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LF: Re: VLF - G3XIZ - Now QRT

To: <[email protected]>
Subject: LF: Re: VLF - G3XIZ - Now QRT
From: "James Moritz" <[email protected]>
Date: Sun, 17 Apr 2011 21:20:37 +0100
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Dear Chris, LF Group,

G3XIZ was visible on my spectrogram between about 0400 and 1630utc today. The attachment uses 715uHz FFT resolution as before.

As for the matching transformer, I would use a large SMPSU type ferrite core, since these are fairly easy to obtain. The ones in PC type PSUs or line output transformers would be too small at this low frequency for several hundred watts throughput. I would suggest getting an ETD59 core assembly - These are stocked in the UK by RS and Farnell and possibly others. The total cost of a core set (2 core halves, 1 bobbin, 2 clips) would be about 20 - 25 GB pounds, including VAT and delivery. They are supplied in "power supply" type MnZn ferrite grades, which are all fairly similar and have low losses at 9kHz. Bearing in mind that

Bpeak = Vrms/(4.44*f*N*Ae)

For a 50ohm winding on such a core, (Ae = 368e-6 m^2) and e.g. 500W throughput, 72 turns would give a peak flux density of 150mT, which is about half the flux required to saturate the core and would result in only a few watts of magnetic losses in the core, so pretty conservative. For other impedances, you could scale the required number of turns according to the square of the impedance ratio. To select the wire, you would aim at sizes such that both windings together nearly fill the bobbin completely, in order to minimise copper losses. For low impedance windings, a bundle of smaller wires in parallel gives slightly less loss and is much easier to wind.

Cheers, Jim Moritz
73 de M0BMU

Attachment: G3XIZ_17_4.jpg
Description: JPEG image

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