Hello Paul,
Thank you! And nice that you are also active at the TX front. This is necessary
to keep the VLF ham interest alive ;-)
In my notes at qrz.com there are some average values, some estimated values and
some measured values. Since the kite and thus it's angle is moving, the C and
thus the resonance is slightly changing. Thus, the antenna current is changing
since the L is fixed and the output power is maximum at resonance... With the
power and antenna current you can immediately calculate the overall losses and
if you know the losses of the coil, you know the earth (and other) losses. The
radation resistance can really be neglected ;-)
For the antenna wire you can assume abt 6pF per meter. My antenna has 580pF and
is 100m, so it seems to be a good estimate.
I do not know the arrangement of your measuring bridge but if it is like that,
described by Alan/G3NYK and Finbar/EI0CF, you will get a minimum output signal
when choosing C = C(antenna) and R = R(earth+surroundings).
If your Z is 70kOhm at 9kHz, you will need about 250pF.
I do not know what " ' " means in meters but if your wire is about (250/6)
meters long, it seems to be reasonably correct.
The house earth can be an important part of your VLF earth. On my hill, there
is not such an earth and i have about 450 Ohm earth losses. But for that, you
may have higher surrounding losses by houses and trees.
What i recommend you, is increasing your antenna capacitance! 250pF is not
much. Sure, it is a good starting point but you will need abt 1,2H for
resonance. This will be an very big coil with much losses. The coil losses may
be higher than the earth losses. The limiting factor will not be the output
power that can be generated but rather the voltage, the wire and insulators can
withstand! 10 kV is nothing here! Spend some time to increase the wire length
or place some wires in parallel (distance >1m) as high as possible and as far
as possible to reduce the needed L. It is the same game as on LF but the losses
and voltages are much higher. But there are some advantages as we saw in the
last weeks (e.g. propagation and possible receivers).
Anyway, you will reach your personal records and it is fascinating to improve
the system all the time.
I wish you good luck and hope i have helped you. Just ask if you have further
questions :-)
73, Stefan/DK7FC
________________________________
Von: [email protected] im Auftrag von Paul A. Cianciolo
Gesendet: Fr 26.03.2010 19:38
An: [email protected]
Betreff: LF: Question about ground impedance at 8.97 KHZ Stefan.
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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