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From: N1BUG <paul@n1bug.com>
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Message-ID: <99a4aabf-81ed-bddc-0380-ce69cb39ad15@n1bug.com>
Date: Mon, 3 Dec 2018 15:10:29 -0500
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Subject: Re: LF: Re: More on unstable antenna R
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Reply-To: rsgb_lf_group@blacksheep.org

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: rsgb_lf_group@blacksheep.org
> 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