Re: KPA-1500 issues

Posted by Wes Stewart-2 on Nov 28, 2018; 7:38pm
URL: http://elecraft.85.s1.nabble.com/KPA-1500-issues-tp7646571p7646612.html

In the link I provided earlier (including some references) I point out some of
the pitfalls and error sources so that is a possibility.  But I still contend
that, with a lossless line anyway, the impedance of a load, resistive or
otherwise, is the same at either end of a 1/2 wavelength line, or integer
multiple thereof.  A quarter-wavelength line has the "magical" property of
inverting impedances, best seen on a Smith chart.  But the rotation is around a
circle of constant SWR, with radius=rho.

Of course better instruments as a rule make better (more accurate)
measurements.  I have several vector analyzers, an N2PK that I built years ago,
an SDR-Kits by DG8SAQ, an FA-VA5 by DG5MK, an AA-55 Zoom by Rig Expert and if it
ever clears US Customs, an FAA-450 by EU1KY.  Any of these, used with
appropriate care, is more than accurate enough for any normal transmission
line/antenna evaluation and remarkably inexpensive.

Wes  N7WS

On 11/27/2018 9:33 PM, K9MA wrote:

> I think the point Mike is trying to make is that many SWR meters don't
> actually measure SWR all that accurately as impedance changes. While the
> length of transmission line doesn't affect SWR (other than the effect of
> loss), the impedance at a given SWR does change with line length, and that may
> cause the indicated SWR to change with some instruments. I wonder if an
> antenna analyzer, which actually measures impedance, and calculates SWR from
> that, would do any better. If it measures impedance accurately, it should.
>
>
> 73,
> Scott K9MA
>
>
> On 11/27/2018 21:34, Wes Stewart wrote:
>> I fail to see what that will prove. The only length that will do anything
>> exciting is the 1/4 wavelength line and even that doesn't affect the SWR. 
>> The other two will just repeat (less loss) on the input side, what terminates
>> the load side.
>>
>> Wes  N7WS
>>
>> On 11/27/2018 3:51 PM, Michael Walker wrote:
>>> If you truly want to understand SWR, I recommend you pick a frequency and
>>> make up a 1/4, 1/2 and 1 wavelength sections of 50ohm coax.
>>>
>>> Then measure the SWR at those points into various different resistive
>>> loads, like 10, 25, 50, 75, 100, 150 and 200 ohms.
>>>
>>> I think you will find the results very interesting.
>>>
>>> Mike va3mw
>>>
>>
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