Hello Marcus,
the cracked branch "connected" my antenna to a repectable birch tree,
so I not quite sure wether the 91 Watt was entirely dissipated by the
leave.
And I think that I wouldn't be the first one to start a fire that way.
Anyway, I alway tune the antenna on low power (25 W) resulting in just
a few 100 mA antenna current. Only after that I switch to full power
(350 W) and as I noticed that the antenna current was low I only
transmitted for a few seconds.
Maybe the alternative solution would have been to give a long 350 W
carrier and just burn the leave.
At this moment it is rainy, so no transmitting tonight.
73, Rik ON7YD
Quoting Markus Vester <[email protected]>:
Hi Rik,
playing with your figures for current and resistance, one finds that
the poor little twig would have dissipated 91 watts. Makes me worry
whether it could have started a bushfire... During a fieldday a
couple of years ago, we ignited bits of dry grass on the LF antenna
wire for demonstration, which did create some respect for RF voltage
amongst the bystanders.
73 and best of luck
Markus, DF6NM
-----Ursprüngliche Mitteilung-----
Von: Rik Strobbe <[email protected]>
An: [email protected]
Verschickt: Do., 29. Mai. 2008, 9:37
Thema: LF: antenna losses
Dear all,
as I tried to tune my 500 kHz antenna last right I noticed that it
was detuned by about 1.5 kHz (from 503 tot 501.5) and that the
antenna current was down to 1.8 A (from 2.1 A).
A first visual control of the antenna revealed no possible cause. So
I started to check the coax cables and variometer, but all was OK
there.
During a second visual control (it was already getting dark) I
noticed that a small branch of one of the surrounding trees was
cracked (probably during a thunderstorm the night before) and just
one leaf was touching the antenna wire. By gently pulling the
downlead wire of the antenna a few times the cracked branch moved a
bit and the leaves were now about 20 cm from the antenna wire. Now
the antenna was back to 503 kHz and the antenna current back to 2.1
A.
Of course I know that the antenna wire should not touch any
greenery, but I was surprised of the rather large effect of just one
(meanwhile dry) leave touching the antenna wire (that has 1mm
plastic isolation).
The loss resistance of my antenna system now is 78 Ohm, including 10
Ohm coil losses (with no touching leaves).
Few weeks ago I increased the height of the top wire from 9 m to 11
m, what slightly increase the loss from 73 to 78 Ohm.
This is would bring the signal about 0.3 dB down, but is more than
compensated by the increase of the radiation resistance (+1.7 dB)
During the first tests in February (with the 9m high antenna) I
measured a loss of 55 Ohm, so the is a seasonal variation almost 20
Ohm (or 1.2 dB).
73, Rik ON7YD - OR7T
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