[Antennas] Re: Hairpin monopoles

Thu, 08 May 2003 10:09:26 -0500

>From: Wes Stewart <[email protected]>
>To: James Duffer <[email protected]>, [email protected]
>Subject: Re: [Antennas] Re: Hairpin monopoles
>Date: Wed, 7 May 2003 17:10:50 -0700 (PDT)
>
>
>--- James Duffer <[email protected]> wrote:
>[snip]
>
>George's explanation is of course right on.
>
> >
> > 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.
>
>Characteristic impedance is dependent on R, L and C
>per unit length.  R, L and sometimes C are frequency
>dependent.

Which is dependent on the physical construction of the transmission line.  
Refer to the formulas for Zo of parallel or coaxial lines (Zo= 276 Log 2S/d 
where S is center to center distance between the conducior and d is diameter 
of conductor[same units], Zo= 138 log (b/a) where b is inside diameter of 
outer conductor and a is outside diameter of inner conductor.

Wouldn't manufacturers of coaxial cable state that the characteristic 
impedance Zo is for example 50 Ohms at 100 Mhz if it was indeed dependent on 
operating frequency.  Sorry but I am from Missouri and you have to "show 
me".

In that line, I am going to visit the URL you referenced and ponder this 
some more, cause if I am wrong I want to know why and I will change my 
thinking when convinced.

73, Jim, wd4air

>As George told you, the characteristic impedance is
>reactive, especially at audio frequencies.
>Terminating a reactive line with a real resistance
>causes the line to be mismatched.  The mismatch is
>frequency dependent.
>
>If you want a simple way to explore this, I suggest
>that you go to:
>
>http://www.qsl.net/ac6la/tldetails.html
>
>and download the program.
>
>Once you install it, select Belden 9913 line and set
>the frequency to 0.001 MHz.  Make the load R=50, X=0
>and then look at the SWR.  You should see that it is
>1.77:1.
>
>If you terminate the line in its characteristic
>impedance; 50 -j28.8, then the line is matched, but
>you have a 1.77:1 mismatch at the input. (Assuming
>realitively short lengths)
>
>
>
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