Hi Paul thanks for the details of your measurements......yes they look
consistant. My guess is that the increased "resistance" and reduced
capacitance may be due to some inductance in the resistors, though it does
look a big difference for that at low frequencies.
Another test is to swap the reference and unknown components across the
bridge to check and see if there are any symmetry problems. The best use of
the bridge is really to see how things change as you make "improvements" to
the antenna system and grounding.
I think the bridge accuravy (or maybe just the sensitivity) of this series
conntion is best when the reactane and resistance are in the same range.
There is always the chance of pick up of srong signals often out of band and
a tuned dector is essential as is a quite high drive level (to overcome the
external signals). A good bridge will give a very sharp and deep null,
butoften the null is actually the level of the interference picked up on the
wire. It can be useful to increase the level of the drive and also switch it
off cmpletely and see what happens to the bridge output.
Great stuff, its can be very interesting....I re-learned a lot of radio when
I started playing LF.
Best Wishes
Alan G3NYK
----- Original Message -----
From: "Paul A. Cianciolo" <[email protected]>
To: <[email protected]>
Sent: Saturday, March 27, 2010 9:01 PM
Subject: 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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