To: | [email protected] |
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Subject: | Re: LF: Re: More on unstable antenna R |
From: | N1BUG <[email protected]> |
Date: | Mon, 3 Dec 2018 15:10:29 -0500 |
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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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