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Date: Tue, 16 Aug 2011 01:05:23 +0200
From: =?UTF-8?B?U3RlZmFuIFNjaMOkZmVy?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: Ferrite RX antennas...Litz wire
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Another question about Litz wire:

I still have no litz wire on hand but what i already have is that wire: 
http://dl.dropbox.com/u/19882028/LF/CLI200.jpg
It is excellent for winding SMPS transformers but ni'm not sure if it 
performs well on LF too. Is the outer serving important to reduce the 
capacity between the turns or will the effect be negligible? If the wire 
has an outer serving it stays round when winding closely while that 
120x0.1 wire becomes a bit flat, you know...

Any experience?

73, Stefan/DK7FC

Am 15.08.2011 23:14, schrieb James Moritz:
> Dear Stefan, LF Group,
>
> I spent some time reading various texts and data books, and doing some 
> calculations. The complete argument is too long and boring to type 
> into an e-mail, but very approximately it seems to be the case that 
> for a ferrite rod antenna and an air-cored loop antenna to have 
> similar signal/noise performance, the rod length needs to be similar 
> to the loop diameter, which seems intuitively reasonable. This means 
> that the air-cored loop is better for larger antennas (a 1m long 
> ferrite rod is very heavy and expensive), but the ferrite rod is 
> better for smaller sizes (a 30cm long ferrite rod is quite reasonable 
> weight/cost, and less bulky than a 30cm diameter loop).
>
> In that case, "is it possible to make a 30cm long ferrite rod antenna 
> and preamp with a noise level below the 136k band noise floor?" is the 
> question to decide if the ferrite rod is worthwhile for /P reception 
> from a low-noise location. Obviously there are many variables, but one 
> can attempt at least a rough calculation.
>
> Assume Stefan assembles his rods into 2 bundles of 7; this would be 
> roughly equivalent to a single solid rod 28cm long x 21mm diameter. 
> Assuming a high permeability ferrite, this l/d ratio will multiply the 
> flux through the winding by a factor of about 70 compared to the same 
> winding without the core (called "mu_core" in Watt's "VLF 
> Engineering"), so the rod antenna will be equivalent to an air cored 
> loop with an area of 0.025m^2. Assuming a noise floor of 0.06uV/m per 
> sqrt(Hz), a single-turn winding with this area would have an induced 
> EMF of 4.2pV/sqrt(Hz). With a low-noise preamp, assume the internal 
> noise level is all due to the resistive losses of the antenna, which 
> depends on the Q. Q of about 250 should be achievable; the inductance 
> of a single turn winding depends on another permeability parameter, 
> mu_rod, which depends on the rod geometry and the permeability of the 
> ferrite; for this rod about 100 from Philips' ferrite data book. L 
> works out to about 0.16uH, and the loss resistance 540 micro-ohms. The 
> noise voltage density is sqrt(4kTR), 3pV/sqrt(Hz). So the internal 
> noise is below the band noise by 3pV/4.2pV = 3dB. Hooray!
>
> So it could actually be feasible. In order to make it work, it will be 
> important to achieve a high Q. Obviously, a single turn winding with 
> picovolt output levels is not very practical. I would aim for a 
> parallel tuning capacitance of e.g. 5nF, so it can be tuned across the 
> 136k band using a 500p variable. This would require L of 270uH, 
> requiring about 41 turns of thick wire, preferably Litz or multiple 
> strands of thin wire. The parallel impedance at resonance with Q = 250 
> would then be 58kohms. Connecting the tuned winding directly to a 
> "miniwhip" type FET input preamp should work well and add negligible 
> amounts of preamp noise. The increased number of turns and the high Q 
> resonant winding winding will increase the 4.2pV/sqrt(Hz)noise floor 
> at the preamp input by a factor of (41 x 250), so 43nV/sqrt Hz. In a 
> 300Hz CW bandwidth, this would be about 0.75uV of noise, so with a 
> reasonably sensitive RX, no further gain would be needed (worthwhile 
> checking if it IS reasonably sensitive though...).
>
> I stress that these are all very rough calculations - you will have to 
> actually try it out to find what the real values are. But they should 
> be a reasonable "first guess", and it seems to show that the expeiment 
> is worth trying.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>