Coax for 70 cm: RG-58, RG-213 or LMR-400

A good antenna on the roof, yet still no contacts? Often the antenna is not to blame — the cable between it and the radio is. On the 70 cm band (in Russia that's 430–440 MHz) coax "eats" far more signal than on HF, and a thin lead that worked great on 145 MHz can halve your power at 433 MHz before it ever reaches the antenna. Let's break down where this loss comes from and which cable to use for an outdoor run.

Where feedline loss comes from

Coax is not an ideal wire. Part of the energy you push into the cable turns into heat: in the resistance of the center conductor and the shield, and in heating the dielectric between them. The higher the frequency, the more aggressively these mechanisms work — which is why attenuation rises with frequency. At 145 MHz the cable is still forgiving, at 433 MHz it already punishes you, and at 1.2 GHz it punishes hard.

The second factor is length. Loss is linear with length: if one meter of cable "eats" X, then ten meters will eat roughly 10·X. That's why a short jumper and a long run up a mast are two completely different cable requirements.

How to read attenuation figures

Attenuation is usually given in decibels per 100 meters (dB/100m) or per 100 feet at a specific frequency. It helps to remember the rough decibel rule:

• 3 dB — that's a loss of half the power;
• 6 dB — a quarter remains;
• 10 dB — one tenth remains.

So if the feedline gives 3 dB of loss at your frequency, only half of your power reaches the antenna. That instantly turns abstract "decibels" into understandable watts.

Three cables for 433 MHz

Below are approximate attenuation values at a frequency of around 433–450 MHz. These are just a guideline. The real numbers depend on the brand, the manufacturer and the quality of the specific batch — with cheap "RG-213-like" cable the attenuation can easily turn out worse than the spec sheet. Always check the datasheet of the specific brand.

RG-58 — thin and convenient, but "deaf"

About 5 mm in diameter, flexible, cheap — handy for routing inside a cabinet. But on 70 cm its attenuation is high: roughly ~30 dB/100 m in the 400–433 MHz region. On a long run this is a catastrophe: already at 10 meters you lose about half the power, if not more. RG-58 is only good for short jumpers — a patch cord from the radio to the tuner, a half-meter to one-meter link. Running an antenna up a mast with it means throwing away watts.

RG-213 — the workhorse

Thick (about 10 mm), with a dense shield. At the same 400–433 MHz its attenuation is roughly ~13–14 dB/100 m — that is, about half that of RG-58. For a typical home run of 5–15 meters it's a perfectly sensible choice: affordable, sturdy, stocked by every shop. It's stiff in the cold and dislikes sharp bends — but as a feedline up to a balcony or rooftop antenna it does an honest job.

LMR-400 — minimal loss

A "low-loss" class cable with foil and braid, diameter close to RG-213. Per the official Times Microwave datasheet, LMR-400 attenuation at 450 MHz is around 8.9 dB/100 m (2.7 dB/100 ft). That's noticeably better than RG-213 and several times better than RG-58. People use it when the run is long (tens of meters) or when every decibel counts — for example, running an antenna up a tall mast. The downsides are price and stiffness; it also needs its own neat connectors.

Connectors and adapters "eat" too

Every joint in the path is a small loss and a potential source of problems. One quality, properly terminated connector adds little (fractions of a decibel), but a chain of adapters SMA→BNC→PL plus a couple of poorly crimped ends together eat up a noticeable amount. On top of that, a bad connector means rising SWR and a point where moisture creeps in.

• Minimize the number of joints: fewer adapters means less loss.
• Solder/crimp carefully: a "cold" solder joint on the center conductor is a classic cause of a "deaf" path.
• Seal outdoor connectors: water in the cable sharply raises attenuation and kills it for good.

What to choose for your situation

1. A short jumper (up to ~1 m) indoors — RG-58 will do, loss over such a length is small.
2. A run to the balcony/roof of 5–15 m — RG-213 as a sensible minimum, LMR-400 if you want to squeeze out the most.
3. A long run (20 m and more) or a tall mast — only low-loss class such as LMR-400. Over such lengths RG-213 already "eats" too much.

And the main rule: the cable should be no longer than necessary. Extra "just in case" meters coiled up are extra decibels of loss, and on 70 cm they are especially expensive.