David,

When you say tuning changes do you mean resonant frequency (where
reactance is zero), antenna system resistance, or both? Mine seems
to stay resonant OK, but the resistance is highly variable, dropping
lower with sustained RF of more than 70 watts into it.

Paul



On 12/3/18 2:26 PM, David Hine wrote:
> I worked with large VLF transmitters, and dampness caused
> leakages on the antenna and in the ATU room will affect the
> tuning. When you said that this changes when the antenna is used
> shows this is the likely cause. The sharper the tuning, the worse
> this becomes, David Hine
> 
> From: Rob Renoud <[email protected]> To:
> [email protected] Sent: Monday, 3 December 2018,
> 19:16 Subject: LF: Re: More on unstable antenna R
> 
> 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