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Date: Thu, 24 Mar 2011 11:10:23 -0700
From: Scott Tilley <sthed475@telus.net>
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Subject: Re: VLF: Resonate the antenna, or not?
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Hi Rik / Piotr

Thank you for taking the time to consider this and provide some very=20
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=20
Stefan's and conduct tests on the viability of the idea.  This should=20
allow me to experiment with current levels up to 20A or so maybe more if=
=20
I parallel FETs etc...

2) Build a Mother of All Very Low Frequency Amplifiers that will allow=20
current levels in excess of 100A.  This will take some time and may not=20
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=20
accurate as the conductor has seven large strands significantly increase=
=20
the surface area.  This resulted in a very low Rac value from the=20
expected on 2200m.

These calculations assume a cylinder of wire.  The wire I'm using is=20
actually 7 large strands and this increases the available surface area a=
=20
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 ef=
fective area of 16.2mm and thus a resistance of 0.174 Ohm for 100m wire (b=
ased on a resistivity for Al =3D 2.82E-8 Ohm*m, see http://en.wikipedia.or=
g/wiki/Electrical_resistivity_and_conductivity#Resistivity_of_various_mate=
rials)
> 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://w=
ww.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: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.=
org] namens Piotr Mlynarski [sq7mpj@toya.net.pl]
> Verzonden: donderdag 24 maart 2011 3:27
> Aan: rsgb_lf_group@blacksheep.org
> Onderwerp: Re: VLF: Resonate the antenna, or not?
>
> Stefan Sch=E4fer 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 (=3D600m^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=3D 8970 Hz
> skin depth: 0.925 mm   so Rac @ 8970 Hz  =3D 0.174 Ohm
> 20m x 30 m rectangular loop - its inductance L =3D 162.85 microHenry
> /RA9MB equation/
> radiation resistance Rrad =3D 0.008994 microOhm  / or using picoOhms as=
 in
> recent examples 8994 pOhm/
> 2*pi*f*L =3D 9.178 Ohm
> 2*pi*f*L/Rac =3D 52.72
>
> using enough pwr to develop 100 Amp current  TX radiated is 89.94 microw=
atts
> loop over ground i.e. twice the Rad along with -0.39dBd loop gain
> results in ERP =3D 164.4 microWatts
> Voltage loop around 918 Volt
>
>                                                             73, Piotr,=
 sq7mpj
>
> qth: Lodz /jo91rs/
>