Hi Stefan,
I agree. The scope match may be an old idea but it is still very
useful. Most VNA's output low power, some as low as -20 dBm, some
maybe up to +10 dBm. They make very accurate measurements, but
cannot see all the possible things that go wrong when we push large
amounts of power into things. ;-) They also don't tell us anything
about our transmitter.
For now I have taken your advice and I'm trying to just forget it.
It is hard for me. It did not do this last winter, so something has
changed. I still do not like unexplained changes in RF systems. :-)
Usually I try to set the taps so it goes through the 50 ohm point
somewhere in the middle of a transmission... except when I get lazy!
73,
Paul
On 12/3/18 4:03 PM, DK7FC wrote:
> 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
>>>
>>
>
>
--
Paul
N1BUG 2200m-2m DXCC Honor Roll
FN55mf ME Piscataquis County
http://www.n1bug.com
http://www.aurorasentry.com
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