LF: Re: VLF - G3XIZ - Now QRT

["James Moritz" <james.moritz@btopenworld.com>]

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

G3XIZ_17_4.jpg
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