As previously stated, you're fine. What you are seeing is quite expected.
Take your cal standard of 51 ohms: Assuming your cal standard is truly is
51.0000000 ohms (DMM measurement tolerance), the SWR against
50.000000...... ohms would be 51.0000000 / 50.000000, or 1.020:1. I doubt
any amateur SWR meter would display this as anything other than 1:1.
Again, not a problem.
Take your coax: Try making an identical measurement with the antenna end
of the line terminated with your 50-ohm cal. load or another known good
resistor (connect with no leads and the resistor must be good at 146 MHz -
no series reactance) in place of the antenna. The ripple is likely due to
your antenna not being 50.000..... ohms over the entire bandwidth you are
measuring. Again, this is quite typical and not a problem.
Dave - W?LEV
toggle quoted message
Show quoted text
On Thu, May 21, 2020 at 3:15 PM Kurt Poulsen <kurt@...> wrote:
Hi Ed
It is quite normal what you see. Please note that the number of point are
only 100 so for the span used 50MHz you have a point for each 0.5MHz.
Reduce the span to e.g. 140 to 150 MHz and see for every 100KHz the
impedance / SWR. The load are OK it is not what you concern should be, It
do not think you Ohm meter are accurate enough to determine the very exact
DC resistance and will only create a very little SWR contribution.
I would guess you antenna does not have 50ohm impedance on 145MHz. The
oscillation you see are natural for reflection from the antenna which is
wandering forth and back in the cable.
Kind regards
Kurt
-----Oprindelig meddelelse-----
Fra: [email protected] <[email protected]> P? vegne af
ed@...
Sendt: 21. maj 2020 07:35
Til: [email protected]
Emne: [nanovna-users] Calibration issue causing ripples in SWR?
#calibration
After calibrating, I connected my nanovna to my VHF antenna via a brand
new 75 foot length of 50ohm DX400MAX coax. The SWR measurement oscillates
up and down every 4MHz or so (see attached). These correspond exactly to
the half-wavelength harmonics of the cable. Looking at the smith chart, it
is clear that the impedance is tracing a circle around about 45 ohms rather
than the constant-SWR circle centered at 50 ohms, resulting in a
frequency-dependent SWR.
Is there is a good way to determine whether this is a problem with my
nanovna, with the calibration standard, or with the cable?
One note: the calibration standard load measures 51 ohms DC resistance,
which seems like a problem to me. Should my standard be exactly 50 ohms dc
resistance?
--
*Dave - W?LEV*
*Just Let Darwin Work*
