John, ON4UN, says:-
>>It's also been claimed by the same experts that lowest SWR is always
>>achieved at resonance for any antenna.
>This 100 % correct.
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.
A good example is the thick dipole in the clear. At resonance it has an
SWR of about 1.5 to 1 in a 50 ohm system. As you higher in freq, the
reactance component increases and the resistive bit gets closer to 50
ohms, so the locus of the impedance tends to move closer to 50 ohms and
then veers off. This only applies to thick dipoles though - length to
diameter ratio of 100 or less.
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.
An inconsequential piece of information. If you have an inverted L
antenna, in which the top is more than twice as long as the vertical
section, then if the product of the current in amps at the feedpoint and
the length of the vertical in metres exceeds 7.5, you can communicate 75
miles by day on 2182KHz by Act of Parliament! If the length of the top
isn't twice the length of the vertical, then the product must be 12 to
attain the same range. That's UK law - Merchant Fishing Vessels Radio
Rules.
73
Peter G3RZP
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