Message-ID: <AA77ECC040694A4D9D94B876DC20D8FF@Rob64PC>
From: "Rob Renoud" <k3rwr@md.metrocast.net>
To: <rsgb_lf_group@blacksheep.org>
References: <0d2ac752-44f5-e503-e2a0-a8b4c841bcee@n1bug.com> <5C052984.8030902@posteo.de> <04207F3E-523E-40B8-A93C-E1AEBE0D3A4A@md.metrocast.net> <b00ba0af-a6f2-8e63-2dc5-68b562e9f28a@n1bug.com> <3c05b896-ecf2-8b93-b6e7-e618fa7263c3@n1bug.com>
In-Reply-To: <3c05b896-ecf2-8b93-b6e7-e618fa7263c3@n1bug.com>
Date: Mon, 3 Dec 2018 19:13:43 -0000
MIME-Version: 1.0
Importance: Normal
Subject: LF: Re: More on unstable antenna R
Content-Type: text/plain;
	format=flowed;
	charset="UTF-8";
	reply-type=original
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org

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