Re: LF: antenna losses

[Markus Vester <markusvester@aol.com>]

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 <rik.strobbe@fys.kuleuven.be>
An: rsgb_lf_group@blacksheep.org
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 
 
Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm 
 

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