Re: LF: Re: More on unstable antenna R

[David Hine <evad_hine@yahoo.co.uk>]

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

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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