Hi Paul,
I think the scope match may be an old method but is advanced because it
shows not only the magnitude of voltage and current and the phase
between them, it also shows the waveforms at the power level of
operation! A VNA, if i'm informed correctly injects a small test signal,
so it is not a measurement at the peak power!
If your antenna R just changes from 58 Ohm to 46 Ohm, then just forget
it! I bet the changes can be much higher on average radio amater setups.
If you like, reduce the number of secondary turns of your output
transformer by 1 so you come out at 50 Ohm after 2 minutes :-)
And when there is some snow, run the system at excessive power levels
and see where the snow is melting :-)
73, Stefan
Am 03.12.2018 21:10, schrieb N1BUG:
Hi Rob,
I would of course agree the ScopeMatch isn't a precision instrument.
I find it reasonably accurate in that if I key up the transmitter
and measure I / E and see that they aren't equal, do the math to
calculate R, translate that to what R would be at the other end of
the coax (in practice with such a short line it isn't significantly
different), calculate a new turns ratio for the antenna matching
transformer to compensate, go out and change the taps I subsequently
find R right where I expected it to be, give or take an ohm.
I don't have a VNA. I can borrow one and have it shipped to me as I
did for initial setup of the system but it wouldn't help with this.
The change I see is very gradual during a transmission but returns
to the "base line" so quickly after RF stops that by the time I
could disconnect the transmitter and connect the VNA I would never
see the lower R condition that developed during transmission.
This is a real head scratcher. As I was saying to Jay a bit ago,
operationally it's not a major problem. I am being heard and the
amplifiers haven't objected. What worries me is that I have no way
of knowing what other antenna, cable, or equipment I might be
damaging! This could be evidence of high RF current or voltage in
places I do *not* want them! Hopefully I am just being paranoid.
73,
Paul
On 12/3/18 2:13 PM, Rob Renoud wrote:
Hi Paul,
Thanks for the info regarding your measurements.
I believe you use a ScopeMatch in the shack to make your tuning adjustments
and voltage/current measurements. I also run a ScopeMatch on LF and MF that
uses a capacitor voltage divider and toroid current sensor. I have found
the ScopeMatch to be an excellent tool for visually tuning the matching coil
to resonance and R=50 ohms as well as monitoring for changes during the
operating period. I have found, however, that the SceopeMatch is not
particularly accurate for determining actual SWR or input impedance (R
and/or X) based on voltage and current measurements using that tool when
compared to a know accurate VNA.
Wonder if you might have access to an antenna analyzer or VNA to measure the
changes that you are experiencing? Think that approach might be more useful
in identifying where to look.
Like you, I also have significant winter to summer changes in the antenna
values that are handled by the tuner. I have not experienced any changes
during the transmitting period, however, so am scratching my head with
you....
Regards,
Rob - K3RWR
-----Original Message-----
From: N1BUG
Sent: Monday, December 03, 2018 5:54 PM
To: [email protected]
Subject: Re: LF: More on unstable antenna R
On 12/3/18 12:22 PM, N1BUG wrote:
At the time of the tests in question, R at the transmitter end of
the line varied from 58 ohms at the start of a transmission to 46
ohms 2 minutes later. I calculated that from measured voltage and/or
current on a 50 ohm line with no reactance.
Uh... not quite. Let's try that again!
Measure voltage, measure current
(I measured peak on the scope and converted to RMS)
Calculate R = E/I
I am assuming that works as long as X = 0. Voltage and current
waveforms were exactly in phase for all measurements.
Correct me if I have gone wrong here. I can always go wrong with math.
73,
Paul
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