[TowerTalk] One off balun question

[Steve Hunt]

I mentioned earlier the importance of the phase information provided by 
the VNA - it not only allows the choke R and X to be identified, but it 
also eliminates a measurement error in |Z| inherent at lower values.

If we throw away the phase information, and only measure |S21| with that 
test arrangement, we cannot distinguish between the infinite number of 
choke impedance values that will produce the same |S21| result. So for 
example, any of these choke impedance will produce an identical |S21| 
indication:

500 + j0      (|Z| = 500)
400 + j325      (|Z| = 515)
300 + j435      (|Z| = 528)
0 + j570      (|Z| = 570)

We have no way of knowing whether the choke is totally resistive or 
totally inductive or something in between; and we only know its 
impedance lies somewhere in the range 500 Ohms to 570 Ohms.

Steve G3TXQ






On 03/11/2014 16:44, Steve Hunt wrote:
>
> With the "Generator/Voltmeter" method you can only easily measure 
> |S21| and therefore the choke Z; but of course the VNA will also give 
> you the vital S21 phase information and the choke R+jX. Plus, the VNA 
> carries out the frequency scan automatically and can be calibrated to 
> take out the effect of the interconnecting leads.
>