Antenna before winter:
a check-up in one afternoon

Trap: good SWR, bad cable

A wet cable turns into a distributed dummy load. The wave reflected from the antenna returns through the cable and loses on the way just as much as the forward wave — so at the radio you see the reflection weakened by double the cable loss.

Example: the antenna has SWR 3, and after getting wet the cable attenuates by 6 dB. At the radio the meter will show SWR about 1.3, and with a dry cable it would show about 2.0. That’s why a suspiciously low SWR that “improved by itself” is just as much a red flag as a rising one.

Cable test with an open end

1. Disconnect the antenna from the cable at the top. Leave the cable end open — with nothing on it — and protect it from rain.
2. Connect an analyser or NanoVNA at the radio end (calibrate the NanoVNA SOL at the end of the lead you use to connect) and measure SWR or return loss at 145 and 435 MHz. Never transmit from the radio into an open cable — SWR 5–9 is a quick way to damage the power amplifier.
3. Cable loss is half the return loss measured at its input. From SWR alone you can read it from the table.
4. Compare with the datasheet. A result clearly worse than in the cable’s spec sheet suggests water — the analyser’s TDR/DTF mode will even show where.

Symptoms

• SWR “wanders” after rain or slowly changes over months — upwards or, worse, downwards.
• Weaker repeater signals and poorer reports at the same SWR — the cable is simply eating the signal in both directions.
• Green or black braid after slitting the jacket by the connector. The copper should be bright.

How to run the cable

Always downwards from the antenna. Water runs along the jacket, so the cable must not carry it back to the connector. Before every place where the cable turns upwards or enters the building — the roof edge, gutter, pass‑through, window — make a drip loop: a gentle “U” from whose lowest point the drops fall.

The loop radius must not be smaller than the minimum bend radius from the cable datasheet — for Ecoflex 10 it’s about 4 cm for a one‑off bend and 8 cm if the cable is to move. Fix with black UV‑resistant ties and don’t tighten them so much that they crush the cable.

Which cable outdoors

The jacket must be black and UV‑stabilised — carbon black in the plastic is exactly the protection against the sun; without it PVC fades and cracks. SSB datasheets state UV resistance for Ecoflex 10 and Aircell 7. The popular Belden 8267 (RG‑213) lists only “Indoor” environment on its datasheet and does not declare UV resistance — for the roof choose a cable that does.

“RG‑213” is a type designation, not a product: one manufacturer rates it down to −40 °C, another to −20 °C. Check the datasheet of the specific cable you’re buying.

In frost don’t bend a PVC‑jacketed cable to its minimum radius and don’t straighten kinks — the plastic stiffens and develops invisible micro‑cracks. Lay a new run in autumn, at above‑freezing temperatures.

The weight we forget

ISO 12494 on atmospheric icing of structures warns that ice can double the normal loading on antennas and their mountings, and the weakest parts are often the small details. Hard rime has a density of 600–900 kg/m³, glaze ice about 900 kg/m³. Then there’s wind: ice increases the area the wind pushes against.

Diamond quotes a wind rating of 50 m/s for the X‑200N, and the manual recommends using guys if the mast is made up of several sections. Remember that wind maps give a 10‑minute mean at a height of 10 m over open ground — gusts, especially on a roof, are distinctly stronger.

Don’t knock the ice off

Ice on structures usually doesn’t melt away but comes off in chunks when it thaws — and damages whatever is below. Don’t go onto an icy roof to knock it off: the whole antenna on the mast is more important than a week of worse SWR. If icicles and overhangs endanger passers‑by, that’s for the building manager and a qualified crew to deal with. Your job is to report the hazard, not to go up there with a broom.

Rotator in freezing weather

The Yaesu G‑450A rotator operates down to −20 °C. In the G‑1000DXC manual Yaesu says the rotator is factory‑lubricated for maintenance‑free operation in any climate and must not be opened — user lubrication is not provided for. With heavy ice and deep frost leave the antenna parked: Hy‑Gain warns that in freezing weather the rotator turns more slowly, and an iced‑up antenna presents more wind resistance.

Corrosion where metals meet

Aluminium and zinc paired with stainless steel will corrode — all that’s needed is moisture, and there’s always plenty of it under washers and clamps. A rule of thumb from stainless‑steel guides: a small stainless bolt in a large aluminium tube is fine; the reverse arrangement is not. A white, powdery bloom on the aluminium by a clamp is a sign the process is under way.

Stainless threads like to gall, and after a year on a roof they also seize. Hy‑Gain and Yaesu recommend greasing bolts and U‑bolts — next year you’ll undo them without an angle grinder.

Guys

Guy tension depends on temperature: in winter the wires are tighter than on a warm day. Don’t crank them up “to the max” on a sunny October afternoon. Check the wire‑rope clips — U‑bolt on the dead end, saddle on the loaded end — and the securing of the turnbuckles.

Chimney clamps

Check that the clamps haven’t loosened and aren’t biting into the chimney render, that there’s no rust on the galvanising, and that the mast is vertical. After every gale it’s worth looking at them again — the TIA standard for masts recommends an inspection after severe wind and icing.

What it looks like in practice

Every cable entering the building goes through its own lightning arrester, and all the arresters sit on a single common plate connected to earth. That’s what the ARRL recommends.

An autumn check is simple: terminals on the plate and on the mast with no verdigris, bolts tightened, the earth strap without breaks and sharp bends. If the arrester has a replaceable gas discharge tube and the manufacturer specifies replacement — check whether it’s due.