[TowerTalk] Topband: Handheld Impedance Analyzer

[StellarCAT]

Hi RIck,

Indeed it is quite true – with yagis depending on the type of design be it OWA vs < 50 ohm with transformation the point where X=0 and minimum SWR can vary and can actually be multiple points! I’m re-tuning an M2 6 element 15 yagi for performance – placing it where I want it to be by measuring on a test range (and finding the stock dimensions to be quite high in the band) ... after some modeling I found a set of dimensions that gives a double dip in SWR making the antenna SWR flatter over the whole band. The R and X form opposite U curves, actually the X is more like a stretched out N so there are 3 points where X=0! Both in the model and measured using a SARK 110. Granted I’ll have to give you that 2 of those X=0 points are close to the two lowest SWR points – but the ‘middle’ one not so much. 

Gary
K9RX 



On 3/28/2016 9:14 AM, Jim Brown wrote:
On Mon,3/28/2016 4:37 AM, KE1F Lou wrote:
Is the lowest SWR is the resonant frequency?
 Resonant frequency is when Xl=Xc.
When Xl=Xc is not necessarily the lowest SWR.
Exactly right. If, for example, the antenna's feedpoint impedance at
resonance is different from the feedline impedance, or if the analyzer
impedance does not match the feedline impedance, the lowest MEASURED SWR

73, Jim K9YC
_________________
I don't believe the above assertion.

Can anyone give a specific example of an antenna having
an SWR at the resonant frequency, (where X=0), that is higher than
the SWR at frequencies slightly above or below the resonant frequency?

I submit that a local minimum in the SWR will always coincide
with X=0.  This is easily proved on a Smith chart, where X=0
corresponds to the X-axis.  Adding reactance or susceptance moves
the impedance farther from the origin in all cases, meaning
SWR goes up.

Rick N6RK