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| To: | <[email protected]> |
|---|---|
| Subject: | LF: Re: Mini-Whip |
| From: | "James Moritz" <[email protected]> |
| Date: | Wed, 24 Mar 2010 20:49:43 -0000 |
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| References: | <[email protected]> <38A51B74B884D74083D7950AD0DD85E82A1B81@File-Server-HST.hst.e-technik.tu-darmstadt.de> <[email protected]> <56800827A5644E1BB3F320CB51E2300E@Black> <26E279AD762E44FB937A8A7DF8F934BC@Black> <8C7F2923D86B4517B1C156BA499E96E4@Black> <F277E8E705B545489E6506C9E10C1225@Black> <bd8.4bf8185f.38d7802a@UNKNOWN> <B25CA16D1AFD4B7E889CB2E75FC2F8FE@White> <29746320.1346347.1269244849281.JavaMail.fmail@mwmweb078> <[email protected]> <[email protected]> <38A51B74B884D74083D7950AD0DD85E82A1B87@File-Server-HST.hst.e-technik.tu-darmstadt.de> <0A742B5B9F6A4E84A802D1F98B670E79@JimPC> <[email protected]> <op.u909m21cyzqh0k@pc1> |
| Reply-to: | [email protected] |
| Sender: | [email protected] |
Dear Roelof, LF Group,Thinking about your mini-whip design, it seems to me it might benefit from modification for VLF use. The circuit is a high Z buffer driven by the capacitive probe - the source impedance might be 10 - 20pF , if one includes the JFET follower input capacitance with the probe capacitance as part of the source. The reactance of 15pF at 9kHz is about 1.2Megohms. This is shunted via the bias resistors, totalling 1.5Megohms, so nearly 3dB loss of signal amplitude will occur relative to higher frequencies at 9k, or more at lower frequencies if one were to use the antenna for "natural radio" purposes - the resistors will also add to the input noise voltage. So for VLF use, higher value bias resistors might be appropriate. Alternatively, the probe capacitance could be increased. The band noise level may be so high that this doesn't make much difference in practice though, as far as SNR is concerned. Cheers, Jim Moritz 73 de M0BMU |
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