Rick is correct.
Any easy way to see this is to use Dan, AC6LA's, TLDetails program.
Wes N7WS
On 4/26/2020 1:38 PM, VE6WZ_Steve wrote:
Rick,
We are talking about sweeping the feedline and looking at the REAL R sweep vs
frequency….NOT impedance Z=R+jX.
We are looking for relative dip in R, not absolute values. We are looking to
see at what frequency this dip occurs.
If I put a 25 ohm resistor and then a 50 ohm resistor at the end of a 55 degree
feedline, are you saying the real R (not Z…no X) will read 50 ohms in both
cases? I think my multimeter would say something different at DC.
If this is true, then you are correct, and the dip in the real resistance seen at the
driver at the parsitic frequency would be muted….the sweep would be flatlined
at 50 ohms across the frequency of interest.
de steve ve6wz
On Apr 26, 2020, at 12:35 PM, Richard (Rick) Karlquist <richard@karlquist.com>
wrote:
On 4/23/2020 7:40 PM, VE6WZ_Steve wrote:
Tom,
Yes.
Just sweeping from the end of the coax in the shack should show the same dip of
R at the correct frequency.
(The absolute R value (in Ohms), and X value will not be correct because of the
feedline transformation, but the frequency of the R dip is ONLY a function of
the reflector tuning.) Try it and see.
I don't see how this would be true for an arbitrary length
of coax, especially if it is an odd multiple of a quarter
wave. For example, if the load on the far end changes
from 50 ohms to 25 ohms, the analyzer in the shack will
see resistance INCREASE from 50 to 100 ohms. A line with
electrical length of 55 degrees will have a resistance
at the shack of 50 ohms whether terminated in 50 ohms
or 25 ohms.
73
Rick N6RK
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