used. This type of cable is difficult to obtain in South Africa, and often expensive when found.

The other variety is known as window line. Unlike ribbon cable it is not a continuous ribbon, but it has rectangular "windows" cut out of the centre. This reduces weight and loss, and increases the velocity factor of the cable (see below). The characteristic impedance is generally 450 ohms, although 460 and 480 ohms types are also manufactured. Unfortunately this type of cable is almost impossible to obtain in South Africa. Generally it has to be sourced from overseas at considerable expense, and it is not cheap to begin with.

The third form, ladder line, is not readily available commercially, but ideally suited for "home brewing" by the radio ham. It uses regular wire which may or may not be insulated, it is inexpensive, and it can take a considerable amount of RF power. The characteristic impedance depends on wire thickness and spacing, but is generally in the order of 600 ohms.

Comparison

The following table compares some typical characteristics for various cable types.

Type
Advantages
Disadvantages
Zc
Vf
Loss
RG-58 coax
Easy to use; medium weight
Significant loss
50
0.77
6.6
RG-213 coax
Relatively easy to use; medium loss
Relatively heavy and inflexible
50
0.66
4.8
Ribbon / tape
Light weight; low loss
Difficult to obtain; expensive; properties change when ribbon collects rain water; takes limited power
300
0.82
0.55
Window line
Low to medium weight; even lower loss; takes medium power; low sensitivity to rain
Very difficult to obtain in South Africa; very expensive when imported
450
0.90
0.48
Ladder Line
Low weight; very inexpensive; extremely low loss; takes high power; insensitive to rain
Requires some simple home-brewing
+- 600
0.97-0.99
0.33

Legend: Zc is the characteristic impedance in ohms; Vf is the velocity factor; Loss is specified in dB per 100m @ 30MHz. Note: some cable types are available with different types of dielectic (e.g. solid vs. foam) which improves their loss figures and changes their velocity factors.

Keep in mind that the typical loss figures for feed lines (like the ones in the table above) assume that the feed line is correctly terminated. If there is a mismatch (e.g. when using a short, non-resonant anttenna) the signal bounces back and forth across the feed line, and losses increase exponentially. If coax is used in this case, only a small fraction of the RF energy actually reaches the antenna, while the rest disappears in coaxial losses! The dramatically lower loss of ladder line really comes into its own here. For more details see the article on random length dipoles.

Installation tips

While coaxial cable can be mounted against walls, steel towers, gutters etc., ladder line should "hang freely". This can be easily achieved by suspending the antenna from a pulley or stand-off at the top to keep it from touching the mast; the bottom end of the ladder line can be easilly suspended from any convenient support with nothing more complicated than a piece string or rope.

In South Africa a popular way of getting coax into the shack is to run it through the hole blocks found in most walls. The same method can be used for ladder line. In this case both conductors are fed through the hole block using different holes with appropriate spacing. Note: if there is a metal screen between the inner and outer hole block, openings in its mesh should be made to prevent the ladder line conductors from touching the screen.

With coax it is easy to run a cable through an open window and then close the window, clamping the cable between the window and the frame. Ladder line, on the other hand, should be kept a short distance away from metal, because the RF field of both wires may couple with metal and therefore unbalance the currents in both wires, causing the ladder line to radiate. In practice this is not a problem: a few simple stand-offs are enough to keep the feed line 8-10cm away from metal. This is pretty much the only drawback of ladder line: you can't tape it to your metal tower or roof gutters like you can do with coax. It is also not advisable to clamp ladder line between a closed window and the window frame, unless the window frame is entirely made out of wood and there is no proximity with any metal parts. Given the fact that most window frames in South Africa are made out of steel, running a balanced feed line through the window and then closing the window to clamp the cable in place is usually not recommended.

If it is impossible or impractical to use ladder line all the way into the shack, the last few meters may be covered using coax. This is not preferable, but sometimes it's the only practical choice. In this case a balun should be used between the coaxial cable and the ladder line to prevent excessive shield currents in (and therefore excessive RF radiation by) the coax. Baluns may be housed in a standard electric box which can be mounted on a pole, tree or wall. Ceramic or plastic insulators, cable ties and/or rope can easily be used in a variety of ways to suspend the ladder line and keep it hanging freely.

The impedance of ladder line is a factor of the diameter of the wires used, and the distance between the wires. Keep in mind, though, that one of the most important properties of balanced feed lines is that the characteristic impedance is not critical! Anything within reason (200 ohms, 300 ohms, 450 ohms, 600 ohms, etc.) will do nicely.

See the baluns page for more tips on baluns.

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