During this very wet spell I noticed that the total antenna resistance has
gone up - probably due to environmental losses. The daily cycling of antenna
resistance does not seem so bad on 136kHz as on 73kHz or should I say that
it is different. I have found no external effect such as tide or weather
that correlate with these antenna current variations.
When the area around the antenna is very wet, and I leave the transmitter on
for testing, the antenna current starts very low then creeps up over a
period of about 5 minutes or more by up to 100%. So much so that I have to
readjust the matching transformer to avoid overloading the transmitter. This
implies that the total antenna resistance has fallen.
When we had a fall of snow a couple of weeks ago this caused a change in the
resonant point although not the total resistance of the antenna. No wonder
the Decca stations had this elaborate auto tuning and matching arrangement.
Just over a week ago (when it was cold and relatively dry) I was using the
transmitter at night and was alarmed by fire and sparks coming from one of
the insulated wires that goes to the top of the mast. In the beam of my
torch I could see a twig that had obviously got caught up with the wire when
the antenna was tilted over during the day. I turned of the transmitter and
hooked the twig down and found it was steaming and nearly too hot to touch.
It shows how lossy wet trees and shrubs are if located close to the
transmitting antenna.
Regards,
Peter, G3LDO
e-mail <[email protected]>
Web <http://web.ukonline.co.uk/g3ldo>
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