[Elecraft] Can I measure antenna impedance with K2?

Per-Tore Aasestrand ptaa at ieee.org
Fri Aug 1 03:28:25 EDT 2014


Hi Don,

Thanks for your explanation.
Yes, I can follow you in this reasoning, and totally agree.
I was clearly not reading carefully enough earlier posts, and was jumping
to conclusions.

Per-Tore / LA7NO




On 1 August 2014 03:32, Don Wilhelm <w3fpr at embarqmail.com> wrote:

> P-T,
>
> No, a conjugate match will assure 100% power transfer.
>
> Look at it this way - what the conjugate match says is that if you cut a
> transmission line at any point, looking one way at that cut point, you will
> have some impedance - example is 70 + j20.  Now look the other way and the
> impedance will be the conjugate match - 70 - j20.  That is the condition
> that exists.
> It also happens to be the condition for maximum power transfer.
>
> Since that cut can be made at any and all points along the transmission
> line - think what would happen if the efficiency at each of those points
> (when connected together) were 50% - nothing would get to the antenna.  So
> we know that 50% is *not* the efficiency of any and all junctions of any
> conjugate match.
>
> Mixing the conjugate match concept with the maximum power transfer theorem
> is getting us into confusion - there are 'holes' in that combination.  Yes,
> they work together, but not seamlessly.
> If a generator has an internal impedance of 50 ohms, the maximum power
> transfer will be only if the load to that generator is also 50 ohms.  That
> says nothing about a conjugate match.
>
> Now to further complicate things, the internal impedance of a generator
> has nothing to do with the efficiency of that generator - especially if we
> are discussing a PA output stage.  Bringing the conjugate match concept
> into the internal design of a PA stage is in error - it just does not work
> that way.  The conjugate match only applies to the output of that amplifier
> stage.
>
> In other words, if we send 10 watts into a feedline (or ATU), all 10 watts
> will go to the load (antenna) except for losses in the feedline.
>
> I am not about to embark on the design and efficiency of a PA stage on
> this reflector, so take the conjugate match and maximum power transfer
> theorem only to the terminals at the PA stage and all will make sense.
>  They do not apply to the internals of that stage.
>
> 73,
> Don W3FPR
>
> On 7/31/2014 7:32 PM, Per-Tore Aasestrand wrote:
>
>> Hi Don,
>>
>> On 1 August 2014 01:16, Don Wilhelm <w3fpr at embarqmail.com> wrote:
>>
>> If one uses the voltage divider example, yes the maximum efficiency is
>> 50%,
>>
>>> But the output of a PA stage is not a resistor, and the collector load
>>> "resistance" is set by the designer to produce the output power desired.
>>>
>>>  I fully agree.
>>
>> But will not a conjugate match also imply a max efficiency of 50%
>>
>> P-T
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