> From: Peter Chadwick <Peter.Chadwick@gpsemi.com>
> To: "'topband'" <topband@contesting.com>
To: <topband@contesting.com>
> Date: Fri, 15 Aug 97 08:48:50 +0000
> >>It's also been claimed by the same experts that lowest SWR is always
> >>achieved at resonance for any antenna.
> I don't agree - at least if we mean 'SWR in a 50 ohm system', AND if we
> define resonance as 'presenting a feed impedance which is purely
> resisitive'. Lowest SWR is achieved where the feed impedance is at the
> point closest to the centre of the Smith chart, and this isn't
> necessarily on the line of zero reactance.
That's right Peter, lowest SWR is NOT necessarily at resonance.
It depends on how fast the reactance changes compared to the
resistance. If the resistance change is slow, and the reactance
change is fast, that statement can be true.
But if the resistance changes rapidly around resonance that statement
can be false.
It's a general rule of thumb that is NOT always true in all
systems, but is true in many or most cases.
> But I never have understood this business of antennas having to be
> resonant to radiate, since that is patently not the case. However,
> there all sorts of myths around.
I can buy a new Corvette someone died in for $500, because they can't
get the smell out. Exxon bought the rights to a carburetor that gets
1000 miles to the gallon and hid the patent. Tank Q needs to be
twelve. Grid current causes more IMD. Resonant radials and antennas
are necessary and good. Four radials are all you need.....
.......and folded monopoles have less ground loss. ;-)
As John ON4UN said, there's nothing like that free lunch. Selling
people on work is difficult. But selling a gimmick idea that does not
even work... but sounds like a "something for almost nothing"
cure.... is pretty easy. It spreads like wildfire.
73 Tom
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