Hardline Coax: Heliax and Rigid Cable for Low-Loss Runs

At 144 MHz a 30 metre run of ordinary coax costs you a tolerable amount of signal. At 1296 MHz the same run can eat more than half your power and spoil the noise figure that you paid so much to achieve. For long runs on VHF, UHF and microwave bands, the usual answer is hardline coax: a cable with a solid or corrugated metal outer conductor, a low-loss dielectric and very little flexibility. It is the cable of cell towers, broadcast stations and the serious weak-signal amateur, and surplus pieces appear regularly at good prices. This guide explains the types, how the loss compares with the flexible cables, how to fit connectors, how to handle the stiff cable and when it is worth the trouble.

What Makes Hardline Different

A flexible coax such as RG-58 or RG-213 has a braided shield, which flexes easily but leaks a little and adds resistance. In hardline the outer conductor is a continuous tube of copper or aluminium, either smooth or corrugated to give some bending ability. The tube gives a shield that is practically perfect, so the cable neither radiates nor picks up noise. The inner conductor is thick, often a copper tube or a copper-clad wire, and the dielectric is made of foam or, in the largest sizes, of air held in place by spacers. The thick conductors and the low-loss dielectric together make the attenuation much lower than that of flexible cable of a similar size. You can find the basics of the cable family in the article on coaxial cable, which includes the section on hard line.

The trade-off is stiffness. A hardline cable cannot be bent around corners like a patch lead. It has a minimum bend radius, often ten times its diameter or more, and a kink ruins it permanently. It is also heavy and needs supports at regular intervals. For this reason hardline is used for the long fixed run, and a short flexible jumper cable connects it to the radio or the antenna.

Types You Will Meet

The most common type is the foam-dielectric corrugated cable that engineers know by the brand name Heliax, which has become a generic word in the amateur world, although it is a trademark of one manufacturer. The sizes are given in fractions of an inch of the outer diameter: one quarter, three eighths, one half, seven eighths, one and a quarter and one and five eighths inches. The half-inch size is the favourite of amateurs, since it has very low loss and is still possible to work with by hand. Larger sizes have an even lower loss but are heavy and difficult to bend.

Smooth-wall rigid line is another type, with a solid tube. It cannot bend at all and is installed with fittings, as in some broadcast installations. Air-dielectric cable has the lowest loss, but it must be kept dry, often by a small supply of dry gas, and it is rarely used by amateurs. Finally, semi-rigid cable of a small diameter, used inside equipment and for jumpers at microwave frequencies, is a hardline of tiny size. The table compares typical choices.

How Much Do You Gain?

The exact attenuation depends on the manufacturer and the frequency, so use the data sheet of the cable you plan to buy, but the pattern is clear. At 144 MHz the difference between a good flexible cable and half-inch hardline is small for a short run and visible for a long one. At 432 MHz and 1296 MHz it becomes large, and on 10 GHz even hardline is too lossy, so the equipment is mounted at the antenna. A rough idea is that half-inch hardline has about half to two thirds of the loss of an LMR-400 type, which means a gain of about 1 to 2 dB on a 50 metre run at 432 MHz. That may sound small, but 1.5 dB is about 30 percent more power at the antenna and, on receive, a gain that no preamplifier can recover once the signal has been weakened in the cable. Our comparison of RG-58, RG-8X and LMR-400 lists the figures for the flexible cables, and the discussion of the receive side is in the article on the mast-mounted preamplifier.

Compare the cost too. A long run of expensive low-loss cable is sometimes cheaper than a mast-top amplifier with its sequencer and power supply, and it has no moving parts to fail. For a repeater site, where reliability counts, hardline is the normal choice.

Connectors: the Hard Part

You cannot solder a PL-259 on hardline. The outer conductor is a tube, often corrugated, and the connector is clamped or screwed onto it. The correct connector for each cable size is made by the cable manufacturer or by specialist companies, and the model depends on the cable, so match it to the exact type. In most cases the connector is an N type, or a 7/16 DIN type for high-power repeater use. The installation uses a few tools: a hardline knife or a pipe cutter, a deburring tool, a tool for removing the foam from the centre conductor, and a wrench set for the connector nut. Follow the instruction sheet of the manufacturer, and practise on a short offcut before you work on the real run. Our overview of RF connectors explains the families of connectors.

Cut the cable square and clean, take away burrs, and do not let metal chips fall into the dielectric. Any damage to the inner conductor or to the foam shows up later as a bump in the SWR. After you have made the connector, check it with an analyser and a dummy load before the cable goes up the mast.

Impedance, Velocity and Power Handling

Most hardline for amateur use has a characteristic impedance of 50 ohms, which matches the radios, but some surplus cables, especially older types from broadcast and cable television systems, are 75 ohms or have another value, so check the marking. A 75 ohm cable in a 50 ohm system works, but it gives a mismatch of 1.5 to 1, which is a loss in itself. The velocity factor of foam hardline is high, commonly 0.80 to 0.88, which is higher than that of solid polyethylene flexible cable at 0.66. When you cut a length for a stub, a phasing line or a feed for a stacked array, use the factor from the data sheet, as described in our article on transmission lines and the guide to velocity factor.

Power handling is another strength. The large conductors carry high power without heating, and at VHF a half-inch cable passes more than a kilowatt with margin. The limit is usually the connectors or the dielectric if the SWR is high, so a good match still matters. Because the cable is so well shielded, you will also find that a hardline run does not pick up local noise, which can be a real advantage in a built-up area where flexible cable acts as an unwanted antenna.

Installation and Handling

Plan the route before you unroll the cable. Measure it carefully, because cutting hardline short is permanent. Unroll the cable from the reel in a straight line, never pull it from the side of the reel, and avoid loops that can form kinks. Bend it only with a large radius, using a gentle curve around a form. Many installers use a bending spring or a shaped former for the tight corners. Keep to the radius specified by the maker, since a smaller one flattens the tube and leaves a permanent mismatch.

The weight is the next matter. A heavy cable on a tower needs clamps at regular intervals, typically every metre or so, to avoid movement and fatigue. On a mast, a stiff cable that hangs unsupported puts a load on the connector at the top and can damage it. Leave a service loop of flexible jumper at the antenna, so that the rotator can turn and the antenna can be changed without disturbing the hardline. At the bottom, a jumper into the shack and a grounding kit at the entry complete the installation.

Grounding and Weatherproofing

The outer conductor of a hardline is a good ground path, and it is customary to ground it at the top, at the bottom and at intervals on a long tower, with the grounding kits that the maker supplies. This reduces the voltages that lightning can create along the cable. Read the notes in our guide to station grounding and lightning protection for the layout of the ground system and the arrestor.

The connectors need good weatherproofing too. Water that enters the end of foam-filled cable can travel along the dielectric, so use sealed connectors, tape and mastic, as described in our article on weatherproofing coax connectors. Air-dielectric cable needs even more care and often a pressurisation or a dry-air arrangement, which is why the foam type is the practical choice for a private station.

Checking the Run After Installation

When the cable is in place, measure it. Connect an analyser at the shack end with the far end terminated in a dummy load, and sweep the band. The return loss should be high over the whole range, and any single sharp dip points to a bad connector or a dent. Then open-circuit the far end and measure the loss of the cable: the result is twice the one-way loss and shows whether the cable matches its specification. Keep the plot and compare it with a new one each year, since a rise in loss warns of moisture long before you can hear a difference. Write the length, the type and the date on a label at each end, because in ten years the person who must replace it, perhaps you, will not remember.

Buying Surplus Hardline

New hardline is costly, but used cable is common, particularly from decommissioned cell and broadcast sites. If you buy surplus, check four things. Look at the outside for dents, kinks and cuts. Ask for the type and the length, and check the markings on the jacket. Ask whether it has been in service and in what conditions. And measure it, if you can, with an analyser: the return loss on a dummy load should be good over the band, and a time-domain sweep will show any damage along the cable. A cable with water in the foam is very lossy, and it is almost impossible to dry. Reject a piece that has been cut open or left with the ends unprotected for a long time.

When Hardline Is Not the Answer

For HF, hardline gives hardly any benefit, since the loss of the common flexible cable is already small, and the stiffness is a handicap. For short runs, below ten metres at VHF, an LMR-400 type is simpler and nearly as good. For a rotating antenna, flexible cable is required in the loop around the rotator. And for the highest microwave bands, even the best cable is too lossy, and the correct approach is to place the transverter or the amplifier at the antenna, as explained in our guide to the 23 cm band, where a short run to the dish is common practice.

The Bottom Line

Hardline coax uses a solid or corrugated metal outer conductor and a low-loss dielectric to give the least loss of any practical cable, which pays off on long runs at 144 MHz and above. Half-inch foam corrugated cable is the favourite for amateurs. Its price is stiffness, weight, special connectors and a need for careful installation, with flexible jumpers at both ends, grounding kits and good weatherproofing. Choose it for long VHF, UHF and 23 cm runs, buy surplus with care and test every piece, and use flexible cable where the run is short or the antenna moves.

Frequently Asked Questions

What is hardline coax?

It is coaxial cable with a solid or corrugated metal tube as the outer conductor and a low-loss dielectric. It has very low loss and excellent shielding, but it is stiff.

Is Heliax the same as hardline?

Heliax is a brand name for a corrugated foam-dielectric type of hardline. Many amateurs use the word for any similar cable.

How much loss do I save with half-inch hardline?

Roughly half to two thirds of the loss of an LMR-400 type, depending on the cable and the frequency. The gain matters most on long runs at 432 MHz and above.

Can I use PL-259 connectors on hardline?

No. Hardline needs special connectors, usually N type or 7/16 DIN, that clamp onto the outer tube and are chosen for the exact cable.

How tightly can I bend hardline?

Only with a large radius, often ten times the diameter or more, as stated by the manufacturer. A kink ruins the cable.

Is surplus hardline worth buying?

Yes, if you inspect it for dents and cuts and test it with an analyser. Water in the foam makes the cable very lossy and is hard to remove.