Direct Burial Coax: Burying Ham Radio Feedlines the Right Way
A cable that runs across the garden to a far antenna is an eyesore, a trip hazard and a victim of the lawn mower. Burying it seems the obvious answer, and many hams do so, but a coaxial cable in the ground lives in a harsh place: water, frost, chemicals, rodents and the weight of the soil all attack it. Some cables are made for this life and many are not. This guide explains what direct burial coax really is, when conduit is the better choice, how to dig and lay the cable, and how to keep water out of the shield, which is the failure that ruins most buried feedlines in the end.
What Happens to Coax in the Ground
The enemy of a buried coaxial cable is moisture. The structure of coaxial cable has a central conductor, a dielectric, a braided or foil shield and an outer jacket. If the jacket is cut or ages, water enters the braid and runs along it by capillary action, like water in a wick, sometimes for many metres. Once the dielectric is wet, the loss rises dramatically, particularly at VHF and above, and the cable becomes noisy and unreliable. Water is not the only problem. The soil may contain acids and salts, the roots of plants push against the jacket, and the freeze and thaw cycle moves the cable and flexes any weak point.
Ordinary coax such as the common RG-58 or RG-8X types has a PVC jacket made to be used in air. PVC contains plasticisers, which leach out over the years in damp ground, so the jacket gets hard and cracks. For this reason such cable is not recommended for burial unless it is placed in a conduit. Polyethylene jackets, in contrast, resist moisture and ground chemicals much better, and cables made for burial use them.
Direct Burial Cable: What the Label Means
A cable sold as direct burial has a jacket of polyethylene or a similar material that is designed to stay in contact with soil. Some of these cables, usually called flooded, also contain a gel between the shield and the jacket, which seals small cuts and stops water from travelling along the braid. These cables are the safest option if you want to put the coax directly in the trench without a conduit. Some of the larger low-loss cable families, for example the 400 size, are available in versions marked DB for direct burial. The specification sheet of the manufacturer states whether the cable is rated for it, and you should check the sheet rather than depend on a general description.
Direct burial does not mean indestructible. Sharp stones, a spade or a rodent can pierce the jacket, and an unprotected cable can also be damaged by settling of the ground. The label means that the material itself will survive the soil, not that you can ignore the trench. If you need more guidance on the properties of the common cable families before you choose, read our comparison of RG-58, RG-8X and LMR-400 coax.
Conduit or No Conduit?
A conduit is a plastic pipe, usually PVC, that you bury and then pull the cable through. It gives mechanical protection and allows replacement of the cable later without digging, which is a decisive advantage, because the antenna and the cable will probably not stay the same for twenty years. The cost is the pipe, the fittings and the time. The main weakness of a conduit is that water can still get in, and since it cannot drain, the cable may sit in a damp tube for years. For this reason a cable in conduit should still have a good jacket, and the conduit should be laid with a slope so that the water runs out, or have a drain at the lowest point. The table compares the three common approaches.
| Method | Advantages | Disadvantages | Best for |
|---|---|---|---|
| Direct burial rated cable, no conduit | Cheapest and fast to lay | Hard to replace, damaged by digging | Short runs in soft soil, low-risk areas |
| Ordinary coax in conduit | Replaceable, protected from stones | Water can collect, jacket may age | Runs under paths, near structures |
| Direct burial or flooded cable in conduit | Best protection and replacement | Highest cost and effort | Long runs, expensive cable, permanent stations |
In practice the third choice is worth the money for any run you cannot easily dig up, and the first suits a temporary or a short installation. Many amateurs lay a spare conduit alongside the first, or leave a pull string in it, so that a second cable, for control or for another band, can be added later at no cost.
Planning the Route
Walk the path before you dig. Mark where the cable will go with a rope or a line of spray paint. Choose the shortest route that avoids large tree roots, drainage lines, existing pipes and electrical cables. Find out what is buried on your property and check the local rules before you start: in many places the utility companies provide a locating service, and digging without it is illegal. Keep the coax apart from the power lines. Many electrical codes demand a minimum distance between low-voltage signal cables and mains cables, and in addition the mains will couple noise into the coax if the two run side by side for a long distance.
Plan the ends as carefully as the middle. At the house, you need an entry point, usually with a lightning arrestor and a grounding bar, as described in our guide to station grounding and lightning protection. At the antenna, the cable should come out of the ground with a gentle bend and a support, so that the weight and the movement of the antenna do not pull on the buried part. Leave a loop of spare cable at each end, in case you need to redo a connector.
Digging the Trench
The depth depends on your local rules and on the frost line. A common recommendation for low-voltage cables is about 30 to 45 centimetres, and deeper where the ground freezes, but check the requirements of your area, because they differ. A deeper trench protects against the spade and the lawn aerator, a shallower one is easier to dig. The width needs only enough space to lay the cable without bends.
- Dig the trench and remove stones and sharp objects from the bottom.
- Put a layer of sand, about five centimetres deep, in the bottom as a bed. Sand drains well and does not damage the jacket.
- Lay the cable or the conduit. Do not pull the cable tight, since the ground will settle and a little slack is helpful. Keep the bend radius above the manufacturer’s value, which is typically ten times the diameter of the cable or more.
- Cover with another layer of sand, then add the soil. Compact the soil gently in layers, not with a heavy machine.
- Lay a marker tape about 15 centimetres above the cable, with the words warning buried cable, so that anyone who digs there later receives a warning.
Take photographs of the open trench with a tape measure and write the route on a sketch of the property, with distances to fixed points. In five years, you will not remember where it goes.
Sealing the Ends and the Connectors
A buried cable is only as good as its ends. The cut end of a cable, as well as any open end of a conduit, is the usual way for water to enter, so do not leave any end open, even for a day. Seal the end of the cable with tape or an end cap while you work. After you fit the connector, protect it with self-amalgamating tape and a layer of mastic or heat-shrink tubing with an adhesive lining, as described in our article on weatherproof coax connectors. N connectors are better than PL-259 connectors for outdoor use because they have a gasket, and our guide to RF connectors shows why.
Where the cable enters a conduit, make the opening face downward or fill it with a non-hardening sealant such as duct seal, to stop rain and insects. At the point where the cable leaves the ground, form a drip loop so that water runs off before it reaches the connector. Any joint that is made underground, such as a barrel connector, is a weak point and should be avoided. If you must splice, put the joint in a weatherproof box above the ground, where you can check it.
Lightning and Grounding of Buried Coax
A buried cable is not safe from lightning. The earth around a strike carries a large current, and a cable in the ground can pick up high voltages, especially if it is long. The shield should be grounded at the entry to the house, and the arrestor should be installed before the cable enters the shack. A ground rod where the cable leaves the antenna, connected to the shield and the antenna base, reduces the voltage that can build up along the cable. Never connect the shield to the ground in a way that creates a loop with the house ground at a different point; use one ground system tied with short, thick conductors.
Testing the Cable and Finding Faults Later
Before you fill the trench, measure the cable with a network analyser or an antenna analyser: return loss, SWR at the working bands and the cable loss. Note the results, because you will compare them with later measurements. Then, once a year, measure again. If the loss increases or the SWR changes without any change in the antenna, water or damage in the cable is likely. A time-domain measurement, as in our article on finding coax faults with a NanoVNA, can locate the damaged point to within a short distance, which saves digging the whole run.
Special Cases: Rock, Clay and Wet Ground
Not every garden has soft soil. In stony ground, a deeper bed of sand, up to ten centimetres on all sides of the cable, prevents sharp stones from pressing on the jacket, and a conduit becomes more attractive because the pipe spreads the load. In heavy clay, water drains badly and the trench can act as a drain, so the cable lies in water for long periods. Here a direct burial rated cable, preferably a flooded type, in a sloping conduit with a drain at the low point is the safe choice. In places where the ground freezes deeply, clay and water expand with frost, and the sand bed and the slack in the cable allow some movement without pulling on the connectors.
Where the route crosses a path or a driveway, use a conduit of strong material, bury it deeper and mark the crossing, since vehicles add heavy loads. Where it runs close to trees, avoid the root zone, because digging cuts roots and the roots later press on the pipe. Where you need to pass a wall, use a proper wall sleeve with a downward slope to the outside, and seal the inside with a flexible sealant. Finally, if rodents are common in your area, a conduit with sealed ends offers better protection than a bare cable, since small animals can chew through a plain jacket.
Common Mistakes
The first mistake is to use ordinary PVC-jacketed cable in the ground without a conduit, and to be surprised two years later by a cable that has become stiff and wet. The second is to leave an end open while waiting for the connectors. The third is to bury the cable too shallow, where the lawn mower or a garden fork will reach it. The fourth is to run the coax together with a mains cable for a long distance. The fifth is to forget to mark the route, so that a later digger cuts the cable. And the sixth is to skip the measurement of a new cable, which makes it impossible to know later whether the loss has changed. Avoid these and your feedline will serve for years.
The Bottom Line
Buried coax fails mainly through water, so choose a cable with a polyethylene jacket made for direct burial, or put ordinary cable in a sloping conduit and accept the extra work. Dig a trench of suitable depth with a sand bed, lay the cable without tension, mark the route with warning tape and seal both ends and every connector against moisture. Ground the shield properly at the entry to the house, and measure the cable before and after burial. A conduit with a spare pull string is the best investment if you expect to change antennas, and it saves the trench the next time.
Frequently Asked Questions
What is direct burial coax?
It is coaxial cable with a jacket, usually polyethylene, rated by the manufacturer for contact with soil. Some types also contain a flooding gel that seals small cuts.
Can I bury ordinary RG-8X or RG-213?
It is not recommended without a conduit. Their PVC jackets can age quickly in damp ground, which lets water into the shield.
How deep should I bury coax?
A common figure for low-voltage cable is about 30 to 45 centimetres, deeper in places where the ground freezes. Check the rules of your area before digging.
Is conduit better than direct burial?
It allows replacement without digging and protects against stones, but water can collect inside. Use a sloping conduit with a drain, and still choose a cable with a good jacket.
How do I keep water out of buried coax?
Seal every cut end, protect connectors with tape and mastic or adhesive heat-shrink, form drip loops where the cable leaves the ground and avoid underground joints.
How can I tell whether buried coax has gone bad?
Measure the return loss and the cable loss when it is new and compare each year. A rise in loss or a change in SWR points to water or damage, and a time-domain measurement can locate it.