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50 Ohms "Real Resistive" impedance a Misnomer?

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July 16th 03, 07:09 AM

Dr. Slick

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(Richard Harrison) wrote in message ...
Dr. Slick wrote:
"But, my point is that you can take a DC measurement anywhere on the
ideal lossless antenna and you will never see 50 Ohms anywhere, only
shorts."

True. D-C resistance of a lossless wire is zero. But, apply 50 watts r-f
power to an antenna adjusted to present 50-ohms resistance at a
particular frequency.

What happens? Our loading, adjusted for 50 watts output, produces 50
volts at an in-phase current of 50 amps.

That would be 2500 watts? I assume you mean 50 volts and 1 amp to
get 50 watts.

Radiation resistance can be readily measured by using an r-f bridge
instrument. The bridge and antenna are excited by a generator operating
at the same frequency the transmitter will use. The bridge will indicate
reactance in the antenna, if it is not resonant. The null detector for
the bridge is typically a good radio receiver.

Radiation resistance is real though it does not heat the antenna. Loss
resistance is real though it does heat the antenna and its surroundings.

A resistance is a volt to amp ratio in which amps are in-phase with the
volts.

The key here is IN-PHASE. If you read my other post, you and Roy
have clarified this for me: Both radiation resistance and dissipative
resistance are the real portion of the impedance.

A resistor is a special type of resistance in which the electrical
energy applied to the resistance is converted into heat.

With very little radiated energy.

Thanks for the input,

Slick

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