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Date: Mon, 03 Dec 2018 22:03:00 +0100
From: DK7FC <selberdenken@posteo.de>
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Subject: Re: LF: Re: More on unstable antenna R
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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: 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
>>      
>