Paul, the values you measured seem to be realistic for me. Especially the measured antenna C does not really differ very much between 137kHz and 9 kHz. It is a good value that allows you to dimension your needed L. The needed L is nearly independent from the earth losses. I generally recommend a variometer part of 1...5% of the fixed L to compensate all effects that influence the antenna C. But think about if it is possible to increase the antenna C instead of building a very huge coil that causes further losses.

 

Good luck and have fun with building up! :-)

 

73, Stefan/DK7FC

---

Von: [email protected] im Auftrag von Paul A. Cianciolo
Gesendet: Sa 27.03.2010 22:01
An: [email protected]
Betreff: RE: LF: Question about ground impedance at 8.97 KHZ **UPDATE

Hello Folks,

Thank you for all the help with measuring my antenna impedance.

Let me explain further what I was using to measure with and then add some updates.
The homebrew bridge I was using was built as the one in the following link.
The diagram at the very bottom of the page is the schematic I used.
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=2285

This really my first venture into things LF and I had really no idea of what to expect for numbers
After posting my results here, and talking to W1VD, he forwarded me a link to Alan and Finbar's bridge experiments here
http://www.alan.melia.btinternet.co.uk/aelossbr.htm

Basically I duplicated that design, making sure that the transformer had 100 mHy of inductance to work at 10 KHz.
Sure enough the bridge worked, and balance with my antenna, but would not balance with my antenna simulator load.

The simulated load is 2 X 390 ohm .1% resistors in series with a 500 pf silver mica.

Reading on Alan talks about stray capacitance.

"The problem with the usual trifilar wound transformer is that if driven from an unbalanced source the secondary suffers unbalanced strays to ground. This would mean that if you were to swap the components on the arms of the bridge the balance would be different"

With that in mind I built another transformer to balance the drive to the bridge.  After doing this the bridge will now balance with the simulated load and the antenna.
See the following numbers for results:

Measurments were taken 10Khz
The "R" and "C" "known" side of the bridge were then measure measured

Sim load =  780 Ohms and  507 PF on  LCR meter

Sim load =  940 Ohms and  460 pf  on the bridge.


Antenna =  1111 Ohms and 330 PF on bridge

Antenna also measured at 137 KHz

Antenna = 85 ohms and 340 pf @137 Khz.


I think these are more in line with what I should be reading????

What do you folks think?

Thank you to everyone who responded to this post and helping me get started.


PauL C
W1VLF












-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Johan H. Bodin
Sent: Friday, March 26, 2010 4:32 PM
To: [email protected]
Subject: Re: LF: Question about ground impedance at 8.97 KHZ Stefan.


Hi Paul,

it sounds like your impedance bridge is measuring the absolute impedance
(that is Z = (R^2 + X^2)^0.5 where R is the real part, resistance, and X
is the imaginary part, reactance). R is probably a 10 to a few hundred
ohms, most of it is ground resistance - it can be neglected - the major
part of your impedance is the capacitive reactance of the antenna (Xc).
At 9 kHz, where C = 1/(2*Pi*9000*Xc), 70 kOhms means 253pF which is in
the ballpark for your 170' wire (about 5pF/m).

73
Johan SM6LKM

----

Paul A. Cianciolo wrote:
>
> Hello All,
>
> Below is a quote from the Stefan's web page concerning his latest "Dreamers Band" DX contact Congratulations again Stefan.
>>From this information alone is it possible to calculate or know the antenna system impedance.
>
>
>
>
> "100m Vertical wire antenna, coil/antenna voltage was abt 15kV rms, average antenna current about 480mA, giving up to 1,7mW ERP. Applied power was up to 250W."
>
> Pertaining to the coil "Technical and mechanical data: L=553mH, Q=82, R(DC)=283 Ohm; 0,4mm diameter enameled copper wire, about 1200 turns! Average diameter 0,55m, height abt 0,5m. So, about 2000m wire! :-)"
>
>
>
> I have built a small impedance bridge that operates at 10 KHZ and shows my sloping 170' wire as 70K Ohms. I can also see approx 500 PF that can be nulled out using the bridge?
>
> Can the 70K value possibly be correct?   (3) 6' ground rods about 6' apart are the ground... plus the electrical ground of the house.
>
> I am hoping to attempt a local transmission on 9KHZ
>
> Any information would be helpful.
>
> Thank you
>
> Paul
>
> W1VLF
>
>
>
>
>

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