Funny springy bit

[RodBearden]

I'm not a radio person, so I'm vaguely puzzled by these 1090 aerials.

I understand that the length of aerials should be a multiple of the wavelength to be received, but why do they have the spiral springy bit towards the base?

Is there a technical explanation, (if there is please keep it simple!) or is it just to stop it getting bent when you knock it?

Rod

[Allocator]

My best guess is that it just keeps the antenna nice and short - same "electrical" length, but physically shorter.

Standing by to be baffled with science :-)

[DaveR]

I think a technical question deserves a technical answer.

When placed on a window ledge and correctly orientated in both the horizontal and vertical plane the spring functions to give the user something intersting to look at when its windy and nothings flying as the aerial moves in the wind ;)

[RodBearden]

Ah - I knew somebody would know!

Many thanks Dave!

Rod

[ChrisGR]

Hi Rod
This RB antenna is divided in two wavelengths.
So the first section below spiral, is one ¼ wavelength, and the spiral with rest wire one 5/8 wave length. Also this spiral works like divider between two wavelengths and as Allocator says in post below.
Con. That's mean this antenna it have a gain of 4db.

Wavelengths calculation.
300000 / 1090 = 275.22 mm /4= 68.8 mm the lower section and 275.22 mm /8*5=172 mm the upper section of antenna.

Chris

[RodBearden]

Thanks for that, Chris.

It's good to have people who understand this stuff around!

Rod