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" <[email protected]> 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
>
>
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>
>
>
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