[Boatanchors] Repeater antenna on a cold mountain top

[Brian Clarke]

Unless the water that freezes on your antenna is laden with conductive 
salts, eg, near an ocean, then the frozen water will be insulating. In that 
case, the main effect will be that your antenna will be electrically longer 
and so the resonance will be lower - but not much.

Have a look at the skin depth of water at your chosen frequencies to 
calculate the electrical effect. At 1 GHz, the skin depth of water is about 
6.8 m when the attenuation will be about 8 dB. Even if you get ice 25 mm 
thick, the attenuation will be negligible. However, if the ice is 
sea-salt-laden, the skin depth is only about 7 mm; attenuation at an ice 
thickness of 25 mm will be about 28 dB.

To withstand the build-up of ice, you may wish to consider tear-drop 
cross-section for horizontal elements. However, if there is also a strong 
horizontal wind component at this mountain top, then circular cross-section 
will be best. You may also want to stuff the elements (if hollow tubing) 
with rope to damp any wind-triggered natural resonance / vibration that will 
result in cracking and fracture - and put bungs over the element ends to 
prevent any wind-induced organ-pipe resonances.

Have a look at what telephone companies do with their cell antennae - they 
put them inside radomes. A 70 cm vertical 5/8 antenna could easily fit 
inside a piece of plastic or fibreglas tubing. Under such conditions, it 
could be economical to have a small re-circulating hot-air blower inside. 
And you could use the single coax feed to carry DC to the heater-blower and, 
via a DC-blocking capacitor, RF to the antenna. As this is a repeater, you 
will need to consider the electrical burden on your repeater electronics of 
warming two antennae.

73 de Brian, VK2GCE.

On Thursday, November 14, 2013 5:47 PM, you said:


> We are installing a 70cm repeater on the summit of a 1600meter mountain.
>
> Temperature there goes down to -20 degree centigrade and everything will
> frozen covered with ice in winter. Antenna there is also expected to be 
> covered with
> thick ice. So antenna performance will be poor, I guess.
>
> How poor will the antenna performance be?
>
> My question is what kind of antenna will be suitable for this kind of 
> environment.
>
> Any body has experience??