Re: LF: Re: More on unstable antenna R

[N1BUG <paul@n1bug.com>]

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 <k3rwr@md.metrocast.net>
>  To: rsgb_lf_group@blacksheep.org 
>  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: 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 
> 
> 
> 
> 
>    
> 

-- 
Paul
N1BUG  2200m-2m        DXCC Honor Roll
FN55mf      ME      Piscataquis County
http://www.n1bug.com
http://www.aurorasentry.com