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Dear OM,

My message of 12:06 apparently was not passed on by the reflector so I now send it anew but without attachment.

73, Dick, PA0SE

----- Oorspronkelijk bericht -----

Van: Dick Rollema

Aan: LF-Group

Verzonden: donderdag 18 oktober 2001 12:06

Onderwerp: Article on LF Coils in QEX

Dear OM,

The ARRL publication QEX (incorporating Communications Quaterly) of September/October 2001 features a seven page article by Paolo Antoniazzi, IW2ACD and Marco Arecco, IK2WAQ: "The Art of Making and Measuring LF Coils" with subtitle "Large, high-quality coils are an important factor in working the LF bands succesfully. This short note on the specific art describes good quality-factor coils (Q around 600) for 136 kHz".

The attached table comes from the article (I hope it survives the Reflector..)

It is interesting that the coils with the lowest Q (205) uses a 16cm PVC tube as a former. I use the same material as a former but obtained Q = 350. But in my case the former was made by bolting four 12.5cm PVC tubes together, resulting in a square former with 25cm sides and rounded corners (see cover of G3LDO's The low frequency experimenter's handbook).

The Italian auhors have made a simple linear amplifier for 136kHz using an audio IC type TDA 7265. On 136kHz it produces 3 - 4W. The authors state: "This power is more than adequate for LF systems tests, generating up to 0.5 A of antenna current".

This means that the total resistance in the antenna-earth-system is no more than 12 - 16 ohms!

The article says: "A single 3 meter rod of 16 mm diameter driven into soil with 100 ohm/meter average resistivity will have a ground resistance (measured at 50 - 60 Hz) of 30 - 50 ohm. Using four parallel rods placed at 10 - 15 m in a square will give a final LF resistance of 10 - 15 ohm".

I think the authors are very fortunate to live on that kind of ground!

The antenna used for the measurements is reported to consist of "vertical (Marconi) rod of 7 - 10 meters isolated from ground using a plastic plate. About 40 - 50 meters (130 - 160 feet) of horizontal wires (hat) are connected atop the Marconi antenna to realize a 450 pF totale antenna capacity".

The same issue of QEX has another article on a related subject, written by George Murphy, VE3ERP: "The Q of Single-Layer, Air-Core Coils: A Mathematical Analysis". This 5 page article tries to find Q of a coil using theory only.

73, Dick, PA0SE