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News · 4 September 2026

SQ4MN’s 70 cm in a new guise

Marian added a second support to his 70 cm Yagi — and the wind no longer has a say. Below is the whole story: what changed, why it works, how the antenna looks from the street and a competition in which a ribbon plays the starring role.

Marian SQ4MNantenna and photos · Warsaw

70 cm Yagi · vertical polarisation 2 support points upper-floor balcony

Competition: where’s the ribbon? Show others

A long 70 cm Yagi with vertical elements on a high-floor balcony; beneath the boom a second supporting tube, in the background a housing estate, a church and the high-rises of central Warsaw

70 cm430–440 MHz band

2support points instead of one

L³that’s how cantilever deflection grows with length

v²that’s how wind load grows with speed

What changed

A boom that stopped dancing in the wind

The new bit hides under the antenna. From one angle it looks unassuming — from another it changes everything a gusty wind does to a Yagi.

A second tube under the boom

Marian SQ4MN sent a photo of his 70 cm antenna in its new guise: a long Yagi with vertical elements hangs on an upper-floor balcony, with the skyscrapers of central Warsaw on the horizon. The novelty is a second, thicker tube: anchored in a hefty clamp, it runs out from the balcony to the end of the boom at right angles and supports the antenna right by the reflector.

Instead of an antenna hanging on a single bracket at the mast we now have a rigid frame with two support points. The clamps stop doing the work, the coax doesn’t chafe on edges, and the end of the boom — until now the most wobbly point of the whole structure — stays put.

Clamp — the thick support tube in a hefty clamp on the black steel arm from the balcony
Flat bar — at the end of the boom it links the support to the antenna right by the reflector
Coax — routed along the tube, with no loose loops catching the wind

Close-up of the additional mounting: a thick strut tube in a heavy clamp on a black steel balcony bracket, above it the antenna boom

Why it works

The difference is bigger than it seems

Three relationships that explain why a second support isn’t mere cosmetics, but a completely different structure.

deflection ∝ L³

Length cubed

The deflection of a cantilever fixed at one end grows with the cube of its length: a boom twice as long deflects eight times as much. That’s why long 70 cm Yagis are so keen to ‘conduct’ in the wind.

wind load ∝ v²

Speed squared

The wind load grows with the square of speed: a wind twice as fast means four times the force at the boom tip. In gusts the clamps get not a vibration but sharp tugs.

order of magnitude

From cantilever to beam

Supporting the tip turns the cantilever into a beam with two support points. For the same force the deflection drops by an order of magnitude — in theory even more than twentyfold. The clamps stop doing the work, and the coax doesn’t chafe on edges.

Specifications

A tube the wave can’t see

Extra metal right next to the antenna sounds worrying. There’s nothing to fear here — and that’s no accident.

Nothing has got worse

The support tube lies horizontally, like the boom — and thus perpendicular to the elements. To a vertically polarised wave such a horizontal conductor is almost transparent, exactly like the boom itself, which in any Yagi with vertical elements lies in the same plane.

The pattern and SWR stayed where they were. Only what you can’t see on the meter has changed: the antenna stopped changing direction with every gust, so the correspondent’s signal stopped ‘floating’.

“I haven’t noticed any worsening of the parameters.”— Marian SQ4MN

Antenna boom end: flat bar linking the strut to the boom, reflector and copper folded dipole, the cable runs along the tube

5 September 2026

The same antenna from the street

A day later a shot from the pavement arrived. From below you can see the whole scene: the Yagi projects well beyond the balcony and sits alongside two satellite dishes, and one floor up a white-and-red flag flutters — as if someone were already testing a wind indicator.

Light-coloured block of flats facade seen from the street; on the central balcony a long 70 cm Yagi sticks out beyond the railing next to two satellite dishes, one floor up a white-and-red flag

Competition · ribbon

Where to attach the ribbon?

SQ5PIT suggested hanging a ribbon on the antenna — a ham radio wind indicator, like a telltale on a yacht’s shrouds. Marian announces a competition: where to attach it? From the street it will be perfectly visible.

On the reflector

So that it streams like a comet’s tail — at the back, right by the strut, where the antenna is stiffest.

On the last director

Like a pennant on the bow — right at the end, where the boom once danced the most.

On the new strut

So you can see whether anything is still vibrating at all. The ribbon is the witness that the strut is doing its job.

Your idea

Got a better spot — or a reason why the ribbon should stay in the drawer? Go ahead.

Reply in the guestbook

SQ4MN

Antenna and photos by

Marian SQ4MN

A radio amateur from Warsaw who works 70 cm from a high-floor balcony — and regularly proves that a beam in an apartment block isn’t a dream, just a matter of good mounting. It’s his Yagi, his strut and his photos. We meet on the air and in person — most recently on 12 August on the Świętokrzyski Bridge, where we watched a partial eclipse of the Sun together.

70 cmYagi from a balconyWarsawtext: Filip SQ5CBA

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