TopBand: Antenna Observations

[Bob Marshall-Read]

Topbanders,

The freezing fog this morning in the Dublin area gave me a unique
opportunity to have a look at the radial system in a new perspective.

I feed my T antenna (slight slope to the centre 15 metres up, 17 metres
each way at the top) from the shack window at the upstairs bedroom,
giving the 10 m long radials about 4 metres elevation at the feed
point.  The radials were covered in an ice film this morning after
sunrise, but the first three metres of the 25 radials were not covered
at all.

Those directly under the vertical portion of the T were not covered
their entire length.  Those on the left and right portions of the
semi-circular arc were not covered in decreasing amounts the closer they
got to the vertical section of the T antenna.

As the fog was reforming as soon as it would melt, I moved the radial
system around to make them closer to the vertical section, and added ten
more in the semi-circle at the outer portion of the arc, until the
‘melting’ portion was uniform to the left and right of the 180 degree
fanout.  The majority of those concentrated under the vertical section
still were completely clear of ice film, and the left and right extremes
were in lesser degrees melted until the furthermost left and furthest
right were only ‘half-melted’.

The melting of the vertical section increased to nearly the top of the T
after the change in the radial system.  At the beginning, the first 8
metres was melted, the rest frozen.

The input resistance declined about 1 ohm (from 10 to 9), and the 1.5 to
1 bandwidth narrowed by about 10 kHz.

Another interesting observation.  The top of the fence bordering the
back of the yard (2.5m high with barbed wire at the top) that goes under
the centre of the T was melted about an eighth wave in both directions.
This fence must be close coupled to the antenna as well.  Will see if it
can be de-coupled.

Anyway, the next few days will tell if the birds are just getting a
warmer place to rest or the RF is leaving the neighbourhood a bit
better.

Conditions the past few mornings have been occasional to the central
USA, and ZL2 was heard this morning.

73

Bob   EI7IU


--
Robert Marshall-Read

Irish Mobile  +353 (0)87 284 3710
Home Number   +353 45 879 086


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