Re: Directional ferrite? - FARMERIK


What I am trying to design in my head would be to block AM radio stations from the sides, I guess that is what you mean by Null. But if it could narrow the direction signals are received from, it might really help DXing. Especially if it could block one direction from the figure 8 pattern too. Maybe a milk jug or filing crate could support the sides, and be easy to move around, like the crate wound loops. Just a thought. - FARMERIK

--- In ultralightdx@..., kevin asato <kc6pob@...> wrote:

There's a short discussion of that antenna in my 9th Edition ARRL Antenna Book in Chapter 14. It may also be in later editions, too, but this is the latest I own.


--- On Sat, 1/23/10, jim_kr1s <jkearman@...> wrote:

From: jim_kr1s <jkearman@...>
Subject: [ultralightdx] Re: Directional ferrite? - FARMERIK
To: ultralightdx@...
Date: Saturday, January 23, 2010, 9:33 AM


Back in the 1980s, the late Doug DeMaw of ARRL Headquarters, just down the road from you in Newington, got interested in the 160-Meter (1800-2000 kHz) ham band. His house was on a small suburban lot, so he invested a lot of time investigating small receiving and transmitting antennas.  One I recall was a tuned ferrite-loop receiving antenna set lengthwise in a pair of aluminum L brackets, forming a U-shaped channel. The purpose of the shield wasn't to improve directivity, though; it was to reduce electrostatic noise pickup.

A long-popular receiving loop design uses a piece of coaxial cable formed
into a large hoop. The center conductor is the antenna, and the ends
are connected to the tuning capacitor, both terminals of which are
above ground. The shield is grounded at either end, but split at the
top center, so as not to form a shorted turn. The shield reduces pickup
of locally generated noise. DeMaw applied the same principle to his
ferrite-core loop.

The only article I could find on ARRL's on-line archives (only open to members) that shows this antenna has a terrible photograph: "Beat the Noise with a 'Scoop Loop,'" QST for July 1977, pp 30-34. There's quite a bit of discussion of this antenna, winding methods for maximizing ferrite-core antenna Q, as well as tuned air loops and preamplifiers. Though oriented toward 160-M use, all of the information is applicable to our interests.

Theoretically, a large shield (needed because of the relatively long wavelengths, about 200-600 meters!) would block signals, but as you know, a loop antenna does not so much provide directivity as nulls. Larger loops, either air or ferrite-core, usually give deeper, narrower nulls.

Depending on what metallic objects are in the walls of your house, sometimes one room favors certain directions, while another favors other directions. In my wiring-and-ducting- infested condo that's certainly the case. It's worth roaming around the house, twisting and turning your portable radio as you go, to see where a particular station comes in best!


Jim, KR1S
http://qrp.kearman. com/ 

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