[Antennas] resonance

Sat Dec 17 16:01:46 EST 2005

Frank, in the real world when you measure the impedance of the antenna over
the ground with some sort of a bridge (or something else) you indeed measure
the sum of antenna impedance proper and impedance of losses. The latter
depends upon the characteristics of the ground and other near by objects
that influence the antenna. Therefore the measured impedance of the system
may not be purely resistive in some cases even though it seems that the
antenna itself cut to be resonant on a given frequency. I have seen many
times that the same antenna being purely resistive over the ground exhibits
a lot of reactance (which is impossible to get rid off by changing the
physical length of the antenna) over the roof made of reinforced concrete.
 Just remember that Z = Z(antenna) + Z(losses) and sometimes the only way to
control Z losses is to raise antenna so high that it feels like in free
space.
73, Igor UA9CDC
> Bill Aycock wrote:
> > My question is: What process are you using to determine that the
> > impedance is purely resistive and at a value of 75 ohms?
> > Bill-W4BSG
> >
> No process. I am blindly trusting what the antenna section
> of the handbook tells me. The only thing I am wondering is
> if XL=XC as the height of the dipole changes. Height
> evidently has a dramatic effect on the feed point impedance
> value according to the chart. It was just not clear how
> height effected the resonance condition if it was effected
> at all.
>> Regards,
> Frank Kamp
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