[Hallicrafters] Snow / Antenna question.

[Kenneth G. Gordon]

On 2 Feb 2008 at 19:54, Bill Gerhold wrote:

> The absorption of RF by snow or ice is negligible.

True. However, for a Yagi, the water between the elements, 
since it has a completely different diaelectric constant than 
air, can have severe effects on the antenna pattern as well 
as SWR.

I know this for a fact, since I have been involved in 
telemetry of seismic data on various frequencies at VHF 
(around 150 Mhz). We use Yagis mounted on mountain-top 
fire towers, etc. to telemeter the seismic signal back to the 
receiving/processing station, sometimes as much as 50 
miles away.

Maximum power into the antenna is usually 100 mW, 
although we have been licensed for 5 watts output for 
certain critical stations.

When the ice around the elements reaches a certain point, 
the pattern of the Yagi is very badly effected. First the 
forward gain drops lower and lower, until finally the pattern 
reverses a full 180 degrees.

I first built several Yagis, using the NBS data on maximum 
gain, using 1/2" steel conduit for the elements, and a very 
solid bit of steel square tubing for the boom. The antennas 
were hell for stout and the directivity and gain were very 
close to what I had calculated from the NBS data.

We put them up during the summer, of course, and the 
signal levels were quite good.

As soon as we started getting some snow on the mountain-
tops, we noticed that received signal strength started to 
drop. After a very severe ice storm up there, our signals 
were unusable, and we concluded that the antennas had 
come down, despite our attempt to make them as strong 
and as secure as possible.

One of our crew managed to snowmobile into one of the 
stations that winter, and radioed back to us that the antenna 
and feedline were still in place, but that the ice-ball on the 
antenna was about 3 feet in diameter, completely 
enveloping the antenna.

We then had a long talk with a commercial builder of 
telemetry antennas, and were told that antennas had to be 
specially designed to deal with that much ice or snow, and 
even the best of their designs only mitigated the signal loss, 
but could not eliminate it. We were told that if the ice built 
up to the levels we were experiencing, nothing could be 
done about the signal loss except by using some method of 
heating the antennas, or enclosing them in a fibreglass 
housing that would prevent ice build-up between elements.

The cost was too high, and we just had to live with the 
problem.

However, in answer to the other question, no amount of RF 
an amateur station could possibly generate would have the 
slightest effect towards melting ice on an antenna. For one 
thing, there isn't enough electrical or RF resistance in the 
ice to cause heating, and the elements themselves certainly 
would never get warm from RF either.

Just adjust the tuner and live with it.

Ken W7EKB