[Antennas] Re: Hairpin monopoles

2003年5月07日 17:00:36 -0500

>It is not a physical resistance, thus it cannot be identified, isolated, or
>directly measured, in a sense. But, it is a useful construct for accounting
>for radiated power.
It has benn stated:
>A similar situation is found in the notion of characteristic impedance of a
>transmission line, expressed in the form R+jX, where R is the resistive
>component and X is the reactive component. Lines operated at HF generally
>have reactance so small as to be negligible so we use Zo as a purely
>resistive value. At audio frequencies, however, X can take on quite large
>values and all the usual transmission line equations must deal with the
>complex nature of Zo.
>>So, this is another instance wherein we use the notion of a resistance to
>define a property although it is not a physical resistor that can be
>identified and isolated. Neither does it dissipate any power! The resistive
>component of Zo is separate and apart from the ohmic and dielectric losses
>of the line which account for its dissipative losses.

An interesting analogy but leaves some questions. If the transmission line 
is terminated in a load that is equal to the transmission lines 
characteristic impedance, then what has frequency got to do with 
characteristic impedance???? The charateristic impedance is dependent on 
the physcial construction
conductor diameter and distance of the conductors. I am beginning to think 
that you are referring to impedances that result from reflections of 
mismatched loads and characteristic impedances. There have been a couple of 
previous post that gave a satisfactory explanation of radiation resistance, 
and your explanation may be just fine, but the analogy leaves me wondering. 
Zo (characteristic impedance is not a figament of someones imagination, it 
is there and relatively constant regardless of impedances resulting from 
mismatched or reactive loads etc.
Now lets hear more about the monopin hairpole.!!
Jim, wd4air
_________________________________________________________________
Tired of spam? Get advanced junk mail protection with MSN 8. 
http://join.msn.com/?page=features/junkmail