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From: "Paul A. Cianciolo" <paulc@snet.net>
To: <rsgb_lf_group@blacksheep.org>
Date: Sat, 27 Mar 2010 17:01:31 -0400
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Subject: RE: LF: Question about ground impedance at 8.97 KHZ   **UPDATE
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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=3D2285

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 driv=
en 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 si=
mulated 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 =3D  780 Ohms and  507 PF on  LCR meter

Sim load =3D  940 Ohms and  460 pf  on the bridge.


Antenna =3D  1111 Ohms and 330 PF on bridge=20

Antenna also measured at 137 KHz

Antenna =3D 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 st=
arted.


PauL C
W1VLF












-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org]On Behalf Of Johan H. Bodin
Sent: Friday, March 26, 2010 4:32 PM
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Question about ground impedance at 8.97 KHZ Stefan.


Hi Paul,

it sounds like your impedance bridge is measuring the absolute impedan=
ce
(that is Z =3D (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 majo=
r
part of your impedance is the capacitive reactance of the antenna (Xc)=
.
At 9 kHz, where C =3D 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:
>=20
> Hello All,
>=20
> Below is a quote from the Stefan's web page concerning his latest "D=
reamers Band" DX contact Congratulations again Stefan.
>>From this information alone is it possible to calculate or know the=
 antenna system impedance.
>=20
>=20
>=20
>=20
> "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."
>=20
> Pertaining to the coil "Technical and mechanical data: L=3D553mH, Q=
=3D82, R(DC)=3D283 Ohm; 0,4mm diameter enameled copper wire, about 120=
0 turns! Average diameter 0,55m, height abt 0,5m. So, about 2000m wire=
! :-)"
>=20
>=20
>=20
> I have built a small impedance bridge that operates at 10 KHZ and sh=
ows my sloping 170' wire as 70K Ohms. I can also see approx 500 PF tha=
t can be nulled out using the bridge?
>=20
> Can the 70K value possibly be correct?   (3) 6' ground rods about 6'=
 apart are the ground... plus the electrical ground of the house.
>=20
> I am hoping to attempt a local transmission on 9KHZ=20
>=20
> Any information would be helpful.
>=20
> Thank you
>=20
> Paul
>=20
> W1VLF
>=20
>=20
>=20
>=20
>=20