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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 -----
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
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