[Elecraft] Antenna Analyser - Phase Angle Sensor

Fri Mar 16 09:19:35 EST 2007

Thanks for the clarification, Don. As I mentioned in my private email, 
to eliminate all ambiguity, you really need to look at the slopes of 
both impedance magnitude and phase (or alternatively resistance and 
reactance). This will cover you for both sides of either series or 
parallel resonance.

This is what I do with my Plot software. You could certainly do it in 
your head, but it's little much to remember. If you have an analyzer, 
like the MFJ, which displays both R and X, it might make sense to print 
a little "what if..." sequence on a laminated card and tape it to the 
back of the unit to help with going through the scenarios when 
calculating sign. Based on my algorithm, I could print up a sample for 
anyone interested. Or you could use the two plots I referenced in this 
discussion to make your own. Remember though, that my plots show correct 
sign, and you have to reverse all the negative values to arrive at the 
correct input data for an analyzer that doesn't show sign.

73,
Larry N8LP

Don Wilhelm wrote:
> Folks,
>
> There is an error in the reply I sent last night (below) that is very
> important - it makes a big difference.
>
> When increasing or decreasing the frequency to determine the sign of the
> reactance:
> One must observe the increasing or decreasing of the IMPEDANCE (Z) and not
> just the reactance.
>
> I had erroneously stated the reactance, and Larry helped get my mind
> straightened out - make that impedance instead of reactance and it will be
> correct.  In other words, when using an MFJ259B - look at the "Z" value when
> increasing or decreasing the frequency, and not the "X" value.
>
> The notes in the MFJ259B manual recommending how to determine the sign of
> the reactance are placed under the instructions for measuring Capacitance
> and Inductance, and they do state that one can look at the reactance value.
> While that works fine for a pure reactance, it does not work for complex
> impedances, where one must look at the magnitude of the IMPEDANCE (Z).
>
> In other words, switch the MFJ259B to IMPEDANCE MODE and do the 'frequency
> increase/decrease thing' before attempting to affix a sign to the value of
> the reactance parameter.
>
> 73,
> Don W3FPR
>
>   
>> -----Original Message-----
>>
>>
>> Larry,
>>
>> I see no inconsistency with Stuart's statement in your plot.  Without
>> looking at the specific phase angle - if the absolute magnitude of the
>> reactance increases with increasing frequency, then the load is inductive,
>> it it decreases, then the load is capacitive.
>>
>> The poles of the black curve on your plot do line up with the
>> zero crossings
>> for the phase.  So if I look at any point on your plot, I see that the
>> absolute value of the magnitude of the reactance will increase with
>> increasing frequency anytime the load has positive reactance (inductance),
>> and the absolute magnitude of the reactance will decrease with increasing
>> frequency any time the load is capacitive.
>>
>> Yes, a simple (or complex) phase angle detector working without regard for
>> the sign will not provide the information required, but 'dithering' the
>> frequency and looking only at the relative magnitude of the reactance
>> between 'dither' points can provide the sign of the reactance.
>>
>> 73,
>> Don W3FPR
>>
>>     
>>> -----Original Message-----
>>>
>>> That won't work, Stuart. I wrestled with this for awhile when developing
>>> my LP-100 Plot program. You also need to know which side of resonance
>>> you're on, and whether it's a series or parallel resonance... at
>>> a minimum.
>>>
>>> Take a look at the plot at www.telepostinc.com/Files/OCF_Z_PH2.jpg. This
>>> is a section of an off center fed dipole as measured by my HP 87510A
>>> VNA. If you take the absolute value of the phase data (ie, have it
>>> bounce off zero as it would with a meter that can't determine phase
>>> sign), then you will see the problem. If you look at the phase in this
>>> plot, starting at the left edge, and remembering that all the negative
>>> values have to be reversed to look like a meter without sign detection,
>>> by your definition, the reactance makes the following changes from left
>>> to right edge...
>>>
>>> C, L, C, L, C, L, C, L
>>>
>>> In reality, as the VNA shows, there are fewer changes. The
>>>       
>> sequence is...
>>     
>>> C, L, C, L, C
>>>
>>> Without the visual aid, it can be confusing, to say the least.
>>>
>>> In my Plot software, I take into account a number of  factors, based on
>>> the slopes of the magnitude and phase of impedance. If you don't do
>>> this, you could easily be exactly wrong ;-)
>>>
>>> 73,
>>> Larry N8LP
>>>
>>>
>>>
>>> Stuart Rohre wrote:
>>>       
>>>> It is not too simple to measure phase with a additional
>>>>         
>>> circuitry.  However,
>>>       
>>>> it is simple to move the frequency of the antenna analyzer and see the
>>>> Reactance change,  IF it is increasing with frequency it is inductive
>>>> reactance, (+); if decreasing with frequency increase, it is
>>>>         
>> capacitive
>>     
>>>> reactance, (-).
>>>>
>>>> Once you get used to that, and make a little reminder note to
>>>>         
>>> paste on the
>>>       
>>>> analyzer case, you can do the check easily and remember which type of
>>>> reactance change you are seeing re frequency.
>>>>
>>>> Stuart
>>>> K5KVH
>>>>
>>>>         
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