From: Rik Strobbe <Rik.Strobbe@fys.kuleuven.be>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
Date: Thu, 24 Mar 2011 20:46:22 +0100
Thread-Topic: VLF: Resonate the antenna, or not?
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Stefan,

have a look here: http://www.seas.gwu.edu/~ecelabs/appnotes/PDF/techdat/CC=
CofCC.pdf
According to this document for a single 2 AWG copper wire in open a curren=
t of 170A will cause a temp rise of 50C above the air temperature and 225A=
 for a 95C temp rise.
As the resistivity of alu is about 1.7 times that of copper I would expect=
  a 50C rise at 130A and  a 95C rise at 170A.
For a 7 strand wire you can take 7 x times the max. current of each strand=
 with a derating factor of 0.7.
If the total area is 35mm^2 (based on 2 AWG) each strand would be about 4.=
7mm^2 =3D 11 AWG and each strand could handle about 40A for 50C rise and=
 60A for 95C rise. For 7 strands and the derating that would be 200A for=
 50C rise and 300A for 95C rise.

So 100A looks OK.

73, Rik  ON7YD - OR7T

________________________________________
Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.or=
g] namens Stefan Sch=E4fer [schaefer@iup.uni-heidelberg.de]
Verzonden: donderdag 24 maart 2011 19:18
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: Re: VLF: Resonate the antenna, or not?

Hi Piotr, Rik, Scott, VLF,

Tnx for the interesting discussion!

Well, 165 uW is quite low for that input power but we have expected that
it's even worse than a  vertical. But the effort is much lower and
Scott's location is not suitable for a vertical....
0.174 Ohm means 1.74 kW at 100 A rms. Still acceptable? :-)

Am 24.03.2011 10:33, schrieb Rik Strobbe:
> 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
>

Hmm, if i imagine a 100m long wire with 6.5 mm diameter where 1.7 kW is
dissapated, it cannot get to warm i think. It depends on the outside
wind and rain and so, of course. But as Scott said, its worth the fun! :-)

Maybe the actual parameters will be different. We shouldn't calculate to
much, the loop and mains is available, a VFO surely too. So just get
started and see what (current) can be achieved :-)

Best 73, Stefan/DK7FC







> ________________________________________
> 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/
>