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To:  <rsgb_lf_group@blacksheep.org>
Date: 29 May 2008 14:52 GMT
Message-ID: <1K1jUP-2DC1qq0@fwd24.t-online.de>
Subject: Re: LF: antenna losses
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Dear Rik, 

when I had been the wireless operator of DCUW in 1957 the sailors once
had set up a sunsail in a way that it touched the downlead of the ship's
T-aerial. It had not been possible to tune the aerial on MF using the
variometer of the MF transmitter! On HF, where the T-aerial has also
been employed, tuning was normal.

We all would regard a sunsail a "non-conducting" device, but for an
aerial with a typical impedance of about 50000 ohms the effect was
obvious.

HW?

73 Ha-Jo, DJ1ZB


"Rik Strobbe" <rik.strobbe@fys.kuleuven.be> schrieb:
> 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
> 
> 
>