On Sun, 10 Jun 2007 08:04:29 +0200, EC1CT Fernando wrote:
>OTOH I´m still trying to
>figure out how the hell the vertical doesn´t work out over the metallic
>roof. It could be that we got so much ground plane?..
I'll bet that the vertical on the metal roof works just fine! Have you
tried transmitting through it? Your mistake is thinking that SWR and
impedance defines how well the antenna works. WRONG! A vertical on a metal
roof SHOULD read 20 ohms or so if it working properly, because the
impedance of the antenna itself is around 20 ohms, and the roof gave you a
very low (GOOD) ground impedance -- a few ohms. When you put the same
antenna on the ground, the earth added 30 ohms of resistance, which wasted
2/3 of your power, but gave you a low SWR, making you think that was good.
But it's BAD!
If you had connected the vertical on the metal roof (using the roof as the
ground) to your transmitter and tuned it with an antenna tuner, you would
probably found that it worked VERY well.
SWR is NOT an indicator of good antenna performance. It is only an
indicator of the MATCH between the antenna and the transmitter. That match
can very easily be fixed by an antenna tuner.
Jim Brown K9YC
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