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
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[email protected]
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [email protected]
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[email protected]
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [email protected]
