To: rsgb_lf_group@blacksheep.org References: <000c01c49078$54a6a860$4ceafc3e@l8p8y6> Date: Mon, 6 Sep 2004 21:07:16 +0100 MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.00.2615.200 X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2615.200 X-SPF-Result: relay.thorcom.net: 217.12.12.138 is neither permitted nor denied by domain of btopenworld.com X-Spam-Score: 1.7 (+) X-Spam-Report: autolearn=no,HTML_20_30=0.474,HTML_MESSAGE=0.001,RCVD_IN_DSBL=1.101,RCVD_IN_SORBS=0.1 Subject: LF: Re: Static Content-Type: text/html; charset=windows-1252 X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Status: No, hits=0.5 required=5.0 tests=HTML_20_30,HTML_MESSAGE autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-Spam-Filtered: by PlusNet SpamCORE (v3.00) Content-transfer-encoding: 8bit

----- Original Message -----

From: hamilton mal

To: rsgb_lf_group@blacksheep.org

Sent: Thursday, September 02, 2004 12:06 AM

Subject: LF: Static

The theory about loop advantage over a vertical is not correct...

73 de Mal/G3KEV

Dear Mal, LF Group,

The supposed interference-rejection properties of loops are based on the fact that a loop responds mostly to the H (magnetic) field component of the electromagnetic wave. Hopefully, E (electric) fields will be rejected. But I don't think this would help a loop antenna reject QRN - the high-voltage lightning discharge will generate an E-field transient, but the kiloamps of current in the stroke will generate plently of H-field transient also. I don't know about the relative intensity of E and H fields close to the strike, but once you get several wavelengths away, the ratio E/H will be constant at 377ohms, just like every other propagating radio wave. So I don't think there is a theretical basis to expect any advantage rejecting QRN or other long-distance interference in using an E or H field antenna for reception.

Where a loop antenna might be an advantage is in rejecting local E field noise, which will not neccesarily have a corresponding H field component. This is an advantage in applications where high voltage, low current phenomena are at play, such as "precipitation static". But I think most of the local QRM experienced by us LF amateurs is largely H-field; noise currents of one sort or another flowing down mains cables, generating primarily magnetic noise fields. Certainly, I have not found that loop antennas are good at rejecting mains noise. A tuned vertical antenna might seem to be a solution, but then again, a magnetic field will be just as effective at inducing currents in the conductors of a vertical antenna as it is in a loop antenna.

Where loop RX antennas do have an advantage for noise rejection is their directional properties - if there is a faily large angle between the QRM/QRN and the signal, the loop can be positioned to null out the noise. In my case, I can null out the Loran noise from Lessay, while receiving signals from most of Europe and North America, which gives about 5 - 10dB improvement in the noise level. Also, because the loops are fairly small, it is possible to move them around to find the position where the local QRM is at a minimum - the bottom of the garden in my case. The end result for me is that loops give substantially better received signals - but they don't reduce the QRN, unless it is in the nulls of the antenna.

Cheers, Jim Moritz

73 de M0BMU