Re the LF article in last mobnth's QEX...

[Andre Kesteloot]

----- 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