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Date: Thu, 29 May 2008 22:36:16 +0200
From: Rik Strobbe <Rik.Strobbe@fys.kuleuven.be>
To: rsgb_lf_group@blacksheep.org
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Hello Marcus,

the cracked branch "connected" my antenna to a repectable birch tree, =20
so I not quite sure wether the 91 Watt was entirely dissipated by the =20
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 =20
a few 100 mA antenna current. Only after that I switch to full power =20
(350 W) and as I noticed that the antenna current was low I only =20
transmitted for a few seconds.
Maybe the alternative solution would have been to give a long 350 W =20
carrier and just burn the leave.

At this moment it is rainy, so no transmitting tonight.

73, Rik  ON7YD

Quoting Markus Vester <markusvester@aol.com>:

> Hi Rik,
>
> playing with your figures for current and resistance, one=C2=A0finds that=20=
=20
>  the=C2=A0poor little twig=C2=A0would have dissipated 91 watts. Makes me=
=C2=A0worry =20
>  whether=C2=A0it could=C2=A0have started a bushfire... During a fieldday a=
  =20
> couple of years ago, we ignited bits of dry grass on the LF antenna  =20
> wire for demonstration, which=C2=A0did create=C2=A0some respect for RF vol=
tage =20
>  amongst the bystanders.
>
> 73 and best of luck
> Markus, DF6NM
>
>
> -----Urspr=C3=BCngliche Mitteilung-----
> Von: Rik Strobbe <rik.strobbe@fys.kuleuven.be>
> An: rsgb_lf_group@blacksheep.org
> Verschickt: Do., 29. Mai. 2008, 9:37
> Thema: LF: antenna losses
>
>
> Dear all,=C2=A0
> =C2=A0
> as I tried to tune my 500 kHz antenna last right I noticed that it  =20
> was detuned by about 1.5 kHz (from 503 tot 501.5) and that the  =20
> antenna current was down to 1.8 A (from 2.1 A).=C2=A0
> A first visual control of the antenna revealed no possible cause. So =20
>  I started to check the coax cables and variometer, but all was OK  =20
> there.=C2=A0
> During a second visual control (it was already getting dark) I  =20
> noticed that a small branch of one of the surrounding trees was  =20
> cracked (probably during a thunderstorm the night before) and just  =20
> one leaf was touching the antenna wire. By gently pulling the  =20
> downlead wire of the antenna a few times the cracked branch moved a  =20
> bit and the leaves were now about 20 cm from the antenna wire. Now  =20
> the antenna was back to 503 kHz and the antenna current back to 2.1  =20
> A.=C2=A0
> =C2=A0
> Of course I know that the antenna wire should not touch any  =20
> greenery, but I was surprised of the rather large effect of just one =20
>  (meanwhile dry) leave touching the antenna wire (that has 1mm  =20
> plastic isolation).=C2=A0
> =C2=A0
> The loss resistance of my antenna system now is 78 Ohm, including 10 =20
>  Ohm coil losses (with no touching leaves).=C2=A0
> Few weeks ago I increased the height of the top wire from 9 m to 11  =20
> m, what slightly increase the loss from 73 to 78 Ohm.=C2=A0
> This is would bring the signal about 0.3 dB down, but is more than  =20
> compensated by the increase of the radiation resistance (+1.7 dB)=C2=A0
> =C2=A0
> During the first tests in February (with the 9m high antenna) I  =20
> measured a loss of 55 Ohm, so the is a seasonal variation almost 20  =20
> Ohm (or 1.2 dB).=C2=A0
> =C2=A0
> 73, Rik ON7YD - OR7T=C2=A0
> =C2=A0
> Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm=C2=A0
> =C2=A0
>
>
> ________________________________________________________________________
> Bei AOL gibt's jetzt kostenlos eMail f=C3=BCr alle.  Klicken Sie auf  =20
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