Hi Rik / Piotr
Thank you for taking the time to consider this and provide some very
useful information.
I'm going to tackle this project in stages:
1) Build a smaller h-bridge TX say on the order of 300-600W similar to
Stefan's and conduct tests on the viability of the idea. This should
allow me to experiment with current levels up to 20A or so maybe more if
I parallel FETs etc...
2) Build a Mother of All Very Low Frequency Amplifiers that will allow
current levels in excess of 100A. This will take some time and may not
be ready very soon and of course is dependent on the results of phase 1.
One note on the wire I'm using, modeling it as a cylinder is not very
accurate as the conductor has seven large strands significantly increase
the surface area. This resulted in a very low Rac value from the
expected on 2200m.
These calculations assume a cylinder of wire. The wire I'm using is
actually 7 large strands and this increases the available surface area a
lot and provided much better performance on 2200m than modeled.
Anyway, we shall see how this goes :-)
73 Scott
On 3/24/2011 2:33 AM, Rik Strobbe wrote:
Hello Piotr, Stefan, Scott,
based on 6.5mm wire diameter and a skin depth of 0.925mm I come to an effective
area of 16.2mm and thus a resistance of 0.174 Ohm for 100m wire (based on a
resistivity for Al = 2.82E-8 Ohm*m, see
http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity#Resistivity_of_various_materials)
What is the same as Piotr's result.
Using IN3OTD's formula I come to a loop inductance of 161 uOhm what also agrees
well with Poitr's calculation.
The ERP values calulated by Poirt seem also correct to me.
The maximum current for a 6.5mm (2 AWG) Al wire is 100A DC (see
http://www.engineeringtoolbox.com/maximum-current-copper-aluminum-wire-d_1690.html).
At 9kHz the current runs only trough about 50% of the wire area (skin effect),
so 100A might be too much.
73, Rik ON7YD - OR7T
________________________________________
Van: [email protected] [[email protected]]
namens Piotr Mlynarski [[email protected]]
Verzonden: donderdag 24 maart 2011 3:27
Aan: [email protected]
Onderwerp: Re: VLF: Resonate the antenna, or not?
Stefan Schäfer pisze:
Wow, nice!
Rik/ON7YD, now its your turn again: What will be the ERP when running
100A in that wire? I think 100A is realistic on such a wire. Skin
depth should be 0.9 mm. So the effective conductor area 19 mm^2.
Assuming 20m x 30m (?) wire (=600m^2) means 100m length, so R ~ 0.15
Ohm. So Scott gets 100 A rms when running 1.5 kW into the antenna (no
problem for the mains), without having problems of winding a large
coil, without resonating, without ferrite parts, without high voltages
(but high currents ;-) ). Am i right, Rik? And what is the ERP? I
could take a look on your website but you can do it so well here! What
is the L and Z and so the needed voltage to get 100A? :-)
Instead of standard FETs, some IGBTs could be used, shouldn't be a
problem at 9 kHz and some 100V. Any recommendations? :-)
Scott, what FETs do you use in your design? The IRFP460 is OK for 20 A
but not much more. One could do the effort and switch 4x 10 of them in
parallel but you'll need a good driver, maybe IXDD430... Interesting
stuff!
73, Stefan EI/DK7FC
Hi Stefan, LF group
Just before i was going to switch off my computers and go to bed
your mails arrived ....
below my quick calculations / Rik , pse, check it out :) /
Al wire diameter 6.5 mm length 100 meters, f= 8970 Hz
skin depth: 0.925 mm so Rac @ 8970 Hz = 0.174 Ohm
20m x 30 m rectangular loop - its inductance L = 162.85 microHenry
/RA9MB equation/
radiation resistance Rrad = 0.008994 microOhm / or using picoOhms as in
recent examples 8994 pOhm/
2*pi*f*L = 9.178 Ohm
2*pi*f*L/Rac = 52.72
using enough pwr to develop 100 Amp current TX radiated is 89.94 microwatts
loop over ground i.e. twice the Rad along with -0.39dBd loop gain
results in ERP = 164.4 microWatts
Voltage loop around 918 Volt
73, Piotr, sq7mpj
qth: Lodz /jo91rs/
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