From: DL4YHF@aol.com
Message-ID: <28.2ba87525.2a994fee@aol.com>
Date: Sat, 24 Aug 2002 17:09:02 EDT
Subject: Re: LF: Active antenna for LF reception.
To: rsgb_lf_group@blacksheep.org
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<HTML><FONT FACE=arial,helvetica><FONT  SIZE=2>Hello Giulio,
<BR>
<BR>
<BR>Here a translation of the text below DF8ZR's diagram:
<BR>
<BR>&gt;&gt;&gt;
<BR>The Circuit
<BR>
<BR>The picture shows the classic design of the three stage amplifier. Only commonly available parts were used. Only the coils for the tank circuit are the usual inductor coil kits by Conrad (YHF: a german mail order company). The ferrite slugs are rated to 100kHz, so Q factors above 200 could not be realized. Pot cores with HF "Litz" wire and matched(?) ferrite can give higher Q, so the bandwidth could be reduced to a few Hz. The amplifier is tuned for maximum gain around 137 kHz. Then the DCF39 carrier at 138.3 kHz is rejected enough to watch the &nbsp;QRSS transmissions at 137.7kHz. Audible CW at the lower band edge can be copied also.
<BR>
<BR>Note: The circuit has been developed for the "VSCW2". If another receiver is connected, the output signal must be reduced because high(?) levels are produced.
<BR>
<BR>
<BR>Final comment (by DF8ZR)
<BR>Used with the VCSCW2 or VSCW1 this combination reaches a similar SENSITIVITY like commercial receivers. With both modules listening to amateur radio signals on longwave is possible with minimum effort. [ .. ]
<BR>
<BR>&lt;&lt;&lt; (end of translation)
<BR>
<BR>
<BR>IMHO, the small coils are ok for a small antenna as mentioned in the article, but I would not connect this high-gain amplifier to a large antenna. The gate-protection diodes at the input will produce terrible intermodulation (there is no selective measure at the gate of the 1st FET !). Use AT LEAST ONE tank circuit before the FET and the limiting diodes, or you hear an ugly mix of DCF, DLF and your "local" LF broadcaster. Reducing the bandwith (and the amplitude of the "sum signal" after the 1st amplifier is too late, at least in this part of DL.
<BR>
<BR>Regards,
<BR> &nbsp;Wolf &nbsp;
<BR> &nbsp;&nbsp;DL4YHF.
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